Organization of the ctenophore tentacular apparatus: Adult Mnemiopsis leidyi lacks a canonical principal tentacle.

Organization of the ctenophore tentacular apparatus: Adult Mnemiopsis leidyi lacks a canonical principal tentacle - PMC Skip to main content An official website of the United States government Here's how you know Here's how you know Official websites use .gov A .gov website belongs to an official government organization in the United States. Secure .gov websites use HTTPS A lock ( Lock Locked padlock icon ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites. Search Log in Dashboard Publications Account settings Log out Search… Search NCBI Primary site navigation Search Logged in as: Dashboard Publications Account settings Log in Search PMC Full-Text Archive Search in PMC Journal List User Guide PERMALINK Copy As a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health. Learn more: PMC Disclaimer | PMC Copyright Notice iScience . 2026 Mar 26;29(4):115508. doi: 10.1016/j.isci.2026.115508 Search in PMC Search in PubMed View in NLM Catalog Add to search Organization of the ctenophore tentacular apparatus: Adult Mnemiopsis leidyi lacks a canonical principal tentacle Gen Dong Gen Dong 1 Department of Biological Sciences, Auburn University, Auburn, AL 36830, USA Find articles by Gen Dong 1 , Dorothy Mitchell Dorothy Mitchell 1 Department of Biological Sciences, Auburn University, Auburn, AL 36830, USA Find articles by Dorothy Mitchell 1 , Anthony Moss Anthony Moss 1 Department of Biological Sciences, Auburn University, Auburn, AL 36830, USA Find articles by Anthony Moss 1, 2, ∗ Author information Article notes Copyright and License information 1 Department of Biological Sciences, Auburn University, Auburn, AL 36830, USA ∗ Corresponding author [email protected] 2 Lead contact Received 2025 Nov 13; Revised 2026 Jan 13; Accepted 2026 Mar 2; Collection date 2026 Apr 17. © 2026 The Authors This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). PMC Copyright notice PMCID: PMC13091417  PMID: 42006375 Summary Ctenophores represent one of the earliest-diverging animal lineages and are distinguished by their exceptional feeding efficiency. The tentacular apparatus of Mnemiopsis leidyi mediates feeding and likely serves diverse sensory roles, yet its structure remains poorly resolved, with conflicting accounts in the literature. Here, we clarify its organization through anatomical and experimental analyses. Light microscopy shows that tentilla bundles arise from a fan-shaped structure previously misidentified as a vestigial principal tentacle. Each tentillum connects directly to the aboral end of the tentacular bulb, contradicting earlier embedding models. Cryosectioning and lesion experiments define the origin and migration of tentilla, while videography reveals fan swaying that bilaterally distributes them. We further show that tentilla can be recycled via entrapment in the transport groove and redirected to the mouth. These findings provide an updated model for understanding tentillar growth, distribution, transport, and recycling within the feeding apparatus. Subject areas: Biological morphology, Natural sciences, Animal physiology, Evolutionary developmental biology, Animal morphology Graphical abstract Open in a new tab Highlights • Adult M. leidyi lacks a canonical principal tentacle; instead, it is a folded tentillar fan • Each tentillum is anchored to the aboral tentacular bulb, not to the groove epithelium • Fan swaying and groove cilia distribute and transport tentilla along the food groove • Tentilla are recycled via transport-groove entrapment and redirected to the mouth Biological morphology; Natural sciences; Animal physiology; Evolutionary developmental biology; Animal morphology Introduction Overview of ctenophores and Mnemiopsis leidyi (L. Agassiz, 1860) Ctenophores, commonly known as comb jellies, are a phylum of gelatinous, carnivorous marine invertebrates, notable for their early divergence within metazoan evolution. 1 , 2 , 3 , 4 Of the approximately 200 described species, most belong to Class Tentaculata, which possess tentacles throughout their life cycle, while the remainder (e.g., Beroe spp.) belong to Class Nuda, lacking tentacles entirely. 5 Ctenophores exhibit biradial symmetry defined by two perpendicular planes, the tentacular plane (associated with tentacle-bearing structures) and the sagittal plane ( Figure 1 D), and are fundamentally built around an oral-to-aboral (O-A) axis, with the mouth at the opposite end from a statocyst-containing “apical organ” at the aboral end ( Figure 1 ). 3 , 6 The defining feature of the phylum is the presence of ctenes or comb plates, which are giant ciliary paddles arranged in eight meridional rows that propel water to achieve locomotion ( Figures 1 A and 1B). 3 , 6 Comb plates, the largest ciliary organs in nature, highlight the unique adaptations of ctenophores that enable their survival in pelagic environments. Ctenophores have emerged as a model system for resolving fundamental questions in metazoan evolution, owing to their phylogenetic position and morphological innovations that challenge traditional views of nervous system, muscle, and sensory organ evolution. 2 , 7 , 8 , 9 Mnemiopsis leidyi ( Figure 1 ), commonly known as the sea walnut, belongs to Class Tentaculata, Order Lobata and is characterized by two large oral lobes (L) and four smaller, orally positioned auricular lobes, or auricles (AUs). The AUs are slender, tapered structures flanking the mouth. 3 , 6 M. leidyi also bears small auricular comb plates, which pump water through the body to collect prey. While the locomotory comb rows are arranged as if they are rungs of a ladder, the auricular comb plates are laterally organized and overlap at a low angle; they are specialized for manipulation of prey rather than propulsion. 3 Figure 1. Open in a new tab Overall appearance and diagram of M. leidyi (A) Viewed from tentacular plane. (B) Diagram from tentacular plane. (C) Viewed from the oral pole. (D) Diagram, viewed from the aboral pole. O-A, oral-to-aboral axis; AO, apical organ; AU, auricle; CP, comb plates; F, food groove; L, lobe; M, mouth; PT, principal tentacle; TB, tentacular bulb; Ti, tentilla; S, stomodeum; ss, subsagittal comb row; st, subtentacular comb row. Scale bars: 1 cm in (A) and 0.5 cm in (B). The ecological success of M. leidyi —particularly its disruptive invasions in the Black and Caspian Seas—is a consequence of its remarkable feeding efficiency, enabled by a suite of specialized morphological and behavioral adaptations. 10 , 11 , 12 , 13 Among these, the tentacular apparatus plays a key role in prey capture, working synergistically with AUs and lobes to maximize prey encounter and retention rates. 14 , 15 , 16 , 17 , 18 Thus, understanding the tentacular system provides an important foundation for understanding how morphological adaptations drive feeding behavior and ecological success in M. leidyi . Tentacular apparatus in M. leidyi : Structure and functional roles The tentacular apparatus of M. leidyi —comprising the tentacular bulb (TB), principal tentacle (PT), and tentilla—is a multifunctional system central to feeding ecology and sensory function. 12 , 14 , 19 , 20 Below, we summarize current knowledge of its anatomical components and their roles in prey capture and processing, highlighting key uncertainties that motivate further investigation. Tentacular bulb The TB arises immediately aboral to the mouth and serves as the base for all tentacular elements and defines the tentacular plane ( Figure 1 B). A bifurcated central canal underlies the bulb and probably serves to provide energy to the bulb, although its detailed functions in M. leidyi remain unresolved. Work in the cydippid ctenophore Pleurobrachia pileus has shown that the tentacular base/bulb region can regulate ciliary beating and prey capture. 21 , 22 Although Pleurobrachia does not undergo a lobate transformation and is, therefore, not directly comparable with respect to lobate-stage remodeling, these studies provide functional context suggesting that the TB in M. leidyi may participate in coordinating tentacle activity and feeding-related ciliary behaviors. The mechanistic basis of such coordination in M. leidyi remains to be demonstrated. Principal tentacle First named by Mayer, 6 the PT is positioned aboral to the mouth and arises from the TB, aligned with the tentacular plane ( Figure 1 B). The nomenclature for this lobate feature has been historically inconsistent, with terms such as central tentacle , primary tentacle , median tentacle , and oral tentacle used interchangeably. 6 , 20 , 23 , 24 The early stage of M. leidyi is considered a “cydippid” due to its shared morphological traits with the Order Cydippida ( Figure S1 ). During development, the tentacular apparatus undergoes a transition from the cydippid form to the lobate form (a process historically termed metamorphosis ). 6 , 23 , 25 The adult PT has been described as a vestige of the cydippid stage, hypothesized to represent a rudimentary structure retained through this transition. 6 , 14 , 26 To date, the detailed structure/function relationships between the cydippid and adult PT and associated tentilla remain unclear. Food groove Four food grooves (F) extend laterally from the mouth to the apical organ, paralleling the AUs ( Figure 1 ). The food grooves are ciliated furrows that are further divided into two functionally specialized epithelial features: the tentacular groove (TeG) and the transport groove (TrG) ( Figure 2 ; Video S1 ). 19 The TeG originates near the tentacle pouch (P). This region is densely populated with tentilla, which are used for prey capture. In contrast, the TrG connects directly to the mouth margin. Its position facilitates prey delivery to the lateral (sagittal) extent of the mouth via ciliary beating within the groove. 12 , 19 Figure 2. Open in a new tab Diagrams of hypothetical organization of tentilla within the food groove (A) Tentilla originating from the subepithelial surface within the tentacular groove. (B) Tentilla originating from a subepithelial tentacle embedded within the tentacular groove. O-A, oral-to-aboral axis; Ti, tentilla; TeG, tentacular groove; TrG, transport groove; Eig, inter-groove epithelium. Video S1. Prey capture process, related to Figure 3 Download video file (4.1MB, mp4) Tentilla Numerous short appendages specialized for prey capture—known in the literature as tentilla , 12 , 19 , 27 , 28 tentillae , 29 small tentacles , 15 , 23 filamentous tentacles , 30 tentacular filaments , 24 or secondary tentacles 14 —line the food grooves of M. leidyi . These structures bear specialized adhesive cells called colloblasts. 28 , 31 Here, we adopt tentilla ([Ti] singular: tentillum) to describe these appendages lining the TeG of M. leidyi ( Figures 1 B and 2). In cydippid ctenophores, including the cydippid stage of M. leidyi , tentilla classically refer to colloblast-bearing secondary branches arising from a PT ( Figure S1 ). Tentilla appear either as thin, extensible filaments in Pleurobrachia and Bolinopsis infundibulum larvae 32 or, rarely, stout, coiled structures in Euplokamis . 33 , 34 Tentilla typically possess an outer layer of dense fibrillar mesoglea and a peripheral cortex. 28 , 35 Accordingly, the adult M. leidyi tentilla superficially resemble those of cydippids. The spatial organization and developmental origin of M. leidyi tentilla are contentious. Some studies describe tentilla as directly emerging from the subepithelial surface of the TeG ( Figure 2 A), 23 , 36 while others propose a cryptic “subepithelial tentacle” analogous to cydippid morphology, with tentilla branching from this axis ( Figure 2 B). 19 , 37 , 38 This concept of a cryptic “subepithelial tentacle” was also debated in the earlier works of Chun 39 and Rolleston and Rolleston 40 based on their own observations. Resolving this ambiguity is critical for establishing functional and evolutionary homologies across ctenophore lineages. Feeding behavior Adult M. leidyi employ two complementary feeding strategies: active ambush predator and passive filter-feeding. 3 , 14 In active ambush predator, mechanosensory detection of larger, reactive prey (e.g., adult copepods) triggers rapid lobe closure, trapping organisms on mucus-coated inner surfaces. 14 , 16 In passive filter-feeding, a slow, hydrodynamically silent feeding current generated by auricular cilia entrains small prey (e.g., copepod nauplii and fish eggs) toward the tentacular apparatus. 14 , 17 , 18 , 29 The tentacular apparatus is central to the passive filter-feeding pathway. Prey entrained by the feeding current encounter tentilla, which ensnare small prey via colloblasts ( Figure 3 ; Video S1 ). 12 , 14 Captured prey are transported to the mouth via ciliary tracts in the TrG ( Figure 3 ). 12 This mechanism enables M. leidyi to feed nearly continuously, contributing to its success as a widespread and ecologically impactful predator. 17 , 41 Figure 3. Open in a new tab Schematic representation of the food groove and prey-capturing process (A) Tentilla contract and drag the prey to the edge of the transport groove (red arrow). (B) The transport groove undergoes a focal eversion (green arrow) to tuck the prey into the interior of the transport groove (purple arrow). (C) The prey is transported rapidly to the mouth (yellow arrow). O-A, oral-to-aboral axis; Eig, inter-groove epithelium; Ti, tentilla; TeG, tentacular groove; TrG, transport groove. Revisiting M. leidyi tentacle morphology: Structure and feeding mechanics Previous studies of ctenophore tentacular systems have focused predominantly on species within Order Cydippida or the cydippid stage of Order Lobata, both of which possess a pair of long tentacles proportional to body size. 32 , 34 , 35 , 42 , 43 In contrast, M. leidyi (Order Lobata) undergoes profound morphological restructuring during cydippid-to-lobate transition: the cydippid stage bears freely extending tentacles ( Figure S1 ), whereas adults possess a tentacular apparatus that is tightly integrated with the body. 6 , 19 , 23 , 25 , 39 , 44 This structural shift aligns with a transition in feeding behavior—from sit-and-wait strategy via trailing tentacles in cydippids to filter-feeding strategy via tentilla in adults. 12 , 20 Understanding the adult tentacular apparatus’ organization is thus critical for unraveling the functional basis of M. leidyi ’s capacity for food capture, and ultimately, its ecological success. Despite its ecological significance, key aspects of the adult tentacular system remain poorly understood, with conflicting descriptions in the literature. Notably, the adult M. leidyi tentacular apparatus does not appear to have been closely examined since its initial description over a century ago. 6 , 45 Here, we provide a comprehensive analysis of the adult tentacular apparatus in M. leidyi , combining morphological, histological, and time-lapse behavioral data. Our findings clarify the process of tentillar outgrowth and the underlying mechanisms of food capture. Results The tentacular apparatus All animals used in this study ranged from 1.5 to 8.5 cm in overall length (from oral to the aboral end exclusive of the lobes); all were adults. The photographs and diagrams are selected to represent the typical organization and structure of an adult M. leidyi, illustrating the relative proportions commonly observed in specimens from the Gulf of Mexico. A tentacular apparatus is already present in the cydippid stage in a simple form (two elongated tentacles), but the analyses presented here focus exclusively on the organization of the adult, body-integrated tentacular apparatus. General organization of the tentacular bulb The tentacular apparatus of a typical adult M. leidyi is shown in Figure 4 A. The TB is located aboral to the lips, with its aboralmost end positioned slightly oral to the attachment of the AU near the oralmost extent of the subtentacular comb plates (st) ( Figure 4 A). The mesoglea layer is thin under the perioral epithelium (Epo), encompassing the triangular region between the TeGs. The aboralmost end of the bulb and its junction with the tentacular canal (Ct) sit at the apex of the triangular Epo ( Figure 4 A). The bulb varies in size but, on average, constitutes approximately 13.2% ± 2.9% (n = 15) of the animal’s overall length (from aboral to oral end). The TB is the darkest structure in the animal. Its color varies from light pink to brown, likely depending on the pigments absorbed from food ( Figures 4 A and 4B). The aboralmost end resides inside a small gable-like fleshy overhang that defines a shallow tentacular pouch (P) at approximately 33.4% ± 9.3% (n = 14) of the bulb’s length ( Figure 4 B). Figure 4. Open in a new tab Light microscopy of the tentacular bulb region in adult M. leidyi (A) Light microscope image of the tentacular bulb. (B) Close-up of the tentacular bulb, with white dashed boxes indicating regions in images (C) and (D). (C) The aboral end of the tentacular bulb. (D) Fan-shaped tissue on the tentacular bulb. O-A, oral-to-aboral axis; AU, auricle; Ct, tentacular canal; Cb, bulb canal; Epo, perioral epithelium; F, food groove; ML, mouth lip; P, pouch; PBF, proximal bulbar field; PT, principal tentacle (=Fan); TB, tentacular bulb; TeG, tentacular groove; Ti, tentilla; TiF, tentillar fiber; Sep, septum. Scale bars: 1 mm in (A and B), 100 μm in (C), and 200 μm in (D). The tentacular canal branches into two parallel bulb canals (Cbs) at the aboral end of the TB ( Figure 4 C). These Cbs are separated by a septum and are blind-ended at their oral end (at the aboralmost end of the bulb itself), as confirmed by video recordings ( Video S2 ). The movement of small particles within the canals shows that they enter the video image from the top (the aboral direction), reverse direction, and exit, driven by ciliary motion ( Video S2 ). This particle movement confirms that the Cbs are closed at the oral end. Additionally, the septum provides structural support for the PT ( Figures 4 B and 4D). Video S2. Bulb canal in the aboral of TB, related to Figure 4 Download video file (1.1MB, mp4) Each tentillum is connected to the aboral end of the tentacular bulb Near the aboralmost end of the TB, we observe fine processes stretching in perfectly straight lines toward the mouth ( Figure 4 C). These lines corresponded to individual tentillar fibers (TiFs), which are visible as thin, elongated structures when the tentilla are fully extended. The fibers extend from the aboralmost region of the TB to the aboral-medial aspect of the TeG, flanking the bulb and mouth in a configuration reminiscent of human nasolabial folds ( Figures 4 B and 4C). Bundled within the groove, each fiber represents an individual tentillum ( Figure 4 C). Contrary to earlier hypotheses, these tentilla do not originate from TeG epithelium 23 , 36 or a cryptic embedded “subepithelial tentacle” 19 , 37 , 38 ; instead, all tentilla arise from and are directly anchored into the aboralmost end of the TB. The extension of the TiF is entrained by cilia, whose sweeping action on the proximal bulbar field (PBF) drags the tentillum into the nearby TeG ( Figure 4 D). This creates tension in the tentillum, which then forms a perfectly straight line from the aboralmost end of the bulb to the TeG. The tentilla bundle (TiB) is pulled together into the TeG as they are put under tension by cilia within the groove ( Video S3 ) and farther from the bulb, which consequently encompasses numerous, very fine tentilla (TiF in Figure 5 B). They are drawn along the TeG by ciliary action but emerge irregularly spaced ( Figure 5 A). The tips of the tentilla are clearly coiled and highly refractile ( Figure 5 C). Tentilla slip in and out of the groove near the bulb, but the tipmost regions lie outside of the groove after they pass the tightest curvature of the TeG immediately aboral to the mouth. Figure 5. Open in a new tab Light microscopy of the tentilla in adult M. leidyi (A) Microscopic view of tentilla extending along the food groove. The asterisk (∗) marks the open side of the groove. Fine linear fibers underlying the epithelial details, extending mainly from the 10-o’clock to 4-o’clock directions, are mesogleal muscle fibers. (B) Bundled tentillar fibers (filaments) inside the tentacular groove. (C) Coiled tentilla tips within dissected tentacular groove (TeG), stained with methylene blue to highlight the colloblast distribution. (D) Cryosection of the aboral end of the tentacular bulb, highlighting tentillar bundles. (E) Tentilla with G-cilia arranged in a “barbed clutch” configuration along the tentacular groove margin. O-A, oral-to-aboral axis; C, canal; CiG, G-cilia; E, epithelium; Ti, tentillum (plural, tentilla); Tip, tentillar tip; TeG, tentacular groove; TrG, transport groove; TiB, tentillar bundle; TiF, tentillar fibers; BL, tentacular bulb lobe; Sep, septum. Scale bars: 200 μm in (A), 10 μm in (B), 50 μm in (C), and 100 μm in (D and E). Video S3. Ciliary tension directs tentilla in TeG, related to Figure 5 Download video file (2.1MB, mp4) G-cilia organization and orientation in the tentacular groove Previous studies have identified motile cilia within the food groove, 19 and our findings further elucidate their organization within the TeG. G-cilia—small, non-motile, compound ciliary organelles—occur in two spatial populations: (1) three to four rows along the inner surface of the TeG, surrounded by hundreds of smaller motile cilia ( Video S4 ), and (2) a fringing row arranged in a comb-like configuration along the groove edge ( Figure 5 E). These edge-positioned G-cilia appear to maintain consistent tentilla spacing and prevent clustering ( Figure 5 E). We observed that ciliary orientation varies with the TeG’s anatomical curvature. Near the bulb region, G-cilia exhibit oral orientation, whereas beyond this curvature, G-cilia orientation shifts so that they point aborally, mirroring the groove’s directional extension ( Figure 5 E). Video S4. G-cilia and motile cilia in TeG, related to Figure 5 Download video file (1MB, mp4) The tentacular bulb generates the principal tentacle and anchors tentilla bundles Cryosections of TBs isolated from adult M. leidyi confirm that the TB serves as the source of both the PT and TiB. Transverse sections stained with methylene blue revealed a trilobed organization—consisting of two lateral lobes and a central septum ( Figure 5 D). Colloblasts, highlighted by their strong affinity for the dye, fully cover the tips of the TiB ( Figure 5 D). At the aboralmost end of the bulb, the TiB emerges from the septal region ( Figure 5 D). The PT also originates from this septum, indicating that the TB anchors both the PT and TiB while functioning as a continuous growth center for these structures ( Figure 5 D). The principal tentacle is a folded fan of tentilla A close examination of the PT revealed that it is a thin fan of longitudinally striated tissue (Figures 4 D and 6 , labels: Fan), which becomes layered, resembling a folded curtain, while at lower magnification without dissection, the fan appears to be a single, thick tentacle. The fan emerges from the septum of the TB and transitions from undifferentiated tissue (TiU) at its oral base—marked by irregular folds and lacking striations—to progressively organized striations aborally ( Figures 6 B and 6C). These striations align with tightly packed TiFs, representing nascent tentilla ( Figures 6 B and 6C). At the aboral edge, free, motile tentilla differentiate fully, completing their maturation within the fan ( Figures 6 B and 6C). This gradient dynamically reflects the developmental morphology of tentilla from TiU to functional, prey-capturing structures (Ti). Figure 6. Open in a new tab Light microscopy of the fan-shaped structure (A) Merged image of intact tentacular bulb. (B) Extensively dissected aboralmost bulb, revealing fine tentillar fiber from the fan. (C) Diagrammatic representation of (B). O-A, oral-to-aboral axis; Ct, tentacular canal; TB, tentacular bulb; TeG, tentacular groove; Ti, tentilla; TiF, tentillar fibers; TiN, nascent tentilla; TiU, undifferentiated tissue; P, pouch; Sep, septum. Scale bars: 0.2 mm in (A) and 0.5 mm in (B). The swaying fan The fan is typically folded but exhibits significant mobility. The aboralmost region of the fan undergoes contractions at irregular intervals, alternating with lateral swaying ( Figure 7 ; Video S5 ). Observations indicate that the PT (=fan) can occupy either side of the PBF. At the most extreme limits of its motion, the fan is transiently drawn into the TeG by the action of cilia that are immediately adjacent to the TeG. In some instances, the fan may remain centrally positioned, although the fan edge consistently orients toward one side or the other. The fan is not anchored into the TeG; instead, its position reflects a balance between ciliary-driven attraction and muscular retraction of the fan ( Figure 7 ). Figure 7 illustrates the dynamic side-switching process of the fan, which proceeds as follows. 1. Initial position ( Figure 7 A): the fan resides on the right side of the PBF, drawn into the right TeG by ciliary movement. 2. Retraction phase ( Figure 7 B): the fan contracts, pulling away from the TeG and retracting toward the aboral end of the bulb. 3. Reorientation phase ( Figure 7 C): the folded fan unfurls and rotates, reorienting toward the opposite side. 4. Completion ( Figure 7 D): ciliary activity draws the fan into the left PBF and subsequently into the left TeG, finalizing the transition from right to left. Figure 7. Open in a new tab The dynamic process of PT(Fan) switching sides (A–D) Representative images from Video S5 . (A) Initial position. (B) Retraction phase. (C) Reorientation phase. (D) Completion. O-A, oral-to-aboral axis; PBF, proximal bulbar field; PT(Fan), principal tentacle (i.e., the fan); TB, tentacular bulb; TeG, tentacular groove; Ti, tentilla. Scale bar, 2 mm in (A–D). Video S5. Swaying fan-shaped tissue, related to Figure 7 Download video file (3.2MB, mp4) This transition occurs rapidly; the process typically is complete within 3 min. Time-lapse Video S5 highlights the fluidity of the motion, underscoring the alternating interplay between fan muscle-based mobility and ciliary activity within the PBF. The tentilla peeling-off process At the aboral end of the fan, near the pouch entrance, a dynamic “peeling-off” releases developing tentilla into the TeG. As the fan sways laterally over time, the aboralmost striation initiates detachment, drawn into either the left or right TeG by ciliary movement in the PBF. Subsequent striations peel free sequentially in a zipper-like manner. This continuous development and detachment from the fan maintains a steady supply of tentilla within the food groove. Multiple tentilla may peel off simultaneously when the extreme-most fan edge forms groups of striations ( Figures 8 and S2 ; Video S6 ). Figure 8 illustrates this group detachment process using still images from Video S6 . Here, the process is described for the fan as it interacts with the upper TeG. The fan unfolds as follows: 1. Intact fan ( Figure 8 A): the fan remains fully integrated, with no visible separation. 2. Initiation of separation ( Figure 8 B): a separation field (SF) forms near the fan edge, marking detachment onset. 3. Expansion of SF ( Figure 8 C): the SF progressively enlarges, widening the gap between the separating fan (FaS) and the main fan body (FaM). 4. Detachment ( Figure 8 D): the SF is no longer evident because the FaS completely separates from the FaM, releasing tentilla (Ti). 5. Post-detachment movement ( Figure 8 E): free tentilla descend into the TeG and extend along its length. Figure 8. Open in a new tab Tentilla peel away from the fan Schematic illustration depicting representative stages from Video S6 , highlighting the sequence of separation and detachment events. (A) Intact fan. (B) Initiation of separation. (C) Expansion of the separation field. (D) Detachment. (E) Post-detachment movement. O-A, oral-to-aboral axis; Fa, fan (principal tentacle); FaM, the main body of the fan; FaS, separating fan; P, pouch; SF, separation field; TB, tentacular bulb; TeG, tentacular groove; Ti, tentilla. Video S6. Tentilla peeling-off process, related to Figure 8 Download video file (5.5MB, mp4) This entire process—from initial separation to tentilla extension—takes approximately 5 min ( Figures 8 and S2 ; Video S6 ). The gradual SF enlargement ( Figures S2 B–S2D) results in the dynamic detachment of tentilla from each other, while the final relocation of Ti into the TeG results in post-peeling structural reorganization. Time course of tentillar regeneration We performed a time-lapse recording of the regeneration following a transversely cut food groove, using an otherwise intact animal. Over approximately 8 h, the groove is progressively repaired. This observation illustrates how tentilla repopulate the food groove. Prior to the cut, tentilla were aligned along the groove, exhibiting high contractability ( Figure 9 A). Upon sectioning, the food groove split into oral and aboral segments. Tissue tension caused the cut to widen, creating a wound approximately 0.5 × 3 mm in size ( Figure 9 B). The incision immediately halted the outward movement of tentilla on the oral (right-hand) side of the cut (TiO) ( Figures 9 C and 9D; Video S7 ). Tentilla (TiA) on the left-hand aboral segment became loosely attached and were released from the groove ( Figure 9 C; Video S7 ). After about 30 min, no tentilla remained in the aboral segment, while TiO continued moving toward the cut site, progressing from the TB ( Figure 9 D; Video S7 ). TiO continued to move outward from the TB and accumulated near the cut ( Figures 9 C–9F; Video S7 ). These tentilla appeared to be flaccid and extended out of the groove, resembling train cars colliding at a broken track, while remaining attached. Figure 9. Open in a new tab Regeneration of tentilla in the tentacular groove (A–I) Show representative time-lapse images from Video S7 documenting the repair process. (A) Intact food groove prior to cutting. (B) Wound (∼0.5 × 3 mm) formed after microsurgical incision. (C) Tentilla on the aboral segment (TiA) detach and shed from the groove. (D–F) Tentilla on the oral segment (TiO) accumulate near the cut site (arrow indicates direction of movement); wound size decreases as repair progresses. (G) Tentilla (TiM) migrate rapidly across the healed wound site (arrow shows trajectory). (H) The aboral segment repopulated with quasi-regularly arranged tentilla. (I) Completed regeneration; tentilla resume normal function. O-A, oral-to-aboral axis; TiA, tentilla on the aboral side of the cut; TiO, tentilla on the oral side of the cut; TiM, migrating tentilla. Scale bar, 2 mm. Video S7. Tentillar regeneration experiment, related to Figure 9 Download video file (9.7MB, mp4) Three hours post-sectioning, the two sides of the TeG had sealed. Rapid movement of orally attached tentilla suddenly resumed across the cut site ( Figure 9 G; Video S7 ). The outward-moving tentilla (TiM) progressed rapidly (∼1 mm/min) until the groove was fully repopulated ( Figures 9 G and 9H; Video S7 ). By 8 h post-sectioning, the wound repair was complete, with the TeG packed with new tentilla. Newly positioned tentilla immediately resumed seemingly normal function, extending outward, as if for feeding ( Figure 9 I). These findings challenge two long-standing hypotheses: (1) that tentilla grow directly from the groove and (2) that they are anchored to a cryptic embedded “subepithelial tentacle.” If either hypothesis were true, tentilla on the aboral side (TiA) would not have been completely lost post-sectioning. Instead, their detachment and absence ( Figures 9 C and 9D) directly contradict these models. Crucially, the O-A directional movement of tentilla demonstrates that tentilla do not regenerate per se, but instead continuously form at the bulb, corroborating our earlier description on the TB and PT. Moreover, the sequential timing of events—specifically, tentilla accumulation at the cut site and their subsequent crossing—only occurred after the TeG regained structural integrity ( Figure 9 G). This highlights the TeG’s indispensable role in coordinating tentillar movement within the structure. Tentillar reabsorption We observed dynamic tentilla reabsorption at the aboral end of the food groove, where tentilla are recycled through focal eversion of the TrG—a localized inside-out turning in which the groove epithelium folds back (cf. Figures 3 B and 10 ; Video S8 ). Under food-deprived conditions, focal eversion occurred spontaneously, sometimes coinciding with lobe muscle contractions. This eversion formed transient folds that drew tentilla into the TrG and trapped tentilla ( Figures 10 B and 10C). Trapped tentilla exhibited two distinct outcomes: (1) escape from the fold and remain attached to the TeG or (2) progressive elongation within the groove, stretching to extreme lengths until breaking ( Figures 10 D–10F). We classify the latter case—where tentilla fractured (Tib) and were transported orally to the mouth—as tentillar reabsorption ( Figures 10 G–10I). Figure 10. Open in a new tab Tentillar reabsorption at the aboral end of the food groove (A–I) Representative images from Video S8 . (A) Normal food groove populated with tentilla (Ti), extending beyond the groove margin. (B) Focal eversion of the transport groove (TrG) under food-deprived conditions. (C and D) Eversion traps tentilla, drawing them into the TrG. (E and F) Trapped tentilla undergo extreme elongation within the TrG, leading to fracture (Tib). (G–I) Broken tentilla fragments (Tib) are transported orally to the mouth (stomodeum) via the TrG. O-A, oral-to-aboral axis; AU, auricle; TeG, tentacular groove; TrG, transport groove; Tib, broken tentilla; Ti, tentilla (singular: tentillum). Scale bar, 1 mm. Video S8. Tentillar recycling in the aboral end of TeG, related to Figure 10 Download video file (2.2MB, mp4) Reabsorption events occurred at irregular intervals, ranging from 10 min to over an hour. The frequency of these events correlated strongly with the spatial organization of tentilla at the aboral end of the groove. When densely packed and extending beyond the groove’s margin, tentilla experienced focal eversion and subsequent detachment more frequently. Discussion Revisiting the tentacular apparatus in Mnemiopsis leidyi The adult principal tentacle lacks cydippid characteristics Classical literature has described the adult PT in ctenophores as a residual structure inherited from the cydippid stage. 36 , 39 , 45 This interpretation was likely constrained by historical microscopy limitations. This concept is accepted by the scientific community, perhaps because it seems to be a logical arrangement. Our findings in M. leidyi reveal a fundamentally different organization in the adult lobate versus the cydippid tentacle. Instead of being a persistent (or residual) cydippid tentacle, the so-called principal tentacle is in fact a very thin, fan-shaped structure composed of highly aligned pre-tentillar structures, folded much like a gathered curtain. We additionally present a revised schematic of the TB region ( Figure 11 ), highlighting its fan-shaped architecture of densely folded pre-tentillar structures—a significant departure from classical interpretations. Figure 11. Open in a new tab Diagram of the tentacular bulb (A) Top view. (B) Top view with an angle. (C) Lateral view. (D) Oral view. O-A, oral-to-aboral axis; BL, tentacular bulb’s lobe; Cb, bulb canal; Ct, tentacular canal; Cp, paragastric canal; E, epithelium; Epo, perioral epithelium; F, food groove; Fa, fan; Fo, fan fold; L, lip; M, mouth; ML, mouth lip; S, stomodeum; Sep, septum; P, pouch; PBF, proximal bulbar field; TeG, tentacular groove; Ti, tentilla; TiB, bundle of tentilla; TiF, tentillar fibers. The remodeling and differentiation of the pre-tentillar fan during tentilla formation by means of an arrhythmic contraction-extension and side-to-side swinging motion is a surprising and novel mechanism underlying cydippid-to-lobate transition. In contrast to our initial expectations and the conclusions of previous work, we found no evidence of morphological continuity between the cydippid tentacle and the adult tentacular structure. This observation prompts a reconsideration of the ontogenetic pathway of the tentacular fan. A common interpretation in prior descriptions is that the adult tentacular fan reflects a reduced or residual form of the cydippid tentacle during the cydippid-to-lobate transition. 6 , 14 , 26 In contrast, our anatomical framework for the adult reveals an integrated tentacular fan and its relationship to tentilla as a structured, adult-specific organization anchoring to the aboral end of the TB. These findings are not readily explained by a simple “residue/shrinking” model of the cydippid tentacle and instead are consistent with stage-specific remodeling and/or adult-specific elaboration of the tentacular apparatus. We, therefore, hypothesize that the cydippid-to-lobate transformation of the tentacular apparatus represents a non-canonical developmental process that warrants targeted investigation. Resolving a century-old debate on the origin of tentilla in the food groove Our study redefines the structural organization and functional dynamics of the TeG and tentilla in M. leidyi . Utilizing high-resolution light microscopy and carefully selected illumination, coupled with meticulous dissection and time-lapse recordings of food groove regeneration, we challenge historical models that posited that tentilla grow directly from the TeG or an embedded tentacle within it. Instead, we demonstrate that tentilla originate from the folded, fan-like matrix (the PT) atop the TB, emerging as elongated fibers at its aboral end. The TeG does not function as a generative structure but rather as a conduit, confining tentilla until their controlled deployment into the food groove, and then at distal locations, where only the terminal regions are directly exposed to the sea water of the food groove. Each tentillum maintains a continuous anatomical connection to the bulb’s aboral end, ensuring structural continuity. These findings resolve a long-standing debate in ctenophore morphology. Chun 39 contested Fol and Buekers’ hypothesis that tentilla attach via a narrow band of epithelial cells and muscle fibers along the TeG wall. 37 , 38 , 40 Instead, Chun traced tentilla to the tentacle base, proposing their direct origin from the tentacular apparatus in lobate ctenophores, with no evidence of secondary branching. 39 Our anatomical evidence conclusively validates Chun’s interpretation, demonstrating that tentilla arise from the TB and remain anchored there—a structural continuity overlooked in earlier models. This framework also clarifies Moss’ previous observations 12 : when reporting the mechanosensory characteristics of the TeG, Moss does not show tentilla in the TeG. In fact, we now know that the absence of tentilla in isolated food groove preparations arose because Moss entirely sectioned across the TeGs shown in that article. Here, our findings indicate that tentilla are not independent structures but rely entirely on their connection to the TB. Severing this connection—as occurred during isolation of the TeG by Moss in his previous work 12 —resulted in their complete detachment from the groove, simply because they are severed, something not evident at the time. By integrating these insights, we present a revised schematic of the food groove ( Figure 12 ), which clarifies the spatial relationship between tentilla and groove, while tracing their developmental origins to the TB. This model fundamentally redefines prior paradigms, reconciling historical contradictions with empirical evidence. Figure 12. Open in a new tab Diagram of the food groove (A) General overview. (B) Close-up view of the tentacular groove. O-A, oral-to-aboral axis; C, canal; CiG, G-cilia; CiM, motile cilia; Eig, inter-groove epithelium; Lin, inner surface of the lobe; Me, mesoglea; Ti, tentilla; TiF, tentillar fibers; TeG, tentacular groove; TrG, transport groove. Reevaluating tentacular structures: Insights into ctenophore phylogeny and morphology Our study revises previous interpretations of the tentacular apparatus in M. leidyi and clarifies its structure and function. These findings may extend beyond this species, revealing potential common traits among ctenophores within Order Lobata and underscoring the need to re-examine tentacular systems across diverse lineages in the phylum Ctenophora, to be able to better understand evolutionary relationships in this group. PTs exhibit striking variability within Order Lobata. For example, Bolinopsis (Lobata) possesses a pair of reduced PTs lacking sheaths but bearing short lateral filaments, a configuration historically interpreted as vestiges of cydippid-stage tentacles. 6 , 24 , 46 , 47 In contrast, Leucothea (Lobata) displays elongated PTs extending beyond the lobe margins. These tentacles may retain lateral filaments ( Leucothea ochracea ) or lack them entirely ( Leucothea multicornis, née Eucharis multicornis ). 6 , 23 Notably, L. multicornis undergoes complete tentacle loss during development, with post-metamorphic regeneration producing novel adult tentacles devoid of lateral filaments—a pattern inconsistent with cydippid-stage tentacle vestige hypotheses. 39 Our findings in M. leidyi suggest that adult PTs also do not represent reduced cydippid structures, urging a systematic re-evaluation of tentacular origins across the Lobata. Tentilla (tentacular filaments) also vary morphologically. Bolinopsis exhibits short oral grooves lined with a single row of small tentilla near the mouth, while Leucothea displays numerous fine tentilla along the oral margin. However, prior descriptions of these structures lack clarity regarding their developmental origins or functional anatomy. Intriguingly, fan-shaped tissues atop TBs—observed in M. leidyi —are occasionally depicted in diagrams of other lobates (e.g., Bolinopsis and Leucothea ) but remain undescribed in the literature. 39 Despite differences in groove depth and tentilla arrangement, we propose that Mnemiopsis- like fans represent an anatomical homology shared among lobates. Beyond the Lobata, the tentacular apparatus of Cestum (Order Cestida) reveals evolutionary conservation despite its highly derived body plan. Cestum retains reduced PTs with sheaths and oral tentilla, mirroring features seen in Bolinopsis and Mnemiopsis . 6 , 39 , 48 This morphological stasis contrasts sharply with the complete tentacle loss observed in Ocyropsis crystallina (Lobata). Molecular phylogenies place Cestum (Cestida) closer to Mnemiopsis and Bolinopsis (Lobata) than to Ocyropsis , 4 , 48 supporting the hypothesis that tentacle loss in Ocyropsis is a derived trait. Collectively, these patterns suggest that fan-shaped tissues or tentacle bulbs represent ancestral features retained in Family Bolinopsidae and all of Order Cestida, while their absence in Ocyropsis reflects secondary reduction. Our morphological comparisons highlight a shared framework of tentacular organization within Lobata, with modifications reflecting lineage-specific adaptations. The retention of tentacle bulbs and oral tentilla in both Cestum and Bolinopsis —despite their divergent body plans—points to deep homology in tentacular systems. The novel tentacle regeneration in L. multicornis and the considerable reduction of the tentacular apparatus in Ocyropsis spp. underscore the plasticity of these structures, even in closely related species. Our findings challenge assumptions of cydippid-stage tentacular vestige homology and emphasize the need to integrate detailed developmental data with phylogenetic analyses. This integrative analysis underscores the necessity of systematically re-evaluating tentacular morphology across Ctenophora. Detailed comparative studies, supported by modern imaging and molecular techniques, including single-cell sequencing, will refine our understanding of evolutionary relationships and functional adaptations within this ecologically important phylum. A system of tentillar growth, distribution, transport, and recycling The tentacular bulb is the growth center for tentilla Our results indicate that the fan-shaped tissue atop the TB serves as the primary growth center for tentillar differentiation. The fan continuously extends from the bulb’s septum. Previous studies have demonstrated that TBs exhibit strong cell proliferation activity across various ctenophore species. In P. pileus and the cydippid stage of M. leidyi , large populations of actively replicating cells have been identified within the TB using EdU ( 5-Ethynyl-2′-deoxyuridine ) staining, which marks cells undergoing DNA synthesis. 49 , 50 , 51 , 52 , 53 , 54 This proliferative activity explains both the continuous growth of tentilla and the rapid tentacular outgrowth observed after transection. The role of fan folds in tentillar distribution The swaying behavior of fans in M. leidyi provides insight into how tentilla are distributed into the two TeGs. This distribution is likely driven by the folding of the fan, which is not a smooth, flattened structure but rather a layered, folded configuration atop the TB ( Figure 11 D). As the fan extends, tentilla peel off from its edges. The final position of each tentillum—whether in the left or right TeG—depends on its original location along the fan’s edge. This inherent folding causes a side-to-side sway, allowing tentilla to be evenly distributed between both grooves. The developmental basis of the folds in M. leidyi fan remains unclear, but observations suggest they form through a programmed developmental process. Investigating their genetic, morphological, or environmental drivers may provide insights into tentacular apparatus development. The ciliary role in tentillar organization and transport Having described the organization and regional orientation of G-cilia within the TeG, we next consider how this ciliary architecture may contribute to tentillar transport and orientation. Contrary to Moss’ original report of uniformly oral-oriented G-cilia and adjacent cilia, 19 our observations demonstrate that ciliary orientation correlates with the TeG’s anatomical curvature. Near the bulb region, G-cilia do indeed exhibit oral orientation in agreement with Moss. 19 However, beyond this curvature, G-cilia orientation shifts so that they point aborally, mirroring the groove’s directional extension. This dynamic orientation is consistent with unidirectional aboral transport of tentilla and may function as a “barbed clutch” to help prevent backward slippage toward the mouth. Intriguingly, while the TeG and TrG are anatomically parallel, their transport directions are antiparallel: the TeG propels tentilla aborally along the groove (i.e., the segment passing beyond the sharp initial oral-proximal curvature), whereas the TrG directs prey orally toward the mouth. This functional divergence likely arises from differences in ciliary orientation between the two grooves, with motile cilia and peristaltic muscular action providing the driving force for prey retrieval. In contrast, the G-cilia and their immediate surrounding regions of motile cilia may stabilize tentilla orientation and maintain the direction of tentillar transport as they extend away from the TB toward the aboral end of the food groove. Tentillar recycling in M. leidyi : A form of autocannibalism Our findings reveal that ctenophores employ a tentillar recycling mechanism that may help sustain an energetically demanding feeding system. In this process, fragile tentilla—densely packed within the TeG—are retracted, orally transported, and reabsorbed after prey capture. During feeding, portions of tentilla frequently break to entangle prey, with these consumable structures later observed within the stomodeum, confirming their ingestion. This self-consumption (“autocannibalism”) may allow Mnemiopsis to reclaim energy-rich cellular resources (e.g., proteins, lipids, carbohydrates) from discarded tentilla, reducing the energetic and biosynthetic costs of replacing these expendable adhesive structures. More broadly, this process suggests a mechanism by which ctenophores can maintain a dynamic, tentilla-rich feeding apparatus while conserving resources under fluctuating prey availability and nutritional conditions. A broader capacity for resource conservation in M. leidyi is suggested by Javidpour and colleagues, who reported larval cannibalism in the Western Baltic. 55 While the triggers and targets differ, both phenomena—consumption of conspecifics and reabsorption of self-structures—highlight a plastic capacity to obtain nutrients from ctenophore-derived tissues in variable environments. This strategy mirrors adaptive behaviors in other marine organisms. Sea cucumbers eject internal organs (evisceration) to evade predators, sacrificing expendable tissues, while regenerating them later. 56 Hydra retract and reabsorb tentacles during starvation, shrinking their bodies to prioritize survival over structural complexity. 57 Octopuses autotomize and consume their own arms under extreme stress, temporarily sustaining energy needs while regenerating limbs. 58 Similarly, planarians cannibalize their own tissues during starvation, degrading non-essential body regions to survive until resources return. 59 Like these organisms, ctenophores appear to very efficiently balance tissue loss with resource recovery, repurposing degraded structures to sustain critical functions. Our observations indicate that tentillar recycling may be part of a broader strategy of tissue recovery and functional maintenance in ctenophores. In summary, we have established an updated anatomical framework for the adult tentacular apparatus of M. leidyi , clarifying the organization of the tentacular fan and the positioning of tentilla relative to the TB and groove region. This refined architecture provides a structural basis for reinterpreting lobate feeding and its stage-specific functional shift, while also pointing to critical future questions regarding the developmental and physiological mechanisms of the cydippid-to-lobate transformation. These include the processes governing tentillar growth, turnover, coordinated movement, and recycling, whose integration may shape feeding performance and ecological interactions. Limitations of the study This study is intentionally focused on the adult-stage tentacular apparatus and, therefore, does not include a developmental series resolving how the cydippid tentacular system transitions to the adult condition. We provide an updated view of adult tentacular architecture; however, how this organization emerges during the cydippid-to-lobate transition remains an important open question. Notably, the integrated adult tentacular fan is not readily explained by a simple residue/shrinking model of the cydippid tentacle, suggesting additional remodeling steps. Future work combining staged morphology with live imaging and quantitative analysis across ontogeny will be needed to test these possibilities. Resource availability Lead contact Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Anthony Moss ( [email protected] ). Materials availability This study did not generate new unique reagents. Data and code availability • Data: Data reported in this paper will be shared by the lead contact upon request. • Code: This paper does not report original code. • Other items: Any additional information required to reanalyze the data reported in this paper is available from the corresponding author upon request. Acknowledgments The authors gratefully recognize the support of the NSF from the EPSCoR via EPS-0447675 and MCB-0348427, a Whitman Fellowship from the Marine Biological Laboratory to A.M., and support over the years from the Dept. Biological Sciences, the College of Sciences and Mathematics, and the Office of the Provost of Auburn University. G.D. was supported by a scholarship from the China Scholarship Council (CSC) and Teaching Assistantship awards from the Department of Biological Sciences at Auburn University. Author contributions Conceptualization, G.D. and A.M.; methodology, G.D. and D.M.; investigation, G.D. (regeneration experiments, cryosectioning, imaging, and videography) and D.M. (cryosectioning); visualization, G.D.; writing – original draft, G.D.; writing – review and editing, all authors; supervision, A.M. Declaration of interests The authors declare no conflict of interest. STAR★Methods Key resources table REAGENT or RESOURCE SOURCE IDENTIFIER Chemicals, peptides, and recombinant proteins MgCl 2 Fisher Scientific CAS: 7791-18-6; Pubchem CID 24644 Methylene blue Sigma-Aldrich CAT. M9140; Pubchem CID 16211647 Low melt preparative grade agarose Bio-Rad CAT. 162-0017; Pubchem CID 4604180 Sucrose Sigma-Aldrich CAT. S-0389; Pubchem CID 5988 O.C.T. compound Tissue-Tek CAT. 4583; Polyvinyl alcohol CID 7348 and Polyethylene glycol CID 25367 Experimental models: Organisms/strains Mnemiopsis leidyi Field collection from the Gulf of Mexico – Software and algorithms Image J-Fiji ImageJ-Fiji https://imagej.net/software/fiji/ Extend depth of field plugin EPFL, Biomedical Imaging Group https://github.com/fiji-BIG/Extended_Depth_Field Photoshop Adobe https://www.adobe.com/products/photoshop Open in a new tab Experimental model and study participant details M. leidyi were collected from the water surface of St. Andrew’s Bay (Panama City, FL), Apalachicola Bay, FL, Dickerson Bay, FL and Dauphin Island, AL using long handle dippers or slow, hand-drawn plankton tows (150 μm mesh, Wildlife Supply Co.). On-site salinity ranged from 20 to 34 ppt. Collected animals were transported to Auburn University, where they were housed in 200 L plankton-kreisels and maintained at 21°C–22˚C in sea water adjusted to the collection site salinity. The animals were fed on alternate days with Artemia salina nauplii (San Francisco Bay Brand), usually supplemented with Brachionus plicatilis (“L-type” rotifer, Reed Mariculture, CA). Artemia were enriched with algal food ( Spirulina powder, Ken’s Aquarium Supplies and RG Complete, AG Breeds) and Selcon (American Marine Inc.). Only animals in excellent health and free from structural defects were selected for structural analyses. A total of over 50 animals were used in this study for observation and experimentation, including 15 animals utilized for morphometric analysis of the tentacular bulb. M. leidyi is a simultaneous hermaphrodite; therefore, no sex-based comparisons were performed. Method details Light microscopy Imaging the tentacular bulb and the food groove The general morphology of the tentacular bulb and food groove in live animals was observed under a dissecting microscope (model SZ-11, Olympus) with an oblique lighting system (model TLB 3.1, Diagnostic Instruments). Images and videos were captured to a computer hard drive using a digital camera (model TCA 10 C, Tucsen, or model MU633-BI, Amscope). To closely examine the structure of the tentacular bulb, selected animals were anesthetized by adding drops of 0.5M MgCl 2 until no obvious muscular responses were observed. The tentacular bulb was carefully microdissected using angled Vannas iridectomy scissors (Storz) as indicated in Figure S3 A. The mesoglea underneath the bulb was manually thinned to reduce the sample thickness. For improved clarity, the dissected bulb then was placed inside a small Petri dish (35 mm × 10 mm, Greiner Bio-One) and examined under an Olympus BHS compound microscope with oblique illumination (40×-200×). Images were captured at different depths of field by Image Pro Plus 7.1 (Media Cybernetics) with a digital camera. Images were stacked using ImageJ-Fiji’s extended depth of field plugin, merged and edited in Photoshop 2020 (Adobe Corp.). Imaging the principal tentacle The principal tentacle (PT), arising from the aboral half of the tentacular bulb, is usually folded inside the tentacular pouch. 6 To access the PT, animals were anesthetized as before, and the tentacular bulb dissected as described in Figure S3 A, with surrounding tissue carefully removed. To expose the PT, the tentacular pouch was slit along the midline ( Figure S3 B). The folded PT was stretched and unfolded using fine-tipped tweezers (Dumont #5) and pinned using minuten pins (Carolina Biological) in a 60 mm Sylgard-coated (Sylgard, Dow Corning) glass bowl. The PT was then examined under the dissecting microscope by oblique illumination and still images and video sequences captured using as described previously. Imaging tentilla in the food groove Tentilla within the food groove are very sensitive to microsurgery and are often missing in reduced preparations. 12 To minimize loss of tentilla during dissection, the full length of the food groove was excised along the dashed line shown in Figure S3 C. The incision extended from the aboral most end of the tentacular bulb to the junction of the auricle and food groove, continuing to the aboralmost end of the food groove. The dissected preparation was then transferred to a small Petri dish (35 mm × 10 mm, Greiner Bio-One) and examined under a compound microscope (Olympus BHS microscope, 200×-1000×). Images and videos were captured as previously described. Cryosectioning of the tentacular bulb To preserve the natural topology and spatial organization of the tentacular bulb for light microscopy, we employed cryosectioning. Standard embedding methods for transmission electron microscopy (TEM) require fixation and dehydration. These processes often introduce tissue contraction, drying, and folding artifacts. Cryosectioning bypassed these steps, better maintaining the bulb’s structural integrity and minimizing distortion—critical for analyzing the delicate general architecture, which traditional embedding techniques struggle to retain. However, fine cellular detail was lost by this method. Our cryosection method was based on a modified agarose embedding method previously used on zebrafish embryos and larvae in the Dunham Lab, School of Fisheries, Auburn University. 60 Tentacular bulbs were dissected as described ( Figure S3 A) and stained with a few droplets of methylene blue (1% w/v) per 10 mL seawater for 30 min to enhance visualization for cells. The stained bulbs were then transferred and immersed in melted 2% low melt preparative grade agarose prepared in 5% sucrose at 70 °C. The sample solidified at room temperature (22 °C) for approximately 1 h. Once solidified, an agarose block containing the bulb (typically a cube 1.5 cm × 1 cm × 0.5 cm) was excised and placed into a 30% sucrose solution, then stored at 4 °C overnight until the block sank and was fully immersed. Before sectioning, the agarose block was coated with O.C.T. compound and frozen on a mounting plate in a cryostat (model 2800N, Reichert-Jung) at −30 °C. Thirty-micron-thick cross sections of the bulb were cut using single-use blades (model 818, Leica) and collected on critically pre-cleaned slides (VWR VistaVision). The cryosections were imaged immediately by brightfield illumination after sectioning using an Olympus BHS microscope. Effects of tentillar resection; tentillar regeneration M. leidyi exhibits remarkable regeneration capacity, 61 , 62 making it an ideal model for studying tentillar formation, development, and outgrowth. To observe tentilla growth, whole animals were immobilized in natural seawater with stainless steel minuten pins (Carolina Biological) in a 60 mm Sylgard-coated glass Petri dish (Corning). The food groove, typically covered by the lobe, was fully exposed by everting the inner surface of the lobe with tweezers and pinning it back ( Figure S4 A). Animals were positioned as shown in Figure S4 A, with lighting adjusted to optimally illuminate a short length of food groove using oblique light. A microsurgical cut (0.5 × 3 mm) was made at right angles across the tentacular groove with very fine iridectomy scissors (Storz) ( Figure S4 B). This procedure was minimally invasive, precisely cutting the tentacular groove with little impact on other tissues. Regeneration of the incision site was monitored using a dissecting microscope (model M8, Wild-Heerbrugg) equipped with a camera adaptor (Martin Imaging, Easley, SC). Wound repair and recovery were recorded via time-lapse intervalometry using a high-definition 1080p, 12-megapixel camcorder (Everio model GZ-HM320BU, JVC Corp., Japan), capturing images at 2 s intervals. Quantification and statistical analysis Tentacular dimensions were quantified using ImageJ-Fiji from scaled images. Descriptive statistics (mean ± standard deviation, SD) were calculated in Microsoft Excel. Statistical details (including exact N) are reported in the Results. N denotes the number of animals. 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Prey capture process, related to Figure 3 Download video file (4.1MB, mp4) Video S2. Bulb canal in the aboral of TB, related to Figure 4 Download video file (1.1MB, mp4) Video S3. Ciliary tension directs tentilla in TeG, related to Figure 5 Download video file (2.1MB, mp4) Video S4. G-cilia and motile cilia in TeG, related to Figure 5 Download video file (1MB, mp4) Video S5. Swaying fan-shaped tissue, related to Figure 7 Download video file (3.2MB, mp4) Video S6. Tentilla peeling-off process, related to Figure 8 Download video file (5.5MB, mp4) Video S7. Tentillar regeneration experiment, related to Figure 9 Download video file (9.7MB, mp4) Video S8. Tentillar recycling in the aboral end of TeG, related to Figure 10 Download video file (2.2MB, mp4) Document S1. Figures S1–S4 mmc1.pdf (558.1KB, pdf) Data Availability Statement • Data: Data reported in this paper will be shared by the lead contact upon request. • Code: This paper does not report original code. • Other items: Any additional information required to reanalyze the data reported in this paper is available from the corresponding author upon request. 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