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Sucker (zoology)
Sucker (zoology)
Sucker (zoology)
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Scolex of Taenia solium with four suckers and two rows of hooks

A sucker in zoology is a specialised attachment organ of an animal. It acts as an adhesion device in parasitic worms, several flatworms, cephalopods, certain fishes, amphibians, and bats. It is a muscular structure for suction on a host or substrate. In parasitic annelids, flatworms and roundworms, suckers are the organs of attachment to the host tissues. In tapeworms and flukes, they are a parasitic adaptation for attachment on the internal tissues of the host, such as intestines and blood vessels.[1] In roundworms and flatworms they serve as attachment between individuals particularly during mating. In annelids, a sucker can be both a functional mouth and a locomotory organ.[2] The structure and number of suckers are often used as basic taxonomic diagnosis between different species, since they are unique in each species. In tapeworms there are two distinct classes of suckers, namely "bothridia" for true suckers, and "bothria" for false suckers. In digeneal flukes there are usually an oral sucker at the mouth and a ventral sucker (or acetabulum) posterior to the mouth. Roundworms have their sucker just in front of the anus; hence it is often called a pre-anal sucker.

Among chordates, some fishes and mammals have suckers, which are used as a holdfast to substrata. Among fishes some members of the order Perciformes have modified fins that form a sucker. Some bats, the Madagascar and the Western sucker-footed bat have unusual suckers on their limbs that are useful during roosting. Some amphibians such as the frog have adhesive pads on their toes to help with their locomotion.

In helminths

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Turbellaria

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In the class Turbellaria, only the species of the order Temnocephalida are parasitic and possess an adhesive disc. The sucker is present at the posterior end on the ventral side.[3] It is lined with syncytial epidermis and numerous microvilli. Beneath the apical membrane are many vacuoles and dense bodies. It is attached to the body through a short stalk. Densely packed muscle fibres link the sucker with the main body through the stalk.[4]

Udonellidae

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Udonellids are symbiotic to fishes, on which body they remain attached using a sucker. The sucker is a membranous extension of the posterior end. It has an indistinct stalk and the anterior surface is lined with microvilli. Some portion of the tegument has interconnected surface extension appearing as loops. The interior is divided into several compartments which are surrounded by interconnected connective tissue. The connective tissues are linked with muscles that extend into the main body.[4]

Tapeworms

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In tapeworms, the sucker is called bothridium (plural "bothridia")[5] to differentiate it from the sucker-like protrusion called bothrium in some species.[1]

Flukes

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Various aspects of anterior sucker of a zoogonid digenean[6]

Among the flukes belonging to class Digenea, there are two suckers, namely an oral sucker and a ventral sucker (often called acetabulum).[7] The oral sucker is at the tip of the anterior body and directly surrounds the mouth. The ventral sucker is located halfway to the middle of the body on the ventral side. They are both used for attachment to intestinal wall and blood vessels. The detailed structure of the suckers, presence or absence of hooks, and their exact position on the body are major taxonomic keys between species.[8]

In the class Monogenea, oral suckers are present in worm parasites of the order Mazocraeidea. They are known to have muscular, glandular, and sensory components thought to play some role in blood feeding. In other species like Anoplodiscus, the sucker is a posterior extension, connected to the main body through a small stalk. The surface is profusely covered with microvilli. It is used for symbiotic association with fishes.[4]

Nematodes

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Parasitic roundworms such as species of Ascaridia and Heterakis possess a single sucker at the posterior end of the body, just in front of anus, hence is often called a pre-anal sucker. Only the male roundworms have them, and are used for attachment to female during mating. The sucker is a protruding cuticle and circular in shape.[9][10]

In annelids

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A medicinal leech with its oral sucker

Annelid worms such as leeches all have an anterior (oral) sucker formed from the first six segments of their body, which is used to connect to a host for feeding. It also releases an anaesthetic to prevent the host from feeling pain while it sucks blood. They use a combination of mucus and suction (caused by concentric muscles in those six segments) to stay attached and secrete an anti-clotting enzyme, hirudin, into the host's blood stream. The medicinal leech (Hirudo medicinalis) has two suckers, one at each end, called the anterior and posterior sucker. The posterior is mainly used for leverage while the anterior sucker, consisting of the jaw and teeth, is where the feeding takes place.[11] During locomotion directional movement of the body is done by successive attachment and detachment of the oral sucker and the acetabulum.[2]

Molluscs

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See also: Cephalopod limb
An octopus displaying its suckers

Cephalopods are characterised by elongated appendages known as cephalopod limbs for locomotion and grasping objects. There are two main types: arms, such as in octopus, bearing numerous suckers along its ventral surface; and tentacles, such as in squid and cuttlefish, having a single sucker at the tip.[12] Each sucker is a circular and bowl-like curved disc. It in turn has two distinct parts: an outer shallow cavity called infundibulum and a central hollow cavity called acetabulum. Both these structures are thick muscles, and are covered with chitinous cuticle to make a protective surface.[13] It is used for grasping substratum, catching prey and for locomotory accessory. When the sucker attaches itself on an object, the infundibulum mainly provides adhesion while the central acetabulum is quite free. The sequential muscle contraction the infundibulum and acetabulum causes attachment and detachment.[14]: 1146–1148 [15]

In fish

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Eumicrotremus phrynoides and Eumicrotremus orbis – lumpsuckers demonstrating adhesive pelvic discs.

Gobies, remoras and lumpsuckers have suckers which are modified fins. These fishes use their suckers to cling to substrata or to bigger fishes. In gobies the disc-shaped sucker is formed from fused pelvic fins. Amphidromous gobies particularly use their suckers for climbing through waterfalls during their developmental migrations.[16][17] In remoras the sucker is a modified dorsal fin. In lumpsuckers, also known as lumpfish, the sucker is formed from modified pelvic fins, located ventrally, and behind the pectoral fins.

A fish family the Catostomidae are known as suckers. These fish have a suckermouth.

In bats

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Certain species of bats such as Madagascar sucker-footed bat and Western sucker-footed bat, are generally called "sucker-footed bats" because of suckers on their limbs. They are members of the family Myzopodidae and endemic to Madagascar. They have small cups of suckers on their wrists and ankles. They roost inside the rolled leaves of palm trees, using their suckers to attach themselves to the smooth surface.[18][19]

References

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Sucker (zoology)

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A sucker in is a specialised attachment organ found in various animals, functioning by creating a partial to adhere to surfaces, hosts, or prey. These organs occur in diverse phyla, including Platyhelminthes (flatworms), Nematoda (roundworms), Annelida (segmented worms), (mollusks such as cephalopods), and Chordata (vertebrates like some , amphibians, and bats). Suckers primarily serve adhesion for parasitism, locomotion, or feeding. In parasitic worms, they enable attachment to host tissues; in cephalopods, they facilitate prey capture and movement; and in some fish and amphibians, they aid in clinging to substrates in flowing waters. The structure typically involves a muscular, cup-like or disc-shaped organ with a sealed rim to maintain suction. Suckers have evolved convergently multiple times across animal lineages, reflecting adaptations to aquatic or semi-aquatic environments where adhesion provides ecological advantages. Their morphology varies, from simple oral suckers in flatworms to complex, ringed suckers in octopuses, often used taxonomically within groups. Fossil evidence and phylogenetic studies suggest independent origins, with early appearances in Paleozoic marine invertebrates.

Overview

Definition

In zoology, a sucker is a specialized muscular organ that enables adhesion to substrates or hosts through mechanisms such as negative pressure generation, muscular contraction, or mucus secretion. These organs are prevalent in various invertebrates, particularly parasitic worms, where they facilitate attachment without penetrating host tissues. The structure typically involves radial, circular, and longitudinal muscle fibers that allow for dynamic sealing and pressure modulation to maintain grip. Suckers are classified into true suckers and pseudo-suckers based on their morphology and strategy. True suckers, such as found in trematodes and some cestodes, feature a cup-shaped with an outer rim (infundibulum) and inner cavity (), enabling the creation of a sealed for suction-based attachment. In contrast, pseudo-suckers, exemplified by bothria in pseudophyllidean cestodes, are elongated, weakly muscular grooves or adhesive discs lacking a true enclosed cavity, relying instead on surface via or . The term "sucker" originated from early observations of attachment structures in parasitic worms and was first formally described in 18th-century literature, notably in Johann August Ephraim Goeze's work on the natural history of intestinal worms, which detailed holdfast organs in cestodes.

Functions

Suckers in serve multiple adaptive functions across various animal phyla, primarily enabling secure attachment, movement, nutrient acquisition, reproductive behaviors, and sensory perception. These roles leverage the muscular hydrostat structure of suckers, which allows for dynamic adhesion and release through contraction and relaxation. One primary function is attachment to hosts or substrates, particularly in parasitic species, where suckers prevent dislodgement by host movements or environmental forces. In trematodes, oral and ventral suckers secure the parasite to host tissues such as ducts or intestinal walls, facilitating prolonged . Similarly, in leeches, anterior and posterior discs anchor the animal to the host during feeding or transit, withstanding forces up to several times the leech's body weight. This is achieved via partial formation and muscular pressure, ensuring stability in dynamic aquatic or host environments. Suckers also contribute to locomotion by enabling alternating attachment and release cycles. In leeches, posterior sucker fixation followed by anterior extension and attachment produces an inchworm-like crawling motion, allowing efficient progression over uneven surfaces. Cephalopods, such as octopuses, use sucker arrays along their arms for substrate crawling, where coordinated contractions propel the animal while maintaining grip. In feeding, suckers facilitate prey capture and nutrient ingestion. Oral suckers in parasitic flatworms and leeches create suction to draw in or tissue fluids, with the anterior disc in leeches specifically adapted for piercing and sucking host . In cephalopods, suckers grasp and immobilize prey like crustaceans, aiding in manipulation before consumption. For mating, suckers or analogous structures provide grip during copulation. In certain nematodes, the male copulatory , a flap-like extension functioning as an attachment organ, grasps the to stabilize positioning and ensure transfer. Additionally, some suckers perform sensory roles, integrating chemotactile detection. In s, sucker epithelia contain specialized chemoreceptors that sense chemical cues and textures via direct contact, enabling "taste-by-touch" discrimination of food or environmental stimuli.

Evolutionary origins

Suckers and related adhesive structures have evolved convergently across multiple phyla, including Platyhelminthes, Annelida, and , in response to selective pressures favoring temporary attachment in aquatic environments. This polyphyletic origin is evident in suction-based organs that appeared independently in at least five metazoan phyla, driven by needs such as host adhesion in parasites and prey capture in predators. For instance, in Platyhelminthes, suckers facilitate parasitic attachment to hosts, while in molluscs, they enable active predation and locomotion. In free-living Platyhelminthes (), the duo-gland adhesive system—comprising viscid () and releasing glands—represents a primitive configuration for reversible attachment. In parasitic lineages, specialized muscular suckers have evolved for host attachment. In , suckers trace back to the coleoid stem lineage, emerging as transverse bulges or primitive infundibula in Permian-Jurassic fossils of early aulacoceratids and phragmoteuthids.

In Platyhelminthes

Turbellaria

In , the primarily free-living flatworms within Platyhelminthes, adhesive structures known as discs are present in certain symbiotic taxa, particularly the order Temnocephalida, where they facilitate non-parasitic attachment to host organisms or substrates. These structures differ from the more specialized suckers found in parasitic flatworms by relying primarily on glandular secretions rather than extensive muscular suction mechanisms. The posterior ventral adhesive disc in Temnocephalida is a key feature, composed of a syncytial —a multinucleated layer without cell boundaries—that covers the ventral surface and enhances surface area through numerous microvilli, which support mucus-based adhesion.48003-0) The disc's integrates with a duo-gland system, including viscid glands that secrete proteinaceous, often acidophilic and releasing glands that enable detachment, allowing for temporary or semi-permanent without penetrating host tissues. In species like Temnocephala, the disc lacks a true central muscular cavity typical of suction-based suckers but incorporates underlying attachment muscles to aid in positioning and stability during . These glands open via ducts directly onto the disc surface, releasing secretions that form a viscous bond, often supplemented by rhabdite-derived for protection and grip. The disc is typically pedunculated, with a ranging from 0.3 to 1 mm depending on species, and is bounded by a peripheral ridge or groove that delineates the adhesive zone.48003-0) Functionally, the adhesive disc in Temnocephalida supports an ectocommensal lifestyle, enabling attachment to freshwater hosts such as crayfish (Cherax species), where the flatworms reside on carapaces, gills, or branchial chambers to access food particles without causing harm or deriving nutrients directly from the host. For example, Temnocephala chilensis uses the disc to maintain position on crayfish exoskeletons, facilitating feeding on detritus and symbiotic benefits like mobility. This structure is diagnostically significant for Temnocephalida, distinguishing the order by the presence of the pedunculated posterior disc alongside features like tentaculate anterior ends and doliiform pharynx. Unlike the holdfast organs in parasitic groups such as Monogenea, these discs emphasize reversible, mucus-mediated adhesion suited to symbiotic interactions.48003-0)

Monogenea

Monogeneans are primarily ectoparasites of and , utilizing specialized suckers for attachment to host or skin surfaces. The primary attachment organ is the posterior haptor, a discoid muscular structure armed with clamps or suckers that enable firm adhesion without deep tissue penetration in most cases. In subgroups like the Polyopisthocotylea, the haptor features multiple discoidal suckers, often asymmetrical and supplemented by marginal hooklets, facilitating precise grip on delicate gill tissues. For instance, in species such as Polystoma integerrimum, the haptor is bowl-shaped with paired discoidal suckers on short stems, allowing effective clinging to amphibian hosts. Certain monogenean lineages exhibit additional anterior suckers for supplementary attachment. In the order Mazocraeidea, a pair of muscular oral suckers surrounds the , incorporating glandular and sensory elements that aid in initial host contact and feeding site selection on . These suckers are typically aseptate and protrude as pseudosuckers, measuring around 37–67 µm in length, as observed in Pseudochauhanea mexicana from hosts. This configuration contrasts with the more generalized prohaptor in other monogeneans, emphasizing the diversity in attachment strategies across subgroups. A unique variation occurs in the family Udonellidae, where the posterior sucker is a membranous extension lacking hooks or clamps, relying instead on glandular secretions for adhesion. Ultrastructural studies reveal this sucker lined with microvilli and subdivided into compartments by connective tissue strands and muscle filaments, enhancing flexibility and secretory function. Udonella species, such as U. caligorum, exemplify this by attaching symbiotically to the exoskeleton of copepod parasites (e.g., Caligus spp.) on fish skin, without penetrating host tissues. This non-invasive mode supports their role as epibionts rather than direct parasites. These suckers primarily function to secure the monogenean against host movements and water currents, with attachment to lamellae or epidermal surfaces enabling uptake via direct absorption or feeding in polyopisthocotyleans. Recent ultrastructural analyses from the highlight sensory papillae on the haptor and body surface, including uniciliated receptors and non-ciliated mechanoreceptors, which detect host cues for site-specific attachment. For example, in diplozoid monogeneans like Eudiplozoon nipponicum, these well-innervated papillae cluster around attachment zones, facilitating host detection and orientation. Such adaptations underscore the evolutionary refinement of monogenean suckers for ectoparasitic lifestyles.

Cestoda

In the class , commonly known as tapeworms, suckers are specialized attachment organs located on the scolex, the anterior holdfast structure that anchors the parasite to the host's intestinal mucosa. These organs enable the acoelomate worms to maintain position within the gut despite peristaltic movements and nutrient flow. The primary types of suckers are bothridia and bothria, which differ in morphology and mechanism of attachment, with their configuration often serving as a diagnostic feature for taxonomic identification. Bothridia are true cup-shaped suckers, characterized by muscular walls that create a for secure through . These leaf-like or semicircular structures, typically four in number and arranged laterally on the scolex, are highly mobile and can pinch or envelop host tissue for enhanced grip. In contrast, bothria are groove-like false suckers, appearing as elongated slits or shallow depressions without full enclosure, relying primarily on and muscular contraction rather than pure for attachment. The muscular nature of both types allows for dynamic adjustment to host movements, but bothria are less efficient at forming airtight seals. Functionally, these scolex suckers resist expulsion by , ensuring nutrient absorption across the tegument while minimizing host tissue damage to prolong infection. For instance, in Taenia species such as the pork tapeworm , the four bothridia provide robust anchorage in the human , often supplemented by a rostellum with hooks for added security. Conversely, in the fish tapeworm Diphyllobothrium latum, two dorsal and ventral bothria facilitate attachment in the , where the grooves align with intestinal folds for frictional hold. The number and arrangement of these organs are phylogenetically informative, with basal cestode lineages like pseudophyllideans featuring bothria, reflecting an evolutionary progression toward more enclosed bothridia in derived groups such as cyclophyllideans for improved efficiency.

Trematoda

Trematodes, commonly known as flukes, exhibit a distinctive dual sucker system that facilitates their parasitic lifestyle within and hosts. The oral sucker is positioned anteriorly around the mouth, serving as a muscular organ for both feeding and temporary attachment, while the ventral sucker, or , is located mid-ventrally and acts as the primary holdfast structure. Both suckers consist of layered musculature enveloped by a syncytial tegument, which provides resilience and sensory capabilities during host interactions. The oral sucker functions mainly in ingestion by drawing host tissues and fluids into the pharynx through coordinated contractions of its radial and circular muscle fibers, while also enabling attachment during larval migration stages such as cercariae penetrating host skin or mucosa. In contrast, the ventral sucker ensures stable fixation to internal sites like intestinal linings, blood vessel walls, or organ tissues, countering host defenses and movements via powerful suction generated by its equatorial and meridional muscles. This division of labor supports the complex life cycles of trematodes, allowing penetration, migration, and long-term residency in definitive hosts. Within the subclass , which encompasses most trematode species, the paired suckers are well-developed, as seen in blood flukes like spp., where the ventral sucker is notably reduced in females to suit their elongated, paired habitation within the male's gynecophoric canal. Basal trematodes, such as those in Aspidogastrea, display Monogenea-like posterior adhesive organs alongside simpler anterior suckers, reflecting evolutionary transitions toward more specialized attachments. These structures share functional similarities with the bothridia of cestodes, though trematode suckers are more mobile and paired for dynamic host invasion. In trematode taxonomy, the size ratio of the oral to ventral sucker—often denoted as oral sucker width or length relative to the acetabulum—is a critical diagnostic trait for species delineation; for example, ratios around 1:1.3 to 1:1.6 are common in allocreadiid genera, aiding identification in morphological keys. Studies from the 2010s, using confocal microscopy and phalloidin staining, have revealed intricate muscle fiber arrangements in suckers, including interwoven longitudinal, circular, and diagonal layers that optimize force generation for adhesion, with radial fibers enhancing suction efficiency during attachment.

In Nematoda

Copulatory suckers

In male nematodes, the copulatory sucker, also referred to as the precloacal or pre-anal sucker, is a specialized ventral structure positioned immediately anterior to the cloaca. This organ consists of a cuticular inflation forming a cup- or sucker-like depression, reinforced by a robust, chitinous or sclerotized rim that provides structural support, and encircled by a muscular sheath enabling contraction and suction. The size and shape of the sucker vary across species, typically measuring 30–100 μm in diameter, with more pronounced forms in certain parasitic taxa exhibiting thicker rims for enhanced grip. The primary function of the copulatory sucker is to facilitate mating by grasping the female's cuticle near the vulva, stabilizing the pair and ensuring effective sperm transfer via the male's spicules, without involvement in host attachment. This reproductive adaptation is absent in females and is a sexually dimorphic trait exclusive to mature males, where it develops progressively during post-embryonic stages, enlarging as the worm reaches sexual maturity. In some groups, the sucker is homologous to cloacal chamber modifications that aid in gamete deposition. This structure is prevalent in both free-living and parasitic nematodes. For instance, in the parasitic species Ascaridia galli, the oval precloacal sucker, supported by a sclerotized ring, measures approximately 210–320 μm and plays a key role in attachment during copulation within avian hosts. Similarly, free-living marine nematodes such as those in the Latronema exhibit sucker-like precloacal supplements that function analogously in reproductive pairing. Unlike parasitic attachment structures, the copulatory sucker is adapted solely for inter-individual reproductive contact.

Parasitic attachment suckers

In parasitic nematodes, attachment to host tissues is often facilitated by anterior structures such as cuticularized buccal capsules, found in various superfamilies including the Heterakoidea. These consist of a hardened, chitinous cavity surrounding the , often reinforced with a thick to withstand mechanical stress from host movement and . In some , the buccal capsule incorporates glandular elements that secrete , facilitating grip on intestinal mucosa or tissue surfaces. The primary function of these structures is to provide anchorage, enabling the to feed on host fluids, blood, or tissues while resisting dislodgement in the . Unlike the more permanent holdfasts in flatworms, nematode buccal capsules allow for intermittent release and reattachment, supporting locomotion and evasion of host immune responses. This mechanism is particularly crucial in intestinal environments, where peristaltic forces could otherwise dislodge the parasite. Glandular secretions from associated esophageal glands further enhance by creating a mucoid seal that resists shear forces. Representative examples include species of Heterakis, such as H. gallinarum and H. dispar, which inhabit the ceca of birds and mammals. These s feature small, non-armed buccal capsules with a massive cuticular lining and osmiophilic reinforcements. Recent ultrastructural analyses, including scanning electron microscopy, have revealed the detailed morphology of these capsules in veterinary parasites like H. dispar, highlighting their role in feeding without dentigerous elements. Such structures are simpler in nematodes compared to platyhelminths and are frequently augmented by stylet-like projections or amphidial secretions in other nematode groups for enhanced grip.

In Annelida

Anterior suckers

The anterior sucker in leeches, such as those in the genus , is a specialized disc-like structure located at the front of the body, formed by the ventral halves of the first four segments. This small oral sucker encircles the mouth opening and consists of a muscular hydrostat reinforced by radial and circular muscle fibers, along with glandular tissues that secrete mucus for enhanced sealing and adhesion during attachment. In species like , the sucker houses a retractable tripartite jaw apparatus positioned behind a thin velum; these chitinous jaws are rigid, lack papillae, and each bears approximately 60 sharp teeth equipped with secretory apertures near their tips for delivering anticoagulants. Functionally, the anterior sucker facilitates blood ingestion by first attaching to the host's skin, where the jaws protrude to puncture the epidermis in a Y-shaped incision, followed by pharyngeal suction to draw blood into the crop. This dual role extends to initial attachment during locomotion, enabling the leech to grip substrates or hosts while the body extends forward. In Hirudo medicinalis, sensory innervation from mechanosensory and chemosensory neurons in the rostral ganglia detects host cues like warmth and chemical signals on the skin, triggering precise sucker placement and jaw deployment for effective feeding. Compared to the larger posterior sucker, the anterior variant is adapted for feeding efficiency, exhibiting a smaller surface contact area that expands by about 52% upon extension to optimize puncture and suction. Biomechanical analyses of verbana (a close relative of H. medicinalis) in 2016 utilized high-speed to document feeding sequences, revealing that the anterior sucker achieves higher attachment tenacity—up to 27.70 mN/mm²—due to its glandular secretions and muscular control, which support rapid underwater or on irregular host surfaces. These adaptations underscore its role in enabling leeches to consume blood meals equivalent to up to 10 times their body weight in a single session.

Posterior suckers

The posterior sucker in leeches, a subgroup of annelids, is a specialized organ located at the caudal end of the body, formed by the fusion of the last seven segments and lacking any oral orifice. This structure features a flexible, joint-like constriction at its base for enhanced maneuverability, an inner surface lined with radially arranged furrows and -secreting glands, and a ring of circular and radial muscles that generate negative pressure to create a strong seal. Unlike the anterior sucker, it serves primarily non-feeding roles, relying on to augment , particularly in aquatic environments where attachment tenacity reaches approximately 26.70 mN/mm² compared to 16.04 mN/mm² in air. Functionally, the posterior sucker acts as a primary during inchworm-style locomotion, where it attaches to the substrate, allowing the body to extend and the anterior sucker to reattach forward, facilitating progression across varied terrains. It also provides leverage and stability during host attachment for feeding, integrating briefly with the anterior sucker to maintain position without direct involvement in . In the absence of chaetae—bristle-like structures present in other annelids that aid in traction—leeches rely on these suckers for locomotion and , compensating for the loss through evolved muscular and secretory adaptations. This organ is present in leeches (class Hirudinea), such as in the family Hirudinidae with the medicinal leech verbana, where the posterior sucker exhibits a seal-off contact area that expands by up to 59% during extension, enabling robust attachment forces exceeding 447 mN. Aquatic species generally feature proportionally larger posterior suckers adapted for swimming and substrate gripping in water, whereas terrestrial leeches, like those in humid forests, prioritize compact designs for crawling on land without submersion capabilities. Experimental measurements have quantified its performance, revealing superior endurance in attachment phases lasting over 1 second, underscoring its role in survival across habitats.

In Mollusca

Cephalopod suckers

Cephalopod suckers are specialized muscular structures located on the arms and tentacles of most , enabling precise attachment and manipulation in aquatic environments. In octopuses, these suckers are sessile, meaning they are directly embedded in the arm tissue without a stalk, whereas in squids and , they are typically stalked, allowing greater flexibility in positioning. Each sucker comprises two primary components: the infundibulum, an outer rim that forms the initial seal against a substrate, and the , an inner cavity that generates negative pressure for . The acetabulum is lined with a complex array of musculature, including radial, circular, and meridional fibers, functioning as a muscular hydrostat to control attachment and detachment. In some species, such as certain squids, the acetabulum features denticles or chitinous hooks that enhance grip on slippery prey. These suckers serve multiple functions central to the predatory of cephalopods, including grasping and subduing prey, facilitating locomotion by adhering to surfaces, and enabling fine manipulation of objects. The muscular arrangement allows for rapid reconfiguration, where radial muscles expand the to create , while circular and meridional muscles contract to release it. Additionally, suckers incorporate chemosensory receptors, such as afferent endings that detect chemical cues from prey or environmental stimuli, allowing octopuses to "taste" surfaces without oral contact. Mechanosensory cells within the sucker also provide tactile feedback, contributing to coordinated movements. In the common octopus (Octopus vulgaris), each of the eight arms bears approximately 280 suckers arranged in two rows, providing immense gripping power for capturing crabs and fish. Developmental studies reveal that sucker formation occurs sequentially along the arm, beginning with buds emerging from a ridge-like sucker field on the oral surface during embryogenesis; this process, observed in related cephalopods like cuttlefish, ensures functional suckers are present by hatching. Sucker development in squids follows a similar proximal-to-distal pattern, with functional structures appearing first near the arm base. Variation exists across cephalopod groups, notably in nautiloids, which lack suckers entirely and rely instead on numerous tentacles for prey capture. This absence highlights an evolutionary divergence, as advanced s like octopuses and squids evolved suckers as a key adaptation for active predation.

Other molluscan attachment structures

In non- molluscs, attachment structures primarily rely on the muscular foot rather than specialized suckers with acetabula, enabling adhesion to substrates through a combination of suction and secretion. Gastropods, such as limpets, utilize a broad, flattened foot that creates a sealed when pressed against rocks, augmented by from pedal glands to enhance grip and prevent slippage. This mechanism allows limpets like Patella vulgata to withstand high wave forces in intertidal zones, where the foot disc conforms to irregular surfaces for temporary or semi-permanent attachment. Unlike the true suckers of cephalopods, these gastropod structures lack a chitinous or cartilaginous and instead depend on hydrostatic generated by foot muscles contracting against the shell, producing forces typically in the range of 10-50 N/cm² under natural conditions. Bivalves employ a different approach with threads—proteinaceous filaments secreted by a glandular foot—that anchor them to hard surfaces or each other, providing temporary rather than suction-based holdfasts. For instance, mussels like Mytilus edulis use these threads to form flexible attachments that resist tidal currents, with each thread capable of tensile strengths around 20-50 MPa. These attachment adaptations in gastropods and bivalves are not homologous to the infundibulum-bearing suckers of cephalopods, despite sharing a common molluscan ancestry; instead, they represent convergent evolutions for sessile or semi-sessile lifestyles. This distinction highlights the pedal origins of non-cephalopod attachments, evolved primarily for environmental stability rather than predation or manipulation.

In Chordata

Fish

In , suckers are specialized structures derived from fins or mouths that enable attachment to substrates or hosts in aquatic environments, primarily through mechanisms that create negative pressure for . These adaptations are crucial for locomotion, feeding, and , allowing species to resist water currents or hitch rides on larger organisms. Common examples include fin-based discs in remoras and gobies, as well as oral suckers in certain benthic like lumpsuckers and . Remoras (family Echeneidae), such as Echeneis naucrates, possess a unique modified into an elliptical, slit-like adhesive disc lined with transverse lamellae—bony, comb-like structures that can rotate to conform to irregular surfaces. The disc is bordered by a fleshy epithelial lip that forms a tight seal, while spinules on the lamellae provide frictional grip to prevent slippage. This structure generates by reducing through muscular contraction of fin rays, enabling attachment to hosts like or for transport and access to food such as parasites. The is maintained via compartmentalized chambers formed by the lamellae, with an anterior cardinal sinus equalizing pressure to minimize leakage and prolong . Gobies (family ) utilize fused pelvic fins forming a ventral adhesive disc, often bowl-shaped with a soft rim for sealing against rocks or . In climbing species like Sicyopterus stimpsoni, the disc's internal skeleton includes rigid basipterygia and branched lepidotrichia that support forces up to 2.5 times body weight, allowing ascent of waterfalls via an inching motion. The mechanism relies on retractor muscles to expand the disc volume, creating a differential for attachment during feeding or evasion of predators. Lumpsuckers (family ), such as , feature a ventral adhesive disc derived from pelvic fins, consisting of modified rays and a soft, papillae-covered margin that enhances grip on varied substrates. This disc creates vacuum adhesion through fin ray flexion, aiding station-holding in strong currents for egg guarding or foraging. In contrast, suckermouth catfish (family ), exemplified by , have an inferior oral disc with rasping lips for suction attachment to surfaces while scraping and . Members of the family, like the Rio Grande sucker (Catostomus plebeius), possess downturned mouths with cartilaginous ridges specialized for grazing by abrading substrates, combining suction for stability with mechanical scraping. Recent hydrodynamic studies highlight the efficiency of the remora disc, showing that its lamellae and lip design minimize drag during host , with attachment reducing the remora's own resistance by up to 50% via effects. Computational models from 2023 confirm that the disc's anisotropic fibrous architecture optimizes pressure distribution for sustained at speeds exceeding 1 m/s.

Amphibians

In amphibians, suckers primarily manifest in larval tadpoles of certain anuran adapted to fast-flowing aquatic environments, where they facilitate attachment to substrates against strong currents. Gastromyzophory denotes the presence of an adhesive abdominal sucker, or belly sucker, forming a specialized ventral disc on the tadpole's below the . This structure is a muscular, adhesive organ composed of glandular that secretes to create suction and grip on rocks or vegetation. The sucker's function enables tadpoles to remain stationary while grazing on , , and biofilms, preventing dislodgement in torrential streams during early development. For instance, tadpoles of the ranid Huia cavitympanum exhibit this abdominal sucker, which supports clinging and foraging in southeast Asian forest streams. A striking example of gastromyzophory occurs in the two Sumatran cascade species, Sumaterana montana and Sumaterana dabulescens, discovered in 2018, whose s possess prominent belly suckers adapted for life in rapid, rocky habitats. These suckers are lost during , as the undergoes dramatic remodeling to become a terrestrial or semi-aquatic adult , eliminating the need for aquatic attachment. In adult amphibians, particularly arboreal tree frogs, toe pads provide analogous attachment capabilities for climbing smooth surfaces, though these structures evolved independently from larval suckers. Toe pads are expanded, disc-like expansions on the digits, featuring an outer epithelial layer of flat-topped hexagonal cells arranged in arrays, separated by deep channels that distribute mucus from underlying glands. In genera like Hyla, these pads generate adhesion through capillary forces from the thin mucus layer and van der Waals interactions enhanced by nanopillars on the cell surfaces, allowing secure grip on wet or dry vertical substrates such as leaves and bark. This adaptation supports locomotion in arboreal niches, with toe pads having evolved convergently across multiple anuran lineages, including hylids and rhacophorids, to optimize climbing performance.

Bats

Certain bats in the families Myzopodidae and Thyropteridae possess specialized adhesive structures on their limbs, known as sucker-footed or disk-winged adaptations, which enable them to roost on smooth, vertical surfaces in tropical environments. These structures are found in species such as Myzopoda aurita, the Madagascar sucker-footed bat endemic to Madagascar's humid forests, and several species of Thyroptera, the disk-winged bats distributed across Central and South America. The discs, with a diameter of approximately 5 mm, represent a rare example of convergent evolution in mammalian adhesion mechanisms. These patagial suckers are located on the wrists (ventral to the proximal of ) and ankles (ventral to the metatarsals), forming plate-like, transversely extended cups with rounded, slightly upturned rims. In Myzopoda, the structures consist of corium tissue, adipose compartments, and specialized glands that produce a thin, watery , creating a uniform fluid film for ; the ventral is thickened to about 70 µm, featuring cone-shaped protrusions and a microstructured surface of pegs, ridges, and pits less than 1 µm in size. In contrast, Thyroptera discs are more concave, supported by a cartilaginous plate and muscles like the flexor pollicis brevis, facilitating a seal for . Both types integrate tendons, such as the palmaris longus in forelimbs and flexor tibialis in hindlimbs, for controlled attachment and detachment. The primary function of these suckers is to provide secure attachment to smooth surfaces, such as the waxy leaves of plants like Ravenala madagascariensis in humid, tropical habitats, allowing the bats to roost in an upright, head-up position within furled or coiled foliage. In Myzopoda, wet adhesion via glandular secretions yields strong shear forces (up to 35.6 mN/mm²) for parallel pulls but weaker lift resistance (3.7 mN/mm²), promoting head-up roosting to prevent passive detachment. Thyroptera species, such as T. tricolor, rely on for clinging to smooth leaves, demonstrating high adherence in over 584 trials on polished surfaces like , though less effective on rough substrates. This supports rapid escape from predators by enabling quick upward flight from roosts. Evolutionarily, these limb suckers are unique among mammals, arising through parallel adaptations in the two families despite their distant phylogenetic relationship within Chiroptera. The wet in Myzopoda likely represents an earlier stage, from which the mechanism in Thyroptera may have derived, as suggested by morphological comparisons. Acoustic studies highlight links to foraging behavior, with Thyroptera using social calls for roost-finding and echolocation adapted to cluttered forest environments where such roosting sites influence insectivory.

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