Species Description: Synaptula hydriformis is a tropical, shallow subtidal holothurian belonging to the Family Synaptidae (Sewell et al 1997). The vermiform (worm-like), tentacle-crowned body plan of this species and other synaptids gives the family the misleading common name 'medusa worms.
The S. hydriformis body wall is thin, semi-transparent and elastic. Living specimens are variable in color, ranging from grayish green to reddish brown, with contrasting patches of miliary granules (a type of body wall ossicle) underlying the epidermis. Juveniles tend to be paler and more transparent than adult specimens. Mature adults have a mouth surrounded by 12 pinnate tentacles with a webbed base. Each of these tentacles is covered with as many as 20 pairs of lateral digits and there is a pair of dark pigment spots at the base of each tentacle, on the inner surface near the mouth (Hendler et al. 1995).
Synaptula hydriformis belongs to the Order Apodida, primarily a burrowing group whose name references the reduction or absence in the group of the locomotory and/or adhesive tube feet (podia) that are the most conspicuous aspect of the water vascular system in most echinoderms. Instead, large numbers (up to 1,500 individuals per square centimeter) of protruding, hooked anchor ossicles in the body wall give the animal the ability to adhere to surfaces in a manner similar to Velcro. Individuals feel sticky and are often found clinging to the neoprene surfaces of wet suits and dive booties after wading or snorkeling through a seagrass bed (Barnes 1987, Hendler et al. 1995, Hendler 2001).
The Apodida, including S. hydriformis, lack the respiratory trees that are the typical gas-exchange organs in sea cucumbers. Instead, cutaneous gas exchange occurs across the body surface (Barnes 1987).
Potentially Misidentified Species: At least three other synaptid sea cucumbers (Epitomapta roseola, Leptosynapta inhaerens, Leptosynapta tenuis) also occur in the IRL. Hendler et al. (1995) note that body wall coloration and, if necessary, ossicle shape, are useful in distinguishing Synaptula hydriformis from other synaptids.
Regional Occurrence: Synaptula hydriformis occurs in shallow marine environments of Bermuda, Florida and portions of the Caribbean and Latin America including the Greater and Lesser Antilles, the Bahamas, Belize, Panama, and Brazil. Museum holdings also confirm the presence of the species in Texas (Hendler et al. 1995).
IRL Distribution: The literature is lacking in detailed IRL distribution information, but Synaptula hydriformis is likely to occur in seagrass habitats throughout the estuary. Although it is considered a tropical species, it is common in Florida as far north as Pensacola (Cooley 1978).
Age, Size, Lifespan: Reported to reach a maximum body length of 10 cm, but most adult Synaptula hydriformis specimens are less than half that size and juveniles are often less than 1 cm (Hendler et al. 1995). The literature is lacking in information regarding the lifespan of this animal.
Abundance: Information is lacking in the literature, possibly owing to the cryptic nature of the species. Congeneric species form the central Philippines can form aggregations so dense as to completely cover the sponges they reside upon (Kerr et al. 2006), but such dense assemblages have not been reported for Synaptula hydriformis.
Reproduction: Compared to other holothuroids, reproduction in Synaptula hydriformis is atypical is several ways. The female broods its young internally, a trait exhibited by approximately 30% of holothuroids but relatively rare in tropical specimens (Hendler et al. 1995, Hendler 2001). Moreover, it is a simultaneous hermaphrodite and is believed to be self-fertilizing as well (Fricke 1998).
Ovulation involves the release of ~250 µm eggs from the germinal epithelium to the lumen of the gonadal tubule, remaining for a short time at the junction (basis) of the tubules before being released (through rupture of the junction wall) into the perivisceral coelom. Eggs removed from the perivisceral coelom are revealed to be covered with sperm, suggesting that the species is self-fertilizing (Fricke 1998).
Fricke (1998) notes individual brood sizes of up to 254 young in populations from the Indian River Lagoon and from Lake Surprise on Key Largo. The study revealed that brooded young were present year-round, corroborating earlier findings (e.g., Clark 1907) that reproduction occurs throughout the year.
Embryology: Synaptula hydriformis is viviparous; rather than a free-living larval stage, eggs and embryos develop while freely suspended within the maternal perivisceral coelomic space until they emerge as small, fully formed juveniles.
Studies by Fricke (1998) also reveal that developing S. hydriformis also exhibit matrotrophy, receiving maternal nutrition well in excess of that supplied by egg yolk. This trait, unusual for an invertebrate, was confirmed experimentally, both through observation of ingestion by brooded young of synthetic microspheres from the maternal perivisceral coelom and also by documenting incorporation of radiolabeled palmitic acid into the tissues of developing young. A greater than one hundred-fold increase in both dry and organic weight from egg to 8-mm juvenile was observed. The additional nutrition is supplied in the form of coelomocytes and organic molecules present in ingested maternal coelomic fluid, and possibly yolk granules or degraded eggs as well (Hickman 1978, Frick et al. 1992, Fricke 1998).
A final reproductive curiosity exhibited by S. hydriformis is the trait of superfetation, in which the brooded young ranging in size from 0.5-10 mm typically belong to more than one cohort (Fricke 1998).
Juveniles, usually 2-3 mm long but sometimes larger, are released through a rupture in the body wall near the anus (Clark 1907, Hendler et al. 1995). The lack of a free-swimming larval stage limits the dispersal ability of this and other brooding holothuroids, but the ability of individuals to cling to seagrass fragments and other flotsam offers an alternative dispersal strategy that is of likely ecological importance.
Temperature: The distribution of Synaptula hydriformis is restricted to tropical/subtropical waters. The northern distributional limits are most likely dictated by thermal tolerance of the species.
Salinity: Synaptula hydriformis is a year-round resident of vegetated estuarine habitats that may experience pronounced seasonal salinity fluctuations.
Trophic Mode: Stomach analysis reveals that the diet is composed primarily of benthic diatoms with a smaller amount of fleshy macroalgae (Martinez 1989).
Predators: As with many holothuroids, synaptids appear to possess chemical defenses against most would-be predators in the form of toxic or noxious secondary metabolites (Kuznetsova et al. 1989, Delbeek and Sprung 1994, Ponomarenko et al. 2001).
Habitats: Synaptula hydriformis is a cryptic species that inhabits coral reefs, seagrass beds, mangrove marshes, and isolated brackish ponds and typically occurs as an epibiont on marine plants such as seagrass blades, mangrove prop roots, and fleshy and calcareous macroalgae. It is a shallow-water animal typically inhabiting depths of 1-7 m (Pawson 1986, Hendler et al. 1995).
Activity Time: Although observational reports are lacking in the primary literature, several marine aquarium hobby authorities support the claim that most medusa worms, including Synaptula hydriformis, are primarily nocturnal (Toonen 2002).
Barnes, RD. 1987. Invertebrate Zoology, 5th Edition. Saunders College Publishing, Philadelphia. 893 p.
Clark HL. 1907. The apodous holothurians: a monograph of the Synaptidae and Molpadiidae (including a report on the representatives of these families in the collections of the United States National Museum). Smithsonian Contributions to Knowledge 35:1-231.
Cooley NR. 1978. An Inventory of the Estuarine Fauna in the Vicinity of Pensacola, Florida. Florida Department of Natural Resources Florida Marine Research Publications publication number 31. 124p.
Delbeek JC, and J Sprung. 1994. The Reef Aquarium, Vol. 1. Ricordea Publishing, Florida. 590p.
Frick, JE. 1998. Evidence of matrotrophy in the viviparous holothuroid echinoderm Synaptula hydriformis. Invertebrate Biology 117:169-179.
Frick J, Ruppert EE, Wourms JP (1992) Nutrition of brooded young in a sea cucumber (Synaptula hydriformis). American Zoologist 32:113A.
Hendler GJ, Miller E, Pawson DL, and MK Porter. 1995. Sea stars, sea urchins, and allies: Echinoderms of Florida and the Caribbean. Smithsonian Institution Press, Washington DC. 390p.
Hendler G. 2001. Short communication: "That's no worm. . . " Science 291:47.
Kerr AM, Netchy K, and AM Gawel. 2006. Survey of the shallow-water sea cucumbers of the central Philippines. University of Guam marine Laboratory Technical Report 119. 56 p.
Kuznetsova TA, Kalinovskaya NI, Kalinovskii AI, and GB Elyakov. 1989. Structure of synaptogenin B, the artifact aglycone of the glycosides of the sea cucumber Synapta maculata. Khimiya Prirodnykh Soedinenii 5:667-670.
Martinez MA. 1989. Holothuroideos (Echinodermata, Holothuroidea) de la region nororiental de Venezuela y algunas dependencias federales. Boletin Insitutto Oceanografico Universidad de Oriente Cumana 28:105-112.
Pawson DL. 1986. Phylum Echinodermata. Pp. 522-541 in: Sterrer W (Ed). Marine Fauna and Flora of Bermuda: A Systematic Guide to the Identification of Marine Organisms. John Wiley and Sons, New York, NY.
Ponomarenko LP, Kalinovsky AI, Moiseenko OP, and VA Stonik. 2001. Free sterols from the holothurians Synapta maculata, Cladolabes bifurcatus and Cucumaria sp. Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 128B:53-62.
Sewell MA. 1994. Reproduction of the intraovarian brooding apodid Leptosynapta clarki (Echinodermata: Holothuroidea) in British Columbia. Marine Biology 121:285-300.
Sewell MA, Tyle PA, Young CM, and C Conand. 1997. Ovarian development in the Class Holothuroidea: A reassessment of the "tubule recruitment Model." Biological Bulletin 192:17-26.
Toonen R. 2002. The medusa worms. Advanced Aquarist's Online Magazine Volume 1, November 2002. Available online.
Aboral: In a direction away from the mouth; the part of the body opposite the mouth.
Anal Cone: In crinoids and echinoids, a fleshy projection bearing the anus at its apex; also known as an anal tube.
Apical System: In echinoids, a ring of specialized skeletal plates, including the genital plates and ocular plates; usually located on the highest point of the test.
Arm: In asteroids, crinoids, and ophiuroids, a movable, jointed ambulacral projection, distal to the disk or calyx that carries a radial branch of the water vascular system and the nervous system; sometimes called a ray.
Basket: One of several types of microscopic skeletal ossicles in holothuroids; minute cup-shaped ossicle, usually with four projections.
Button: One of several types of microscopic skeletal ossicles in holothuroids; minute ossicle with four perforations; may be smooth or knobbed.
Disk: The round or pentagonal central body region of ophiuroids and asteroids; see also Terminal Disk.
Distal: In a direction away from the center of the body; for example, toward the tip of the arm in asteroids or the tip of a spine in echinoids.
Dorsal: In echinoderms, this term is variously applied; in asteroids, ophiuroids and echinoids it usually refers to the surface of the body that is opposite the mouth, the surface that is uppermost; in holothuroids, with mouth and anus opposite ends of the cylindrical body, the uppermost surface is considered dorsal; in crinoids, the surface opposite the mouth in considered dorsal by convention, even though it is functionally the ventral (lower) side.
Echinulate: Something spiny or prickly, usually referring to the microscopic texture of a skeletal element such as a spine.
Hermaphrodism: A condition in organisms whereby one individual possesses both functional male and female reproductive structures; hermaphroditic individuals may express both sexes simultaneously, alternately, or sequentially.
Interambulacral Area: An oral or aboral section of the body lying between two ambulacra; in interradius; also known as an interambulacrum.
Interradial: Referring to interambulacral areas of the body; interradius and interradii also commonly used.
Oral: In a direction toward the mouth; a part of the body on the same surface as the mouth.
Oral Papillae: In ophiuroids, small plates at the edge of the mouth, attached to the edges of the jaw plate and/or to the aboral shield; may be variously shaped, from spine-like to scale-like.
Papillae: In holothuroids, specialized dorsal tube feet that lack a suckered tip; in ophiuroids, certain skeletal elements of the jaws or disk.
Papillate: Covered with papillae.
Papillose: Covered with papillae.
Pedicellariae: Small stalked or unstalked pincer-like organs on the body of asteroids and echinoids, used for defense and grooming.
Peltate: Shield-shaped; used to describe the tentacles of some holothuroids.
Perforated Plate: One of several types of microscopic skeletal ossicles in holothuroids; sieve-like and widespread; may also be found in other echinoderm classes, especially in juvenile individuals.
Periproct: In echinoids, a flexible region surrounding the anus, which consists of a membrane containing embedded plates and often bearing spines and pedicellariae.
Plates: One of several types of skeletal elements in echinoderms; tabular structures with a characteristic shape and a fixed position.
Primary Plates: The first-formed plates on the dorsal side of the disk; in ophiuroids, these are the central and five radial plates; in adults, they may form a rosette of scales near the center of the disk, or they may be separated by numerous secondarily developed scales.
Radial: In a direction toward the central axis of an arm or ambulacrum; a part of the body near an arm or ambulacrum.
Radial Shields: Pairs of plates on the dorsal surface of the ophiuroid disk, which lie near the base of each arm; usually relatively large and conspicuous, but may be hidden by granules or superficial scales.
Rods: One of several types of microscopic skeletal ossicles in holothuroids; commonly found as supporting structures in tentacles or tube feet.
Scales: One of several types of skeletal elements in echinoderms; flat, thin structures that are overlapping, tessellate, or haphazardly arrayed.
Sole: In some holothuroids, the flattened ventral part of the body, either covered with or surrounded by tube feet.
Spines: One of several skeletal elements in echinoderms; movable, articulating structures that are long, slender and attenuated.
Teeth: In ophiuroids, small plates or spines attached to the dental plate on the inner edge of the jaw, a series of them extending into the mouth; in echinoids, the five hard, sharp, and movable ossicles incorporated in Aristotle’s lantern; the term also refers to five movable ossicles that surround the anus of some holothuroids.
Tentacle Scales: Small, movable spines or scales, associated with ophiuroid tube feet, which are attached to the ventral arm plate and/or lateral arm plate; may cover the tentacle pores and protect the retracted tube feet.
Tentacles: In holothuroids, feeding structures in the form of highly modified tube feet arranged in a ring around the mouth.
Terminal Disk: Round portion on the end of the tube foot in many echinoderms; usually employed for attachment to substrates.
Tube Feet: Fluid-filled, fingerlike extensions of the water vascular system that protrude through the openings in the skeleton or between skeletal elements; muscles and nerves in the shaft of the tube feet control their movements; glands, and sometimes a muscular sucker, at the tip function in adhesion; specialized tube feet are used for locomotion, feeding, burrowing, respiration, and a combination of functions.
Ventral: In echinoderms, this term is variously applied; in asteroids, echinoids and ophiuroids, it is the surface of the body that carries the mouth; this surface is in contact with the substrate; in holothuroids, with mouth and anus at opposite ends of a cylindrical body, the ventral surface is lowermost, in contact with the substrate; in crinoids, the ventral surface carries the mouth and is functionally the uppermost surface.