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).
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