Part 1:  The Medusa Worm (Synaptula hydriformis)

Story & Photos By Eric S. Cole, Biology Department, St. Olaf College, Northfield, Minnesota

Trigger warning: this creature’s habits are so foreign, they may offend the senses.

These “naked sea cucumbers” are inhabiting an algal garden in a salt pond.

One of the most marvelous life-forms I’ve encountered in both The Bahamas and Turks & Caicos islands is a tiny sea cucumber named Synaptula hydriformis. Barely three inches long and resembling a worm, Synaptula is related to the more familiar starfish and sea urchins encountered in near-shore environments. Unlike its echinoderm relatives (Echino-derm = spiny skin), Synaptula has given up the bony plates that characterize its relatives in favor of a truly flexible, worm-like existence.

A ring of sticky tentacles (as many as 12 in adults) encircles the mouth, tentacles that multi-task for locomotion and feeding. A tentacle reaches out and adheres to the substrate (mud or vegetation), attaches, and pulls the animal forward. A second tentacle reaches further, then a third, and a fourth. As the fourth tentacle begins its pull, the first detaches, bringing with it a “handful” of local sediment. The cucumber’s circular mouth gapes wide, and the animal reaches a sediment-laden arm down its throat. The mouth constricts like a sphincter, and as the arm is pulled back, sediments are wiped clean from its sticky surface to be ingested. “Feed-walking” is the awkward term I might suggest.

Oddly, these tiny sea cucumbers are relatives of a giant (two meter long) snake-sea-cucumber that one may encounter over the reef on a night dive. For an impressive video of one of these monsters “feed-walking,” see https://www.youtube.com/watch?v=WRq_5zm_zK4&ab_channel=NationalGeographic.

A brief history

Synaptula has a fascinating history in the Caribbean islands. It was first discovered by Charles Alexandre Lesueur who deserves to be remembered for his extraordinary gifts as a natural history illustrator. In 1800, and at the age of 21, Lesueur joined a four-year expedition to explore “New Holland” (Australia) for Napoleon Bonaparte. The Baudin expedition boasted nine naturalists, charged with documenting the wildlife along the coast of Australia (Bonaparte’s wife Josephine is said to have been fascinated with the Australian continent). Lesueur was hired as an assistant gunner! With no formal artistic training, his amateur illustrations rapidly earned him a place as illustrator for Nicolas Baudin’s travel journals (1800–1804).  Lesueur’s elevation to this position was secured as the other nine formally hired naturalists of the expedition died or deserted en route. (It was a rough journey.)

I may be forgiven for finding his illustrations to rival, or even surpass those of John James Audubon. After Napoleon’s abdication (1815) Lesueur cast about for employment, finding a partner in William MaClure, a Scottsman who became the first American geologist to survey all of the southerneastern U.S. On a subsequent journey through the Caribbean (1815–1816), Lesueur collected the first specimen of Synaptula from the island of Guadeloupe (he would describe it in an 1824 publication). Sixty years later (1886), America launched the USS Albatross, the first research vessel designed specifically for marine science exploration. The Albatross set off to collect data from The Bahamas. While perusing San Salvador Island (formerly known as Watling’s Island), the USS Albatross obtained a second specimen of this sea cucumber from a mangrove on the northern shores. It was stored in the Smithsonian Museum for years and later identified as Synaptula hydriformis (Lesueur 1824) when Hubert Clark examined it in the 1900s.The Smithsonian obtained a more recent sample from The Bahamas in 1996.

In 2022, my research team and I, accompanied by TCI’s John Galleymore and assisted by one of the excellent boats from the Big Blue Collective (Mark Parrish), were able to visit Lake Catherine, a saltwater pond on West Caicos. Lake Catherine is most notable for the Flamingo colonies that frequent its shores, and several enormous “conduits”:  underwater caverns that connect Lake Catherine to the sea, and whose marine connection produces a “boiling water” effect as the incoming tide roils the surface above them.

These images show the 1890s railroad causeway bisecting Lake Catherine on West Caicos.

In the 1890s, sisal was the principal product of West Caicos. This short-lived enterprise involved construction of a railroad causeway that bisected the lake, and remains to this day. Coincidentally, construction of the causeway created an experiment in habitat manipulation. On the one hand, it separates the part of the lake fed by two enormous conduits. The north half has a loose, carbonate sediment, and modest biodiversity. The south half, fed by tidally-active conduits, supports a rich sea-grass bed, with again, only modest biodiversity. Curiously, the steep banks of the causeway have created an artificial reef in the middle of Lake Catherine that is rich in marine life! It was here that we found Synaptula, tangled amongst the luxuriant green and red algae beds.

An extraordinary life history, a magnificent “colonization” machine

By now the reader might be excused for wondering why this unarguably unattractive creature has captured the author’s attention (feed-walking notwithstanding). The fascination has to do with the creature’s reproductive life history. (Alert: candid description of a thoroughly atypical sex-life follows.) First, Synaptula is a rare example of a creature that undergoes self-fertilization. Each individual is in possession of a gonad that simultaneously produces both eggs and sperm and releases them into the animal’s own body cavity where fertilization occurs. This produces embryos that are sloshing about in the fluids that bathe the sea cucumber’s organs.

Rich algal-gardens colonize the causeway and provide refuge for a diverse marine biota.

Second, development is fast and direct! Most sea cucumbers release gametes (eggs and sperm) to the ocean waters where fertilization occurs, and development produces a swimming larvae that later metamorphosis into a juvenile “cuke.” Synaptula embryos skip the planktonic larvae and develop directly into a tiny cuke, complete with a ring of tentacles and a complete gut, and all within about 24 hours of fertilization.

Third, baby cukes “feed-walk” around the mother’s/ father’s insides (pronouns get difficult), slurping down whatever loose tissue and organic material they encounter. This process is referred to as “matrophagy.” It does not seem to harm the parent, (perhaps even cleansing their body wall) and we have found as many as 40 juveniles charging about the innards of a single, three-inch long parental cuke. By manipulating the light/dark regimen we were able to synchronize a round of self-fertilization, and by exposing the adult to a cold-shock we could induce “live birth.” This let us photograph the entire embryonic life of these natural curiosities.

An ingenious colonizing lifestyle

This is a mature Synaptula, note the tentacles on the right.

As a final note, it is notable that these creatures are extraordinarily “sticky.’ We first discovered them clinging to our dive booties after a pond-swim. Peeling them off we initially reacted to them as a nuisance. A wading bird would undoubtedly collect these sticky rascals on their feet and distribute them to every marine body of water they visited. Remarkably, as super-colonists, Synaptula need not arrive as a male and female pair to establish a colony. A single, virgin cuke will rapidly begin churning out baby cukes through self-fertilization/fast-development/matrophagy and live birth within days of arrival in a suitable habitat.  

At left and middle: You can see juvenile “cukes” inside their mother’s body, and their mother giving birth (at right).

A cautionary tale

The salt ponds of the Turks & Caicos (sometimes referred to as “anchialine ponds”) are special. The caverns that feed them are home to some of the world’s strangest and most facinating animals, and the ponds themselves (the “black lagoons”) are refuges for marine organisms that can escape the predation and competion of the open sea, and serve as micro-laboratories of fast evolution. The author is grateful to the Turks & Caicos for extending protection over these rare and extraordinary habitats and exploring ways to identify and further secure these national treasures.

Professor Cole teaches and conducts research with undergraduates at St. Olaf College in Northfield, Minnesota. He is part of a small team of researchers who propose to conduct a biodiversity survey to deepen understanding of the natural history of the interior of East Caicos.