Holothuroids are either deposit feeders or suspension feeders. Approximately 33 percent of species are suspension feeders, nearly all of them within Dendrochirotida. This group has richly branched tentacles that are lightly coated in mucus and extend into currents to capture algae, planktonic animals, or organic matter. Food is captured passively on mucus-coated sites on the tentacles or mechanically. The tentacles are brought into the mouth one at a time and are wiped clean by contracting muscles encircling the pharynx. While they are being withdrawn from the mouth, the tentacles may be reprovisioned with mucus by small glands in the pharynx. The pelagic elasipodan Pelagothuria captures settling floc with a circumoral funnel of webbed papillae. Most holothuroids, however, feed on bacteria, algae, or detritus in surficial deposits. The variety of ways in which holothuroids feed is reflected in the diversity of tentacle form. Epibenthic aspidochirotes and elasipodans shovel in, or mop up, sediment with peltate tentacles. Synaptid apodans lash the surface with plume-like tentacles. Other apodans, as well as dactylochi-rotes and molpadiians, are burrowers and have digitate tentacles that probably aid ingestion of the surrounding sediment. One enigmatic molpadiian, Ceraplectana, has short tentacles in sclerous, claw-like sheaths of unknown function.
Numerous animals, including sharks, rays, other large fishes, crabs, gastropods, sea stars, and marine mammals such as walrus, occasionally feed on adult holothuroids, However, holothuroids are a regular part of the diets of only a few fishes and sea stars. Only gastropods in the genus Tonna appear to specialize on holothuroids. These large snails engulf holothuroids with an extensible proboscis or, if the holothuroid is too large, rasp out circular sections of the body wall of prey. Harpacticoid copepods are voracious predators of holothuroid larvae in culture and may therefore also be important predators in the wild.
While sea cucumbers have few specialist predators, they host numerous types of commensals and parasites. These include species living on the body surface, tentacles, and in the anal opening such as portunid and pinnotherid crabs, palaemonid shrimp, polynoid polycheate worms, and flattened polyctene comb jellies (Ctenophora). Eulimid snails (Gastropoda) burrow into the body wall. Turbellarian flat-worms and eel-like carapodid pearlfishes may live within the body cavity. Pearlfishes feed externally, but seek refuge in the sea cucumber during the day by entering through the anus and a tear in the rectal wall. Juvenile pearlfishes may feed on the sea cucumber gonad.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.