The sea cucumber is a marine invertebrate belonging to the class Holothuroidea, a group within the phylum Echinodermata, which also includes sea stars and sea urchins. Unlike its spiny relatives, the sea cucumber has a soft, elongated, cylindrical body. These animals are found on the seafloor worldwide, from shallow coastal areas to the deepest trenches, functioning primarily as bottom-dwelling scavengers. The more than 1,700 known species range greatly in size, from a few millimeters up to three meters in length.
Anatomy and Movement
The sea cucumber features a thick, leathery outer skin containing a reduced internal skeleton composed of microscopic, calcareous plates called ossicles. It lacks a distinct brain, instead possessing a nerve ring around its mouth. The body is oriented horizontally, with the mouth and a ring of feeding tentacles at one end and the anus at the other.
Movement is powered by the water vascular system, a hydraulic system that extends throughout the body and operates the numerous tube feet, or podia. This internal system is often filled with coelomic fluid rather than seawater. The tube feet are typically arranged in five rows along the body, with the three rows on the underside (the trivium) used for slow crawling and attachment to the substrate.
Unique Survival Strategies
The sea cucumber sustains itself as a deposit feeder, using its specialized feeding tentacles to ingest large amounts of sediment from the ocean floor. It digests organic matter—such as detritus, bacteria, and microscopic algae—within the sediment and then expels the cleaned sand or mud. In high-density populations, a single sea cucumber can process over 19 kilograms of sediment per square meter each year.
When threatened, some species employ a defensive behavior called evisceration, expelling parts of their internal organs through the anus. This jettisoned mass can include the digestive tract, respiratory trees, and sometimes sticky, toxic strands known as Cuvierian tubules. The expulsion serves as a distraction, allowing the animal to escape while the predator is tangled or repelled.
The ability to survive this self-mutilation is due to an extraordinary regenerative capacity. Following evisceration, the sea cucumber can completely regenerate all lost internal structures, including the digestive tract and respiratory organs, within weeks to months. This process involves the division and differentiation of specialized cells to reform the lost viscera.
Ecological Importance and Commercial Value
The constant ingestion and expulsion of sediment results in a process called bioturbation, which aerates and mixes the substrate. This prevents the accumulation of excess organic matter and enhances nutrient cycling by releasing inorganic nitrogen and phosphorus back into the water column, supporting local primary producers like algae.
The feeding habits of some species contribute to ocean chemistry by increasing seawater alkalinity through their digestive processes. This alkalinity buffers the effects of ocean acidification in surrounding areas, particularly on coral reefs.
The sea cucumber’s value to humans is primarily commercial, as many species are harvested for use as a delicacy in certain Asian cuisines and in traditional medicine. Due to their high market value and ease of collection, many wild populations have been subjected to overharvesting, leading to significant conservation concerns and a reduction in their ecological services.