Sea cucumbers are unusual inhabitants of the ocean floor, resembling their namesake vegetable. Despite their appearance, these soft-bodied creatures are members of the echinoderm phylum. Their classification is based on shared biological features that link them to more familiar ocean dwellers.
The Defining Traits of Echinoderms
Echinoderms are characterized by several distinct features. One recognizable trait is pentaradial symmetry in their adult stage, meaning their body parts are arranged in fives around a central axis. This symmetry is evident in animals like starfish, with five arms, and sea urchins, displaying five rows of tube feet. Another characteristic is the presence of a unique water vascular system, a network of fluid-filled canals that power their tube feet.
This system facilitates locomotion, feeding, and gas exchange. Echinoderms also possess an endoskeleton, an internal skeleton made of calcareous ossicles. These plates, composed of calcium carbonate, vary in size and arrangement, forming the rigid structure of a sea urchin’s test or the interconnected plates of a starfish’s arms. Examples of echinoderms include starfish, sea urchins, brittle stars, and crinoids.
Sea Cucumbers: An Echinoderm Despite Appearances
Despite their elongated, worm-like appearance, sea cucumbers exhibit echinoderm characteristics. Their body plan, though outwardly appearing bilaterally symmetrical, retains internal pentaradial symmetry, with five rows of tube feet often running along their body. This internal radial organization indicates their echinoderm lineage. The water vascular system is present and functional in sea cucumbers.
Their tube feet, extensions of this system, are often modified as feeding tentacles around the mouth or scattered across the body for slow locomotion. Sea cucumbers also possess an endoskeleton, significantly reduced compared to other echinoderms. Instead of large, fused plates, their calcareous ossicles are microscopic, scattered within their leathery body wall, providing structural support without impeding flexibility. This allows for their characteristic soft and pliable texture while still fulfilling the criteria of an echinoderm skeleton.
Unique Adaptations and Lifestyle
Sea cucumbers have unique adaptations that distinguish them within the echinoderm phylum. Their elongated, soft bodies allow them to navigate and burrow through seafloor sediments. Many species are deposit feeders, using specialized oral tentacles—modified tube feet—to sweep up organic matter and detritus from the seabed. Other species are suspension feeders, holding tentacles outstretched to filter plankton and other particles from the water column.
These animals employ defense strategies when threatened. Some species can rapidly expel their internal organs, a process known as evisceration, to distract or deter predators. The expelled organs regenerate over several weeks to months. Other sea cucumbers can release sticky, thread-like Cuvierian tubules, which entangle and incapacitate potential threats, providing an escape mechanism.
Their Role in Marine Ecosystems
Sea cucumbers play a significant role in marine ecosystems, primarily as detritivores. They consume decaying organic matter, bacteria, and algae found in sediments, recycling nutrients back into the marine environment. By ingesting sediment and expelling cleaner, oxygenated material, sea cucumbers contribute to bioturbation, the disturbance of sediment layers. This process aerates the seafloor, preventing anoxic conditions and promoting the health of benthic habitats. Their continuous processing of sediments contributes to the productivity of marine ecosystems.