The sea cucumber is a marine invertebrate belonging to the class Holothuroidea, making it a relative of sea stars and sea urchins. These organisms possess an elongated, soft body with leathery skin, often resembling the vegetable for which they are named. Sea cucumbers are benthic animals, living on the ocean floor in nearly all marine environments worldwide. They play a significant role in the marine ecosystem as detritivores, sifting through sand and sediment to consume decaying organic matter. This process cleans the seafloor and is important for nutrient recycling.
Who Hunts Sea Cucumbers
Despite their slow movement and soft bodies, sea cucumbers have few dedicated predators due to their unappealing texture and chemical defenses. Specialized threats include large, predatory mollusks, such as the tun snails (Tonna galea and Tonna perdix). These snails use a powerful paralytic poison to subdue the sea cucumber before swallowing it whole.
Opportunistic predators include various fish species, crustaceans, and other echinoderms. Fish such as triggerfish, pufferfish, tuna, and cod sometimes prey on sea cucumbers, often consuming only parts of the body due to toxins or calcified internal structures. Crustaceans, including crabs and lobsters, use their claws to tear and consume the tough exterior. The multi-armed sunflower starfish (Pycnopodia helianthoides) also preys on them, sometimes extending its stomach externally to begin digestion.
Passive and Physical Defenses
Sea cucumbers employ several strategies to avoid detection and consumption. Many species use camouflage, blending into the sandy or muddy seafloor. A more active behavioral defense is burrowing, where the animal digs into the sediment until it is almost completely concealed, leaving only its feeding tentacles exposed. This technique prevents predators from accessing the main body mass.
The physical structure also provides protection against attack. Their tough, leathery body wall is reinforced with microscopic, calcified plates called ossicles or sclerites embedded beneath the skin. Furthermore, their tissues contain mutable collagenous tissues, which allow the body wall to rapidly change its mechanical properties from soft and flexible to stiff and rigid. This ability to stiffen the body makes it difficult for many predators to pierce or tear the skin.
The Dramatic Act of Self-Defense
When severely stressed or physically attacked, the sea cucumber employs autotomy, or self-amputation, known as evisceration. This process involves forcefully expelling a significant portion of its internal organs through a tear near the anus. The expelled organs, which can include the digestive tract, respiratory trees, and gonadal tubules, often distract the predator, allowing the sea cucumber to escape the encounter.
The most specialized defensive structures expelled during evisceration are the Cuvierian tubules, sticky, thread-like strands found in some species. When ejected, these tubules rapidly elongate and become highly adhesive, effectively entangling and immobilizing small attackers. The release of these sticky filaments is often accompanied by a chemical defense in the form of a toxin called holothurin.
Holothurin is a saponin-like neurotoxin concentrated in the Cuvierian tubules and the body wall. This compound is toxic to many marine organisms and acts as a powerful deterrent, sometimes killing smaller attackers. The sea cucumber can later regenerate all of its lost internal organs, a process that can take a few weeks to several months depending on the species and the extent of the damage. Regeneration of Cuvierian tubules alone takes about five weeks to complete.