Sea cucumbers, members of the phylum Echinodermata, resemble soft, cylindrical bags resting on the ocean floor. As slow-moving bottom-dwellers, these invertebrates lack the hard armor of their relatives, the sea urchins and starfish, making them vulnerable to predators. Despite their unassuming appearance, sea cucumbers have evolved some of the most dramatic defense mechanisms in the animal kingdom, often involving the deliberate expulsion of their own body parts. These strategies range from physical distraction to chemical toxicity.
The Ultimate Sacrifice: Evisceration
When severely stressed or directly attacked, many sea cucumber species engage in a process known as evisceration. This defense involves the forceful expulsion of a significant portion of their internal organs through either the anus or, in some species, a rupture near the anterior end. The organs typically ejected include the entire digestive tract, the respiratory trees (which function as gills), and sometimes even the reproductive organs.
The mechanism is triggered by a neural signal that causes the body wall muscles to contract sharply, while the tissues anchoring the viscera are simultaneously softened. This muscular contraction and tissue weakening create a high internal pressure that forces the organs out of the body. This leaves a mass of viscera behind to distract the attacker, allowing the sea cucumber to escape the immediate threat while the predator remains occupied.
Sticky Barriers: Expulsion of Cuverian Tubules
Certain species, particularly those in the genus Holothuria, involve the expulsion of Cuverian tubules. These are fine, thread-like structures attached to the base of the left respiratory tree. When the animal is mechanically stimulated, it discharges a small cluster of these tubules through the anus.
Upon contact with seawater, the expelled threads rapidly expand, sometimes elongating up to 20 times their original length. They become adhesive, forming a dense, sticky network that can effectively entangle and immobilize potential predators like small fish or crabs. The stickiness is due to an outer mesothelium and a core of mutable collagenous tissue, which stiffens irreversibly after expulsion, ensuring the threads withstand the predator’s struggle.
Chemical Warfare: The Role of Toxins
Beyond physical defenses, sea cucumbers employ a chemical deterrent in the form of saponin compounds, the most well-known of which is holothurin. Holothurin is a triterpene glycoside, a type of natural detergent, which is stored in the body wall and often concentrated within the Cuverian tubules. This toxin serves as an antipredatory defense against fish and invertebrates that might attempt to consume the sea cucumber.
The toxin’s mode of action is based on its ability to act as an anionic surfactant, disrupting the cell membranes of other organisms. Specifically, holothurin interacts with delta 5-sterols in biological membranes, causing the membranes to leak and resulting in the rupture of red blood cells, a process known as hemolysis. Holothurin can also function as a neurotoxin, having a blocking effect on nerve tissue, which further incapacitates a would-be attacker.
Some sea cucumber species release this toxic compound into the surrounding water as a general chemical deterrent. Others rely on its direct contact toxicity when the predator encounters the expelled Cuverian tubules or attempts to ingest the animal. The presence of this chemical makes the sea cucumber an unappetizing or dangerous meal. The concentration and specific type of toxin vary between species.
The Power of Recovery: Regeneration
The ability to discard internal organs or specialized defensive structures would be a fatal strategy without a mechanism for replacement. Sea cucumbers possess a high regenerative capacity, which is the final component of their defense cycle. Following evisceration, the animal rapidly seals the wound and initiates the regrowth of its lost viscera.
This regenerative process involves the dedifferentiation of existing cells. These cells then proliferate and migrate to form a blastema-like structure, which serves as the foundation for the new organs. The digestive tract and respiratory trees can be fully reformed, with the entire process taking anywhere from a few weeks to several months, depending on the species and the extent of the damage.
For species that expel Cuverian tubules, the regeneration is often much faster, sometimes taking as little as 17 days to five weeks for the threads to be fully replaced. This cellular plasticity ensures that the self-sacrificing defense mechanisms are temporary setbacks rather than permanent injuries.