Sea cucumbers are marine invertebrates belonging to the class Holothuroidea, known for their soft bodies and slow, sedentary existence on the seafloor. These creatures, part of the Echinoderm phylum alongside starfish and sea urchins, possess active defense mechanisms. When threatened by a predator, the sea cucumber can deploy a specialized response involving the ejection of internal structures. This strategy allows the animal to escape danger by distracting, entangling, or confusing its attacker.
The Primary Defensive Weapon
The most common defense mechanism involves specialized organs called Cuvierian tubules. These are clusters of fine, sticky, thread-like tubes found only in certain species, such as those in the genera Holothuria and Bohadschia. The tubules are attached internally to the base of the left respiratory tree and float freely in the coelomic cavity.
When a sea cucumber is stressed or physically stimulated, it contracts its body wall muscles sharply, forcing a portion of these tubules out through the anus. Upon contact with seawater, the expelled tubules undergo a rapid transformation, expanding and lengthening significantly, sometimes up to 20 times their original size. This creates a large, white, tangled mass that is highly adhesive.
The tubules are highly adhesive, allowing them to form strong bonds to surfaces in less than ten seconds. The adhesive is primarily organic, a mix of proteins and carbohydrates, which gives the threads their exceptional tenacity and ability to immobilize small predators like crabs and fish. Some species’ tubules also contain a toxic saponin known as holothurin. The sea cucumber then detaches the tubules and crawls away, leaving the predator entangled in the sticky, potentially toxic mass.
The Extreme Measure: Evisceration
Evisceration is a defensive action often triggered by severe stress or physical damage. This process involves the autotomy, or self-amputation, of major internal organs, which are forcibly expelled from the body. Evisceration involves the ejection of the entire digestive tract, including the gut and associated vessels, and often the respiratory trees and gonads.
The expulsion of these viscera can occur either through the anus or through a rupture in the anterior body wall, depending on the species. The process is initiated by the weakening and softening of the body’s collagenous connective tissues at the points of attachment. The significant muscle contraction that follows increases internal pressure, causing the organs to be ejected, effectively presenting the predator with a large, confusing distraction.
Evisceration is a complete loss of the digestive system, serving as a last-resort measure to ensure survival. In some species, up to 28% of the animal’s body mass may be expelled during this traumatic event. The immediate consequence is the loss of the sea cucumber’s feeding and respiratory structures, yet its body wall remains intact, allowing it to survive the initial assault.
The Aftermath: Remarkable Regeneration
The ability to survive evisceration is possible due to the sea cucumber’s extraordinary regenerative capacity. Once the organs are expelled, the body seals the rupture, either by muscular contraction or by forming a plug of connective tissue. It then begins the process of regrowing its lost internal structures.
Regeneration times vary among species. Some, such as Holothuria leucospilota, can regenerate a functional digestive tract in about 20 days, while the full regrowth of all lost organs can range from seven to 145 days. During this recovery period, the animal must rely on stored energy reserves and often ceases feeding.
The biological mechanism behind this regrowth involves cellular dedifferentiation. Cells at the site of injury revert to a less specialized state, allowing them to proliferate and form a new set of tissues. New organs, including the digestive tube, respiratory trees, and gonads, are formed from a mass of undifferentiated cells that arise at the wound site, ensuring the sea cucumber returns to full functionality.