Evisceration is a method of

autotomy Autotomy (from the Greek ''auto-'', "self-" and ''tome'', "severing", αὐτοτομία) or self-amputation, is the behaviour whereby an animal sheds or discards one or more of its own appendages, usually as a self-defense mechanism to elude ...

involving the ejection of

internal organ In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

s used by

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s as a defensive strategy.

Sea cucumbers Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian ...

(Holothuroidea) eject parts of the gut in order to scare and defend against potential predators such as crabs and fish. The organs are regenerated in a few days by cells in the interior of the sea cucumber.

Description

When stressed, the sea cucumber faces away from the attacker and contracts its body wall muscles sharply. This causes the wall of the cloaca to tear and the anus to gape. The evisceration process in '' Eupentacta quinquesemita'' proceeds as follows: #Three main structures weaken over a period of about 1–3 minutes, become soft and transparent, and eventually separate from their attachments. These are the basal part of the

tentacles In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainl ...

, the attachment points of the introvert retractor muscles to the longitudinal

muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...

(there are 10 of these), and the junction of intestine and cloaca. The softening is a state-transformation of the collagen components in the tissue. #Parts eviscerated include the gut, associated haemal vessels, tentacles, and introvert (the dexterous anterior extensible portion of the body wall). The gut tears away from the

mesenteries In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms com ...

that suspend it within the

coelomic cavity The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...

. #Most of the gonad stays behind. Only strands of gonad tangled in the gut are eviscerated. The paired respiratory trees and cloaca also remain (although they may be expelled in other species) #The introvert changes from being firm and opaque to being soft and translucent. The body-wall muscles contract and the increased pressure forces coelomic fluid and viscera into the introvert. It enlarges like a balloon and soon ruptures, expelling the fluid and viscera. #This takes about 20 minutes, with final detachment of the tentacles and introvert sometimes taking as long as 12 hours. #The anterior rupture seals, at first by muscular contraction and then by healing as a plug of connective-tissue.

Function

During evisceration in some species, several hundred

Cuvierian tubules Cuvierian tubules are clusters of fine tubes located at the base of the respiratory tree in some sea cucumbers in the genera ''Bohadschia'', ''Holothuria'' and ''Pearsonothuria'', all of which are included in the family Holothuriidae. The tubules ...

(part of the respiratory tree) may be expelled. Water from the respiratory tree is forced into these tubules causing a rapid expansion and they elongate by up to 20 times their original length. They have great tensile strength and become sticky when they encounter any object. The adhesive is unique among marine invertebrates and a firm grip is obtained in under ten seconds. The mass of threads can entangle and immobilise potential predators such as small

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

or crabs. The threads become detached from the sea cucumber which crawls away. The tubules are readily regenerated, a process that takes about 17 days in ''

Holothuria leucospilota ''Holothuria leucospilota'', commonly known as the black sea cucumber or black tarzan, is a species of marine invertebrate in the family Holothuriidae. It is placed in the subgenus ''Mertensiothuria'' making its full scientific name ''Holothuria ...

'' and five weeks in ''

Holothuria forskali ''Holothuria forskali'', the black sea cucumber or cotton-spinner, is a species of sea cucumber in the family Holothuriidae. It is found at shallow depths in the eastern Atlantic Ocean and the Mediterranean Sea. It was placed in the subgenus ''Pa ...

''. The tubules contain a toxic saponin called

holothurin The holothurins are a group of toxins originally isolated from the sea cucumber '' Actinopyga agassizii''. They are contained within clusters of sticky threads called Cuvierian tubules which are expelled from the sea cucumber as a mode of sel ...

, which is also present in the body wall in some sea cucumber species.

Occurrence

The giant California sea cucumber ('' Parastichopus californicus'') will often eviscerate due to rough handling, temperature shock, or other stressful treatments. The event occurs through the anus and the eviscerated parts are mainly the respiratory trees. ''Holothuria arenicola'' is described as a sea cucumber suitable for keeping in aquaria as it does not eviscerate, whereas the "Australian" sea apple (''Paracucumaria tricolor'') frequently eviscerates.

Similar behaviour

Some starfish evert their stomachs through their mouths to eat their prey. The starfish then retracts its stomach back inside of its body.

References

External links

Video of evisceration
{{Ethology Antipredator adaptations Zoology