Defenses & predators
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  Evisceration & regeneration
  Defenses of evisceration & regeneration will be considered in this section, while THICK BODY WALL/OSSICLES, SWIMMING, WITHDRAWAL INTO CREVICES/"CATCH" CONNECTIVE TISSUES, TOXIC CHEMICALS/UNPALATABILITY, and PREDATORY SEA STARS will be considered in other sections.
 
Research study 1
 

histogram showing seasonal incidence of sea cucumbers Parastichopus californicus with complete viscera in the San Juan Islands, WashingtonEvisceration, or forceful expulsion of the internal viscera, occurs in some west-coast sea-cucumber species and, in some cases, possibly on a regular seasonal cycle.   The first report of this phenomenon on the west coast seems to be for Parastichopus californicus in San Juan Islands, Washington.  Collections over the period Sept-Mar show a state of “gutless-ness” during Oct-Dec and the author concludes, although without direct evidence, that this results from a process of seasonal evisceration. Swan 1961 Science 133: 1078.

NOTE  in British Columbia a small fisheries exists for Parastichopus californicus and harvestable age is about 6yr.  Winter fisheries yield may be affected by a 25% reduction in body-wall mass associated with seasonal "visceral atrophy".  Fankboner & Cameron 1988 p. 796 In, Echinoderm Biology (Burke et al., eds.)  A.A. Balkema, Rotterdam

NOTE  while P. californicus will often eviscerate through rough handling, temperature shock, and other stressful treatments, examples of eviscerations in the field are so rare as to be nonexistent. The graph uses sample sizes of 9-35 averaged from the author's data (some months involve 2 collection)

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Research study 2
 

graph showing % individual sea cucumbers Parastichopus californicus with intact and absent viscera in Indian Arm, British ColumbiaLater studies on specimens of Parastichopus californicus collected in Indian Arm, British Columbia suggest that rather than evisceration, the absence of viscera is a result of seasonal atrophy.  By recording the condition of gut, gonad, respiratory trees, and circulatory system in 430 individuals collected over a 40-mo period, the authors show that the viscera degenerate during autumn, regenerate in late autumn/winter, and are present and stable during spring/summer. Visceral loss is preceded by cessation of feeding and locomotion and onset of torpor.  During this period the organs are resorbed, a process accompanied by phagocytic activity of coelomocytes (producing characteristic “brown bodies” in the tissues), and sometimes scavenging activity of "endosymbionts" (certain protists and flatworms).  During resorption the coelomic fluid becomes cloudy from the presence of cellular debris and microorganisms. The gut tube regenerates first and involves a uniform thickening of tissue primordia along the animal’s length.  Cavities appear at either end of the gut, and expand towards the front and back until they meet.  Feeding begins soon afterwards.  Next, the respiratory trees, gonad, and circulatory system differentiate.  The authors hypothesise that visceral atrophy in P. californicus is an expression of seasonal dormancy induced by reduced food availability.  Fankboner & Cameron 1985 Can J Zool 63: 2888.

NOTE  the reader should now be totally confused. Is it evisceration or atrophy, or are they separate events? The answer is that there are 2 causes of loss of viscera in Parastichopus: the first is visceral atrophy, which occurs seasonally and which is not a defensive response to predators; the second is evisceration, which is known to occur in response to stress, including predatory

NOTE  the authors use the term diapause, which is usually used in reference to insects, so the term dormancy, as used here, may be more accurate

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photograph of a sea cucumber Parastichopus californicus about to eviscerate taken from a video

CLICK HERE to see a video of P. californicus eviscerating. Note that the event occurs via the anus and that the eviscerated parts are mainly the respiratory trees.

NOTE  the video replays automatically

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Research study 3
 

map of southern British Columbia showing collection locations of sea cucumbers Parastichopus californicus in a study of eviscerationhistograms showing % sea cucumbers Parastichopus californicus with intact, degerating, and regenerating viscera at different ages and in different seasonsA later study by the same authors of Research Study 2 above at several locations in British Columbia show that visceral atrophy occurs in juvenile (<5yr of age) Parastichopus californicus in the autumn.  Collections in early August indicate that almost 100% of juveniles have intact viscera, while collections in late October show that most juveniles have degenerating or regenerating viscera. Cameron & Fankboner 1989 J Exp Mar Biol Ecol 127: 43.

NOTE individuals are aged by matching sizes (length/width) with other individuals, presumably laboratory-reared, whose ages are known

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Research study 4
 

photograph of a sea cucumber Eupentacta quinquesemitagraph showing % individuals of sea cucumbers Eupentacta quinquesemita with viscera lacking over different seasonsEvisceration is known to occur seasonally in the small west-coast holothuroid Eupentacta quinquesemita, although its function, like that of visceral atrophy in Parastichopus californicus, is unclear. Studies at the University of Victoria, British Columbia show that evisceration occurs in about half the population in autumn after a summer’s feeding.  Less food is available in suspension for Eupentacta during the winter so, if you must get rid of your guts, this may be the least disadvantageous time to do it.  As to its function, the author proposes that evisceration in Eupentacta functions to rid the body of waste-laden (i.e., “brown body”-laden) digestive tracts after a summer of feeding. The assumption is that retaining viscera laden with accumulated wastes, or brown bodies, is a greater metabolic load than that of regeneration itself.  Byrne 1985 Ophelia 24: 75.

NOTE the composition of these wastes is unclear

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Research study 5
 

drawing of a sea cucumber to illustrate the process of eviscerationphotograph of eviscerating sea cucumber Eupentacta quinquesemitaThe evisceration process in Eupentacta quinquesemita proceeds as follows:
1.  three main structures weaken over a period of about 1-3min, become soft and transparent, and eventually separate from their attachments.  These are the basal part of the tentacles, the attachment points of the introvert retractor muscles to the longitudinal muscles (there are 10 of these), and the junction of intestine and cloaca. These parts are shown in light blue on the drawing on the Right and labelled with "1". The softening is a state-transformation of the collagen components in the tissue.

2. parts eviscerated include the gut, associated hemal vessels, tentacles, and introvert (these parts are shown in pink on the drawing).  The gut tears away from the mesenteries that suspend it within the coelomic cavity. 

3. interestingly (from an adaptive point of view), most of the gonad stays behind.  Only strands of gonad tangled in the gut are eviscerated. The paired respiratory trees and cloaca also remain behind.

4.  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.

5.  this takes about 20min, with final detachment of the tentacles and introvert sometimes taking as long as 12h.

6.  the anterior rupture seals, at first by muscular contraction and then by healing as a plug of connective-tissue.  Byrne 1985 Ophelia 24: 75; Byrne 2001 J Exper Biol 204: 849.

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Research study 6
  Evisceration in Eupentacta quinquesemita is thought to be mediated by the nervous system via neurosecretions.  The active molecule has been tentatively identified as a small molecular-weight peptide of 30-50 amino acids in size, and can be isolated from the discarded coelomic fluid after evisceration.  The process can be mimicked in vitro by increase in potassium concentration.  Byrne 1985 J Exper Biol 117: 69; Byrne 2001 J Exper Biol 204: 849.
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Research study 7
 

drawings of sea cucumbers to show how the gut regenerates after eviscerationRegeneration of the gut begins almost immediately after evisceration and, in populations of Eupentacta quinquesemita in the Victoria region of British Columbia, is completed within 2-4wk. 

In some species, such as Stichopus (and, by close relationship, Parastichopus), a thickening develops along the mesentery edge that later hollows out to form the gut (see drawings on Right).  The anterior part of the mesentery is actually contiguous with the healed tissues of the stump end of the pharynx.  The torn ends of the radial water canals regenerate and unite to form the oral ring canal of the water vascular system.  The end of the pharynx, now a blind-ending tubule, grows posteriorad along the free edge of the mesentery.  It meets and fuses with the newly forming gut tube along the mesentery edge. The anterior ends of the longitudinal muscle bands split off branches that become the introvert retractor muscles.  Description of regeneration from Hyman 1955 The Invertebrates: Echinodermata Vol. IV McGraw-Hill; observations on Eupentacta from Byrne 1985 Ophelia 24: 75; drawings of regeneration in Stichopus japonicus adapted from Leibson 1992 Monogr Developmental Biol 23: 51 (p. 51 In, Keys for Regeneration Karger, Basel.

NOTE in Stichopus japonicus, a species in Japan closely related to Parastichopus californicus, the gut tube is completed after about 12d (temperature not specified)

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Research study 8
 

photograph of a sea cucumber Leptosynapta clarki courtesy of the Barcode of Life Data SystemsAn interesting observation on sea cucumbers Leptosynapta clarki collected from Bamfield Inlet and Sooke Harbour, British Columbia indicates that gut regeneration involves a gradual transformation of the dominant cell type in the stomach to the dominant type in the intestine. The transformation takes place in about 28d, but requires about 12d to get started (at 12oC).  Regeneration in Leptosynapta, therefore, is accomplished by redifferentiation of stomach tissues directly into intestinal tissues.  Gibson & Burke 1983 Can J Zool 61: 2720.

NOTE  the authors do not use eviscerated specimens in their study; rather, they bisect experimental specimens at various locations along the length (and width) of the body and check out what is happening

Leptosynapta clarki from Bamfield, British Columbia 2X; photo courtesy of the Barcode of Life Data Systems

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While evisceration is included here as part of PREDATORS & DEFENSES, might there be other possibilities for its function? Think about the following, then CLICK HERE for explanations.

Ridding of waste matter. 

Diverting predators. 

Ridding of parasites. 

Reducing metabolic costs in adverse conditions. 

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