Abstract
The ejection of internal organs, i.e., evisceration, is a well-known phenomenon in sea-cucumbers. We report the ability of a member of the Chordate phyla, the tropical ascidian Polycarpa mytiligera, to eviscerate and regenerate its gut within 12 days, and to rebuild its branchial sac within 19 days. Evisceration occurred within 4–43 seconds of gentle mechanical pressure exerted on the tunic in 47% of the tested P. mytiligera. Individuals were able to discard up to 3/4 of their digestive tract via the incurrent siphon by rupture of the branchial sac in this area. Although chemical analysis revealed no significant levels of toxic compounds, the eviscerated guts were unpalatable to the triggerfish and pufferfish on which they were tested, suggesting evisceration as a defense mechanism. Given the close affinity of ascidians to vertebrates, the regeneration pathway of the viscera and branchial sac of ascidians suggests its potential beneficial application in soft tissue regeneration research.
Highlights
The ejection of internal organs, i.e., evisceration, is a well-known phenomenon in sea-cucumbers
We report the ability of a member of the Chordate phyla, the tropical ascidian Polycarpa mytiligera, to eviscerate and regenerate its gut within 12 days, and to rebuild its branchial sac within 19 days
By sampling eviscerated individuals from the field at weekly intervals, we were able to follow the progress of the branchial sac and gut regeneration, examine the physiological state of the animal, and follow the cascade of events leading to expulsion of the gut
Summary
The ejection of internal organs, i.e., evisceration, is a well-known phenomenon in sea-cucumbers. We report the ability of a member of the Chordate phyla, the tropical ascidian Polycarpa mytiligera, to eviscerate and regenerate its gut within 12 days, and to rebuild its branchial sac within 19 days. Wiley (1897)[14] commented that in another closely-related species, which he named Styeloides eviscerans, the animals survive the evisceration procedure, suggesting that regeneration of the eviscerated body parts (referring mainly to the branchial sac) occurs from the endostyle[15], a deep ciliated groove along the ventral-mid line of the branchial sac which secrets mucus. Our documentation of evisceration response of the solitary ascidian Polycarpa mytiligera (Savigny, 1816), a conspicuous member on Indo-Pacific coral reefs[16,17], provides a unique opportunity to deepen our knowledge and revive the study of evisceration in ascidians, establishing a solid platform from which to study regeneration of the digestive tube from molecular, cellular, and developmental aspects Despite the fascinating opportunities to study evisceration in ascidians, this phenomenon was only anecdotally mentioned by taxonomists in the following decades in several other species (see Supplementary Table S1), leaving basic questions unanswered, such as: How are the viscera ejected? Does the animal survive following evisceration? Does it rebuild its viscera; and if so - how? Our documentation of evisceration response of the solitary ascidian Polycarpa mytiligera (Savigny, 1816), a conspicuous member on Indo-Pacific coral reefs[16,17], provides a unique opportunity to deepen our knowledge and revive the study of evisceration in ascidians, establishing a solid platform from which to study regeneration of the digestive tube from molecular, cellular, and developmental aspects
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