Predators & defenses
 

Among world species of sea cucumbers, defensive features of adults include a leathery body wall, swimming, withdrawal into crevices, evisceration/autotomy, skin ossicles, toxic chemicals, nocturnal and/or cryptic behaviour, and sticky, noxious tubules released from the cloaca.  All but the last defensive strategy is found to one extent or other in west-coast sea cucumbers.  

NOTE  chemicals of possible defensive function include saponins (soap-like substances) and various terpenoids, some with known feeding-deterrent capability.  When damaged by a predator or when deliberately crushed into a tidepool for fishing purposes, some Indo-Pacific species have sufficient toxins in the body to kill or incapacitate fishes. Bryan et al. 1997 J Exper Mar Biol Ecol 210: 173.

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  Thick body wall/skin ossicles
  Defenses of thick body wall/skin ossicles will be considered in this section, while SWIMMING, WITHDRAWAL INTO CREVICES/"CATCH" CONNECTIVE TISSUES, EVISCERATION & REGENERATION, TOXIC CHEMICALS/UNPALATABILITY, SEA-STAR PREDATORS and SEA-OTTER PREDATORS will be considered in other sections.
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Research study 1
 

photograph showing close view of the rubbery tubercles of the sea cucumber Parastichopus californicusThe sea cucumber Parastichopus californicus crawls about the sea bottom openly exposed to potential predators. Its first line of defense is a heavy body wall with large rubbery tubercules. The thick body wall of a sea cucumber is mostly collagenous connective tissues, with little musculature.  A special feature of this collagen is the ability to transform instantly from a stiff state providing a tough, defensive “shell”, to a soft and flexible state allowing withdrawal into crevices and locomotion.  Laboratory tests on Parastichopus japonicus (a Japanese species) show that mechanical stimulation of the body-wall collagens increases stiffness by a factor of 2.  Motokawa 1984 Comp Biochem Physiol 77A: 419.

NOTE  these collagens, known as “catch” connective tissues, are considered in another part of DEFENSES & PREDATORS: WITHDRAWAL INTO CREVICES/"CATCH" CONNECTIVE TISSUES

Close view of the tubercules of P. californicus. Note that all tubercules open to
the outside via a small pore. The function of these pores is not known, although
individuals have been observed in the field venting gametes through such a pore 3X

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Research study 2
  Within the dermal layer of the skin of sea cucumbers are numerous ossicles made of calcium carbonate.  Each is a single crystal that, depending upon species, may be in irregular globular form or in distinct species-specific form.  The ossicles are the remains of the ancestral skeleton, present in contemporary echinoderms variously as test (sea urchins, sand dollars), spines (sea urchins, sea stars), and ossicles (sea cucumbers, sea stars, brittle stars).  Coincidental with evolution to a softer-skinned, flexible, crawling life style in sea cucumbers has been selection for a reduced and more loosely articulating skeleton.  Except for species of Psolidae (e.g., Psolus chitonoides), it seems unlikely that skin ossicles are an effective defense against predators, especially against sea stars with their their ability to digest prey externally. 
 
photograph of ossicles in close view of a holothuroid
Mixed skin ossicles from unidentified holothuroids photographed from a prepared microscope slide. . Neither the origin nor the size of these ossicles is known
scanning e-microscopical view of the skin ossicles of the sea cucumber Eupentacta quinquesemita courtesy Phil Lambert, Roy BC Museum
Skin ossicles from Eupentacta quinquesemita. The largest of these is about 100um in size. Photo courtesy Phil Lambert, Royal BC Museum
scanning e-microscopical view of the skin ossicles of the sea cucumber Pseudocnus curatus courtesy Phil Lambert, Roy BC Museum
Skin ossicles from Pseudocnus curatus, measuring about 85um in diameter. Photo courtesy Phil Lambert, Roy BC Museum, Victoria
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Research study 3
 

photograph of the sea cucumber Psolus chitonoidesscanning e-microscopical view of a calcareous skin plate of the sea cucumber Psolus chitonoides courtesy Emlet 1982 Biol Bull 163: 264
Skin ossicles of Psolus chitonoides are porous for greater strength. The pores are thought to stop cracks from spreading and also to enable surface repairs to be made.  The photo on the Right shows a skin ossicle from the soft sole on which the animal sits. Other ossicles in Psolus are elaborated into large overlapping protective plates that would seem to be quite effective in defense, especially against biting predators such as fishes and crustaceans (although no-one seems to have investigated this). The tentacles protrude from an opening in these plates at the top/front of the animal and, when withdrawn, are protected by 5 plates that close over.  Similarly, the cloacal/anus region also closes by means of 5 anal plates. Emlet 1982 Biol Bull 163: 264.

 

Sea cucumber Psolus chitonoides. The sturdy plates that
cover the upper surface are each several mm in size and are
covered by epidermis. The plates form as aggregates of
many smaller ossicles of the type featured above Right 1X

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