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  Defenses & predators
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  Withdrawal into crevices/"catch" connective tissues
  Topics of withdrawal into crevices/"catch" connective tissues will be considered in this section, while THICK BODY WALL/SKIN OSSICLES, SWIMMING, TOXIC CHEMICALS/UNPALATABILITY, EVISCERATION & REGENERATION, and SEA-STAR PREDATORS will be considered in other sections.
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Research study 1

photograph of a sea cucumber Cucumaria miniata within a group of red sea-urchinsSea cucumbers such as Cucumaria miniata that live among tumbled rocks or in crevices can pull in their tentacles and withdraw their bodies for protection.  The first is enabled by 5 tentacle-retractor muscles; the second, by contraction of 5 sets of longitudinal muscles.  The tentacles are hollow and their spaces communicate with the water-vascular system.  During withdrawal, the fluid within the tentacles moves into special containment sacs suspended in the general body cavity.  The body wall contains both circular and longitudinal muscles.  Contraction of both sets of muscles increases the internal fluid pressure in the body and hydraulically inflates the tentacles to extend them into feeding mode.

NOTE  known as Polian vesicles.  There may be one or several of these, depending upon species


A single Cucumaria miniata feeds within a group of red
sea-urchins Strongylocentrotus franciscanus 0. 25X

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Research study 2

Just as connective tissues function in our bodies for support, binding, elasticity, and protection, so they are abundant in sea cucumbers, especially in the dermal part of the body wall. A special feature of body-wall and other connective tissues in sea cucumbers is that the collagen component can transform from one state to another. Collagen is a fibrous protein, and in sea cucumbers the fibrils are discontinuous and can slide past one another.  The transformation is controlled by the nervous system and happens quickly. In one state, the collagen is stiff and non-elastic, and provides toughness and rigidity to the body. In an alternate state, the collagen is soft and fluid, and imparts deformability to the body. This allows the sea cucumber to squeeze through tiny cracks and openings between rocks. State-transformation of collagen is also involved in evisceration and autotomy in holothuroids. Photograph courtesy Ron Long, Simon Fraser University , Burnaby, B.C.

NOTE  lit. “glue” G. Collagen is a protein of long, strandular molecular makeup, and is ubiquitous in our bodies.  In various forms it is the main component of vertebrate cartilage, tendons, ligaments, corneas, and lenses. It is found in bones and teeth, and contributes elasticity to blood vessels and skin.  Its degradation with aging in humans leads to skin wrinkling.  A comparable ability to liquefy our collagens would leave us as bone-bumpy puddles of skin on the floor with no ability to move. The collagen-rich body walls of sea cucumbers makes them highly nutritious as an additive to soup or an ingredient of stir-fries, and explains their sale in powdered form as “ginseng of the sea”  (directed at the same market as is purported sore-joint "curing" glucosamine and chondroitin sulphate)

photograph of a sea cucumber Cucumaria miniata courtesy Ron Long, Simon Fraser University, Burnaby, British Columbia
"Plumped-up" version of Cucumaria miniata
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Research study 3

drawings showing locations of "catch" connective tissues in different Classes of echinodermsMutable collagens are unique to echinoderms, but are not restricted to sea cucumbers.  Similar tissues are found in all supporting systems in echinoderms, for example, spine and tooth-jaw ligaments of sea urchins, arm ligments of brittle stars, cirral ligaments of crinoids, inter-ossicle ligaments of sea stars, and elsewhere.  When the collagenous connective tissues are in their “catch” state, the supporting systems become hard and immovable, and maintain the animal’s posture.  When the catch tissues transform, these “supportive shells” relax and become flexible, thus allowing deformation and locomotion.  Viscosity changes in these tissues may be 100- to 1000-fold.  Motokawa 1985 p. 69 In, Echinodermata (Eds. Keegan & O’Connor) A. A. Balkema, Rotterdam; diagram modified from Wilkie 1983 Mar Behav Phys 9: 229.

NOTE in several South-Pacific island countries, eviscerated intestines of the sea cucumber Stichopus variegata are eaten raw, or after pickling or cooking.  By poking or cutting a hole in the body wall and hooking out the guts, the sea cucumber is induced to eviscerate.  The hole is easily made with a finger because the poking induces state-transformation in the body-wall collagens. The harvesting is thought to be sustainable because the eviscerated animals are believed to regenerate their lost parts within a few days. 

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Research study 4

graph showing state-transformation of "catch" connective tissues in the sea cucumber Stichopus japonicus through addition of the neurosecretion holokinin 1State-transformation of the “catch” connective tissues is under nervous control, but is not mediated by nerve impulses; rather, by agents secreted by the nerves, including in some species neuropeptides of small molecular mass.  Research on a related Japanese species Stichopus japonicus has identified 4 such neurosecretions, of 5-16 amino acids in size.  Two of these, termed by the authors holokinin 1 and  holokinin 2, act to soften the tissues.  The graph on the Right shows the effect of holokinin 1 (10-6 M) on stiffness of sea-cucumber dermis, measured as change in viscosity (i.e., extent of “creep”; that is, “puddling”).  Note the speed of the response. Holokinin 2 (10-6 M) has a similar effect.graph showing stiffening effect of the neurosecretion pentapeptide on the "catch" connective tissues of a sea cucumber Stichopus


A third neurosecretion, termed pentapeptide for its content of 5 amino acids, has an opposite effect to that of the holokinins; that is, it stiffens the tissues. The graph on the Left shows the effect of pentapeptide 1 (10-6 M) on stiffness of sea-cucumber dermis. Within a few moments of washing the tissues in clean seawater, the effect is lost.


The fourth neurosecretion, termed stichopin, acts to inhibit another neurotransmitter, acetylcholine, itself graph showing inhibitory effect of the neurosecretion stichopin on another neurosecretion acetylcholine in the sea cucumber Stichopusan agent of stiffening.  In one experiment a section sea-cucumber dermis is treated with stichopin (10-6 M) and its viscosity measured every few seconds.  There is no significant change in viscosity (graph on Left). After 3min acetylcholine (10-5 M)  is added, with no effect. 

graph showing stiffening effect of the neurosecretion acetylcholine on connective tissues in the sea cucumber StichopusThen, the same tissue is washed free of both chemicals and tested again.  After 3min, application of acetylcholine causes an abrupt increase in viscosity (graph on Right). The authors note that this is the first report of the controlling agents in phase transformation of sea-cucumber “catch” connective tissues.  Birenheide et al. 1998 Biol Bull 194: 253.



NOTE  lit. “row” G., derived from the genus name Stichopus

NOTE  lit. “whole movement” G.

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Research study 5
  photograph of a sea cucumber Cucumaria frondosa courtesy of the Barcode of Life Data SystemsA different chemical “plasticising” agent seems to be involved in the sea cucumber Cucumaria frondosa, an Atlantic species, and may therefore be in our west-coast species of Cucumaria.  Studies in Maine show that the agents may be proteins released not from nerves, but directly from cells in the dermis.  Calcium appears also to play a role in Cucumaria because in calcium-free seawater the tissues become soft and compliant.  When calcium is restored to normal levels the tissues again are stiff.  Koob et al. 1999 J Exper Biol 202: 2291; photo courtesy of Barcode of Life Data Systems.
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Research study 6

drawing showing anal pores in sea cucumber Leptosynapta clarkiStudy of sea cucumbers Leptosynapta clarki at Friday Harbor Laboratories, Washington reveals the presence of 5 pores around the anus (see drawing).  Each pore is surrounded by muscle fibres that act as miniature sphincters. The pores appear to allow coelomic fluid to be released from the main body cavity, although the function of this action is not understood.  However, since release of this fluid results in a rapid reduction in size to about one-quarter of original length and seems to be stimulated by touch or vibration, the author thinks the pores may function in defense.  Following deflation, an individual’s original body volume is restored withing 15-30min.  The path of re-entry of water is not known, but tests with methylene blue and fluorescein dyes do not identify the anal pores as the route.  Anderson 1966 Can J Zool 44: 1031.

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