Predators & defenses
 

Fishes, sea stars, crabs, and lobsters are known to prey on ophiuroids, and their first line of defense is to withdraw from contact.  For example, a basket star Gorgonocephalus eucnemis when disturbed will pull in its arms to form a tightly inter-tangled knot, a strategy that may deter potential predators.  When disturbed, brittle stars, also known as “serpent” stars, will slither away to safety under rocks or in crevices.

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Escape behaviour

  Other defenses considered in this section are autotomy and low nutritional content.
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Research study 1
 

photograph of brittle star Ophiopteris papillosa courtesy Bill Austin, Khoyotan Marine Laboratory, Victoria, BCAn unusual behaviour is seen in Ophiopteris papillosa, a shallow-water inhabiting species found along the west coast. When attacked by fishes an intended prey brittle star may be carried off the bottom by the predator and, if an arm is autotomised or if the fish releases its hold on the prey, the brittle star drops through the water.  As it drops it often raises its arms above the disc in a tulip-like position, and it holds this position until it lands.  Individuals in the tulip position fall about 50% faster than ones not in this position, likely because of reduced hydrodynamic drag.  When tested in the field at Santa Catalina Island, California, tulip-position individuals are eaten about 25% less frequently by fishes than ones not in a tulip position.  The author concludes that adoption of this behaviour may increase the likelihood of a brittle star reaching protective cover of the sea bottom before being attacked again.  Pomory 2001 Mar Freshw Behav Physiol 34: 171. Photo courtesy Bill Austin, Khoyotan Mar Laboratory, Victoria.

NOTE of 5 species of fishes observed to make contact with test brittle stars Ophiopteris experimentally dropped through 6-7m of water to the sea bottom, only kelp bass Paralabrax clathratus and sheepshead Semicossyphus pulcher actually attack and attempt to eat the brittle stars

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Research study 2
 

An informal report by two west-coast scientists is interesting for its description of brittle stars and other invertebrates attempting to escape, not from predators, but from unsuitable environmental conditions. The study area is near the wharf at Coupeville, Whidbey Island, Washington at about 8m depth. Over a 2wk period in Sept 2014 it became a “dead zone”, characterised by anoxic sediments, black hydrogen-sulphide residues, and grey slime from deaths of hundreds of brittle stars, cockles, and sea cucumbers. For a day or so the brittle stars attempt to leave the sediments and some are seen standing “tippy-toe” to elevate their central discs above the toxic sediment. A week later the water above the sediment becomes cloudy. After 10d the the water has become clear, most or all of the dead brittle stars are gone, and after a month or so the area appears normal. The authors, however, remark that it may in fact take several more years for the invertebrate populations to recover. Such areas, usually on smaller scale, are occasionally noticed by SCUBA divers, but their causes are still a mystery. Shimek & Kocian 2014 Reef to Ranforest Media. Photographs courtesy Jan Kocian.

NOTE hydrogen sulphide-reducing anaerobic bacteria obtain energy not from metabolism of sulphates to hydrogen sulphide (H2S). Hydrogen sulphide is black and stinks of rotten eggs. Many clams and other tube-dwelling invertebrates may tolerate the anaerobic conditions in their sediments as long as they are able to pump in, or respire in, clean seawater. Clams with blackened shells are usually symptomatic of possible sewage or other organic pollution entering their habitat

 
photograph of brittle stars attempting to escape from toxic effects in dead zone in Whidbey Island, WA photograph of two cockles Clinocardium nuttallii blackened by effects of hydrogen-sulphide toxic sediments at Whidbey Island, WA photograph of dead and dying invertebrates from effects of toxic sediments in "dead zone" at Whidbey Island, WA
Brittle stars Amphiodia periercta raise up to escape the toxic sediments. In healthy sediment they would be buried with only their arms extending upwards to feed Cockles Clinocardium nuttallii would normally be buried with only their siphons protruding. The black coloration is from activity of hydrogen sulphide-metabolising bacteria Organisms buried in or living on the sediment surface are most affected. Note the tubeworms Eudistylia sp. living on a rock above the sediment seem largely unaffected
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  Autotomy
 
Research study 1
 

photograph of brittle star Amphiodia urtica courtesy Bill Austin, Khoyotan Marine Laboratory, Victoria, BCUnder duress from a potential predator, brittle stars readily drop their arms.  This is facilitated by instant transformation of the collagen comprising the inter-ossicle ligaments from a stiff to a weakened, highly viscous state.  Contraction of arm muscles at this spot causes the connection to rupture and the arm drops off.  Note the 2 missing arms in this photo of Amphiodia urtica. It is not known whether the arms in this or other west-coast species drop off near the base, as suggested by the photo, or whether they can break anywhere along an arm. The autotomised arm may move around for some time, perhaps distracting a predatory fish and permitting the previous owner to crawl to safety.  Unless the disc is damaged, the brittle star can survive such attacks and regenerate the missing parts. Photo courtesy Bill Austin, Khoyotan Marine Laboratory, Victoria.

NOTE the special properties and functions of these so-called “catch” connective tissues are reviewed elsewhere in the ODYSSEY: LEARN ABOUT SEA CUCUMBERS: PREDATORS & DEFENSE

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  Nutritional content
 
Research study 1
 


Even with their long arms extended into the currents when feeding and therefore seemingly vulnerable, ophiuroids appear to suffer little loss to predators.  This may owe, at least in part, to the fact that the arms may have little nutritional content, being comprised of up to 2/3rds calcareous ossicles

NOTE  more on this subject can be found in other parts of the ODYSSEY: LEARN ABOUT NUDIBRANCHS: DEFENSES: NUTRITIONAL CONTENT and LEARN ABOUT FEATHER STARS: PREDATORS & DEFENSES: NUTRITIONAL VALUE

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