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  Predators & defenses
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There are few predators of west-coast adult sea stars.  Sea gulls eat smaller individuals of ochre stars Pisaster ochraceus and juvenile king crabs Paralithodes camtschatica are noted to be  predators of Evasterias troschelii, but only one species, the sea star Solaster dawsoni, appears to be a true “asteroidivore”. There is also anecdotal evidence that sea otters will bite off arms of Pisaster giganteus, and remove and eat the pyloric ceca and gonads from within. Dew 1990 Can J Fish Aquat Sci 47: 1944.

 
Research study 1
 

map of study area for sea-otter researchhistrograms comparing sea-star biomass on Attu Island, Alaska before and after sea otters returnedIt may be a surprise to know that sea otters will eat sea stars, and in some areas their depredations can severely impact population numbers of asteroids.  Early observation of this came in the decades from 1983-1994 when sea otters recolonised Attu Island, Alaska after a long absence, eventually reaching a population density of about 2750 individuals.  Researchers from the Institue of Marine Science, University of California present data from before-and-after surveys taken in areas of the island showing a significant drop in numbers of sea-star species, most notably Evasterias retifera (see graph).  Correlative with the demise of sea stars is a significant increase in survival of sea mussels Mytilus trossulus, evidenced in parallel experiments in 1983 when sea otters are absent and in 1994 after their return.  The authors are well aware of the correlative nature of their data relating to sea-otter predation on sea stars, but support their thesis with direct observations in the field of such predatory behaviour and of the presence of many damaged individuals appearing shortly after the return of sea otters to certain areas.  Vicknair & Estes 2012 Mar Biol 159: 2641.

NOTE  fur traders drove the sea-otter populations in this area to near extinction during the boom years of the 18th-19th Centuries.  From the early part of the 20th Century when federal regulations prohibited the harvesting and sale of otter pelts, the population slowly began to increase, but held back during this time by increased predation by killer whales

NOTE  these include Evasterias retifera, Leptasterias hexactis, Lethasterias nanimensis, Henricia sp., and Crossaster papposus
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Spines & ossicles

  Defenses of asteroids include spines & ossicles, considered here, and PEDICELLARIAE, ESCAPE BY CRAWLING, RIGHTING RESPONSE, DISTASTEFUL CHEMICALS, MUCUS, AUTOTOMY, and CAMOUFLAGE, considered in other sections.  No research seems to have been done on the defensive role of ossicles in west-coast asteroids.
 
Research study 1
 

drawing representation of ossicle system in a sea starSpines and ossicles are a characteristic feature of many sea-star species.  They are made up of crystals of calcium carbonate. Technically, spines are ossicles, but ossicles also take the form of body plates that interjoin and articulate. Not only do they make the intended prey unwieldy and hard to manipulate, but their abundance in some species (up to 65% dry mass in the ochre star Pisaster ochraceus) must make the effort of catching, ingesting, and digesting them unprofitable for many potential predators.

 

 

Ossicle system of a representative sea star. The bumps
at the top represent the spines that would protrude
from the body surfaces. These spines would be
covered in a thin, protective epidermis

 
Here are a few photographs to illustrate ossicles and their possible roles in defense (in no particular order): photograph of a seagull eating an ochre star Pisaster ochraceus
Seagull eating an ochre star Pisaster ochraceus. The next stage will see large bulges in the neck of the bird as the prey is moved through the esophagus to the crop
photograph of a dead seastar pecked by seagulls
A dead sea star, possibly Pisaster ochraceus, being pecked at by seagulls. The sea star was dead when it was cast up on this sand beach 1X
photograph of sea star Poraniopsis inflata courtesy Roland Anderson and Seattle Aquarium
Poraniopsis inflatus responds to contact with sunflower stars by shrinking its arms so that the spiny ossicles stick out more prominently (see Research Study 2 below 0.5X Photo courtesy Roland Anderson & Seattle Aquarium
photograph showing spines of sea star Orthasterias koehleri
Orthasterias koehleri with spines erected in response to an unknown disturbance. The pedicellariae are also extended in tufts around each spine, which accentuates the spiny appearance of this individual 0.7X
photograph of sea star Orthasterias koehleri showing detail of spines
Orthasterias koehleri in a more "normal" state with spines less visible and pedicellariae tufts withdrawn. It's not clear with this species whether the spines actually move, or whether the body volume decreases 1X
photograph of sun star Solaster dawsoni eating another sea star Lepstasterias hexactis courtesy Dave Cowles, Walla Walla University, Washington
Ossicles in this prey sea star Leptasterias hexactis are no obstacle to its being consumed by the asteroid-eating Solaster dawsonii. Photo courtesy Dave Cowles, Walla Walla Univ, Washington rosario.wallawalla.edu
photograph of juvenile sea star Leptasterias hexactis
Juvenile Leptasterias hexactis with ossicles and spines showing through the transparent skin 0.15X
photograph of a juvenile sea star mounted on a microscope slide to show spines and ossicles
Juvenile sea star prepared and mounted on a microscope slide to show spines and ossicles. The tube feet are also visible 0.1X
   
 
Research study 2
 

drawing of response of sea star Poraniopsis inflatus to contact with a potential predatory sea starThe subtidal sea star Poraniopsis inflatus lacks pedicellariae or other overt defenses against predatory sea stars.  An earlier observation on a single specimen reveals that it responds to contact with sunflower stars Pycnopodia helianthoides and bat stars Asterina miniata by shrinking its arm (see drawing).  This causes the spiny ossicles or “thorns” stick out more prominently, possibly acting as a deterrent to being consumed. The authors measure Poraniopsis’ escape-crawling velocity at only about 3cm per minute, much slower than most other sea stars, and thus likely not a defensive option.  Later tests with a second individual show that of 18 sea-star species tested, 12 cause some deflation, and 4 elicit strong deflation.  Thus, to the “strong-inducing” species listed above can be added Crossaster papposus and Solaster dawsoni.  Of the 4, only S. dawsoni is a confirmed “asteroidivore”. The mechanism of deflation is not known, but loss of coelomic fluid in the affected arm, perhaps by being shunted to other parts of the body, is certainly involved.  Deflation time after contact with a “strong-inducer” is about 5min. The authors discuss the limitations of their results and propose several interesting follow-up research questions. Anderson & Shimek 2010 The Can Field-Naturalist 124 (3): 199.  Drawing courtesy the authors and Marla Coppolino.

NOTE  so uncommon is this species that the researchers have to wait another 15yr before a second specimen is procured for study

NOTE  because of possible risk to their single specimen, the authors do not allow a test to proceed beyond initial reactions.  Thus, we're not sure whether arm deflation works in defense or not, and this would be something for future research (were an adequate supply of specimens to be located).  Apparently, the first specimen was partly consumed by a Crossaster papposus and later died, so we know that in at least one encounter, arm deflation was ineffective  

 
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