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

Researchers at Simon Fraser University, British Columbia report that diets of glaucous-wing gulls Larus glaucescens in the Stanley Park and Roberts Creek areas of southern British Columbia include about 60% representation of ochre stars Pisaster ochraceus.  In “prey offer”-type trials with gulls and 4 size-classes of ochre stars, medium-small individuals are selected first (see histogram).  This size-class provides significantly greater energy return to the gulls than either smaller or larger size-classes when ingestion and digestion times are factored in.  Interestingly, when other gulls are present, with the risk of kleptoparasitism or food theft increasing in proportion to their numbers, then a single gull’s preference shifts significantly to the smallest size-class of prey, presumably the better to swallow it as quickly as possible (within seconds) before it is snatched by another gull. Suraci & Dill 2011 The Auk 128 (4) 643-650.

It 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

Anyone who has been on an intertidal west-coast rocky shore in the company of sea gulls is likely to have seen one eating an ochre sea star Pisaster ochraceus (see also RS1 above). It all looks very cute until you consider it from the sea star’s point of view. In one study over a series of 8 low-tide periods in summer at Roberts Creek, British Columbia, researchers from Simon Fraser University observe 8-11 glaucous-wing gulls Larus glaucescens eating 265 juvenile sea stars P. ochraceus (<6.5cm overall diameter), estimated to be equivalent to 117kg live mass of sea stars. Each prey item requires about 7min handling time. No sea star >10cm diameter seems to be eaten. Such behaviour, if repeated at the same intensity elsewhere, must have serious negative effects on ochre-star population numbers. Suraci & Dill 2012 Behav Ecol vol?: 280. Photograph courtesy Jeff Masters.

NOTE it is not clear from the authors’ description exactly how many gulls are involved in this statistic. However, based on other published data a live ochre star of, say, 6cm arm length would weigh no more than about 50-60g, making the total mass consumed closer to 13-16kg, not 117kg as the authors report. This, however, in no way changes the comment about the severity of gull predation on sea stars. See Feder 1963 Ecology 44: 505 for arm length/live mass data for ochre stars

What comes next? A sea gull Larus glaucescens may
take much longer than 7min to consume a sea star,
especially if the prey sits crosswise in its beak

Spines 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

The 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