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  Predators & defenses
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  Distasteful chemicals

Defenses of asteroids include distasteful chemicals, considered here, and SPINES & OSSICLES, PEDICELLARIAE, ESCAPE BY CRAWLING, RIGHTING RESPONSE, MUCUS, AUTOTOMY, and CAMOUFLAGE, considered in other sections. 

Research study 1

photograph of mixed array of west-coast asteroidsBright colours and general "warning" conspicuousness of sea stars suggest that distasteful chemicals may be involved in defense.  Saponins are abundant in asteroids, including the sunflower star Pycnopodia helianthoides and the pink star Pisaster brevispinus.  Tests show that the substances are hemolytic and/or neurologic to selected fishes, copepods, and whelks, and interfere with development of sea urchins.  Rio et al. 1963 Am Zool 3: 554; Mackie et al. 1977 Comp Biochem Physiol 56B: 9.

NOTE  soap-like steroid-glycoside chemicals abundant in asteroids, present in holothuroids (where they are known as “holothurins”), but absent or only in low levels in ophiuroids, crinoids, and echinoids?

NOTE  3 different and novel saponins are found in aqueous extracts of each of P. helianthoides and P. brevispinus collected in the Gulf of California.  No additional information is available on their function.Bruno et al. 1989 J Nat Prod 52: 1022; Zollo et al. 1990 J Nat Prod 53: 1000.

Mixed array of west-coast asteroids. Pycnopodia helianthoides is at the 1130
o'clock position; Pisaster brevispinus is partially in view at the 6 o'clock position

Research study 2

What effects might these saponins have on scaleworms symbiotic with sea stars?  This is investigated in Tacoma, Washington using dilutions of saponins isolated from the sea graph showing effects on oxygen consumption in scaleworms of different concentrations of saponinsstar Evasterias troschelii to assess physiological effects on oxygen consumption in 2 species of Arctonoe scaleworms, photograph of sea star Evasterias troschelii with commensal scaleworm Arctonoe fragilisone a regular associate of E. troschelii; the other, not a regular associate.  Results show tha oxygen uptake of Arctonoe pulchra, not usually found on the sea star, is deleteriously affected by the saponins, while A. fragilis, a regular symbiont of Evasterias, seems unaffected.  The authors remark that the saponins may play an important role in the determination of symbiotic populations by strongly selecting for A. fragilis and against the similar A. pulchra Patterson et al. 1978 J Exp Mar Biol Ecol 33: 51. Photograph courtesy Chris Gunn, Campbell River, British Columbia

Evasterias troschelii with symbiotic scaleworm Arctonoe fragilis

Research study 3

What about sea-star larvae? They lack any evident structural defenses such as possessed by other invertebrate larvae (shell, spines, setae) and may be good candidates for possession of chemical defenses. Results of experiments at Friday Harbor Laboratories, Washington in which several types of invertebrate larvae, including bipinnaria (sea star Pisaster ochraceus) are fed to 2 types of filter-feeding adult invertebrates (mussels and sea squirts) suggest a degree of chemical protection. Survival of bipinnaria when exposed to Styela gibbsii (sea squirt) and Mytilus trossulus (mussel) is 70 and 20% under the experimental conditions imposed. In comparison, average survival of larvae of other invertebrates (sand dollars, sea urchins, goose barnacles, limpets, and worms), all possessing apparent structural defenses averages only about 40%. The authors note that low survival of all larvae in the mussel treatments may in large part relate to the virtual non-selective intake of particles by the mussels, followed by rejection of unwanted items after coating and transporting them in mucus.  By this time most delicate larvae such as sea-star bipinnaria, even if ultimately rejected as food, are too photograph of bipinnaria larvae courtesy Tim Rawlings, Cape Breton University, Nova Scotia tangled in mucus to survive. The poor overall survival of all larvae suggests to the experimenters that physical features of the larvae are generally ineffective in defense, at least to these particular filter-feeders.  These features include barbed setae (Sabellaria worms), spines (barnacle nauplii), skeletal rods (echinoplutei), shells (veligers), and so on. In other tests, where larvae are homogenised and pipetted into the branchial-siphon region of the tunicates, more siphonal closures occurred in response to homogenates of echinoderm larvae than to other larvae, also suggesting the presence of distasteful chemicals. Cowden et al. 1984 Mar Ecol Progr Ser 14: 145. Photograph of bipinnaria larvae courtesy Tim Rawlings, Cape Breton University, Nova Scotia.

Bipinnaria larvae of an unidentified
west-coast sea star 70X

Research study 4

photograph of juvenile bat star Patiria miniata courtesy Dave Cowles, Walla Walla University, WashingtonLaboratory experiments in which juvenile bat stars Patiria miniata (2-5cm diameter) are exposed to 7 macroinvertebrate and one fish predator from their immediate habitat over periods ranging from 2-6wk, lead to little mortality from the predators.  Only sea stars Pycnopodia helianthoides eat a few of the juvenile Asterina.  The possible presence of distasteful chemicals in Patiria is not discussed by the author, and this might be a good subject for further research.  Rumrill 1989 Mar Ecol Progr Ser 56: 37. Photograph courtesy Dave Cowles, Walla Walla University, Washington

NOTE  the predators include sea stars Patiria miniata, Evasterias troschelli, Dermasterias imbricata, Pisaster brevispinus, and Pycnopodia helianthoides; crabs Cancer productus and C. magister; and lingcod Ophiodon elongatus.  Sizes and numbers of predators are not provided in the paper, but only 3 juvenile Patiria are eaten, and these all by Pycnopodia

Juvenile bat star Patiria miniata 1X

Research study 5

photograph of an injured sunflower star Pycnopodia helianthoides in a tidepoolA study at the Bamfield Marine Sciences Centre, British Columbia shows that sunflower stars Pycnopodia helianthoides exhibit a classic alarm response to, and will move away from, the scent of body parts and tissue fluids of injured conspecifics.  The movement is always in the direction of the prevailing current and takes place generally within 2-5min.  Positive escape responses are obtained in 25 of 34 tests on different individual sunflower stars.  Lawrence 1991 Mar Behav Physiol 19: 39.



Injured sunflower star Pycnopodia helianthoides in a
tidepool, presumably leaking alarm-response chemicals 0.25X

Research study 6

histogram showing consumption of larvae of a sea star Solaster stimpsoni by 3 predatorshistogram showing percentages of developmental stages of sea stars eaten by sculpins in comparison with control pieces of sea-urchin gonadsEggs of lecithotrophic sea-star species are often brightly coloured, suggesting that they are warning of the presence of defensive chemicals. Indeed, studies at Friday Harbor Laboratories, Washington show that most of these eggs and the larvae that develop from them are rejected as food by sculpins Oligocottus maculosus, and sea anemones Anthopleura elegantissima and Metridium senile.  Note in the histogram on the Left that the 3 predators eat a significantly lower percentage of 14-d larvae of the sea star Solaster stimpsoni than they do control pieces of sea-urchin Strongylocentrotus purpuratus ovaries. Similarly, sculpins eat almost no developmental stages of the sea stars Leptasterias sp. and Henricia leviuscula, but readily consume sea-urchin ovaries (histogram on Right). The authors note that because these stages lack obvious structural or behavioural defenses, the most likely cause of the deterrence by the fish and sea anemones is their content of saponin chemicals.  The authors conclude that predation is unlikely to be a primary cause of larval mortality in these lecithotrophic sea-star species.  Iyengar & Harvell 2001 J Exp Mar Biol Ecol 264: 171.

NOTE  other data in the study on deterrent properties of ovaries, larvae, and juveniles, and information on other species of sea stars and predators are not presented here.  The study is valuable because so many species and developmental stages are tested against a wide range of different predators