The biochemical composition, energy content, and chemical antifeedant defenses of the common Antarctic Peninsular sea stars Granaster nutrix and Neosmilaster georgianus

The sea stars Granaster nutrix and Neosmilaster georgianus are conspicuous members of benthic communities along the Antarctic Peninsula. An analysis of the proximate composition of somatic body components of nonreproductive adults indicates the nutrient storage organs (pyloric caeca) are rich in both protein (60.7 and 60.6% mean dry wt, respectively) and lipid (25.4 and 29.8% mean dry wt, respectively). Body-wall tissues, while containing small inconspicuous skeletal ossicles, are also comprised of significant levels of organic matter (33.5 and 55.7% mean dry wt, respectively), attributable primarily to protein. Both the pyloric caeca and body-wall tissues are relatively rich in energy (mean energy levels=24.8 and 26.5 kJ g⁻¹ dry wt; 8.4 and 14.1 kJ g⁻¹ dry wt, respectively). Despite the availability of these nutrients and energy neither sea star is preyed upon by the sympatric omnivorous sea star Odontaster validus, a common predator of other Antarctic sea stars. Laboratory feeding bioassays indicate that O. validus rejects live intact individuals and body-wall tissues of both sea star species while readily consuming dried krill. Alginate food pellets containing a krill powder and tissue level concentrations of organic methanol extracts of body-wall tissues were also rejected by O. validus. Moreover, the copious mucus released from the body wall of N. georgianus was deterrent in O. validus food pellet bioassays. Thus, both sea stars evidently possess defensive secondary metabolites that defend against predation and are likely to play a role in mediating materials and energy transfer in the Antarctic benthos.