The palatability of 35 non-encrusting, subtidal macroalgal species collected from the vicinity of Palmer Station, Antarctica (64°46' S, 64°03' W), was determined in laboratory bioassays utilizing sympatric sea stars and fish known to consume macroalgae in nature. Overall, 63% of the macroalgal species offered to sea stars and 83% of the macroalgal species offered to fish in thallus bioassays were significantly unpalatable. This included all of the ecologically dominant, overstory brown macroalgae in the region. When organic extracts of unpalatable macroalgal species were incorporated into artificial foods, 76% of the species unpalatable as thallus to sea stars were also unpalatable to them as extract, and 53% of the species unpalatable as thallus to fish were also unpalatable to them as extract. If either sea stars or fish rejected thallus of a macroalgal species, palatability of organic extracts of that species to herbivorous amphipods was determined: 63% of such algal species were unpalatable as extract to the amphipods. It was concluded that antarctic macroalgae are commonly unpalatable to sympatric consumers and that much of this unpalatability is the result of chemical defenses. As a whole, neither thallus toughness nor a variety of nutritional quality parameters appeared to be related to macroalgal palatability. We also tested the hypothesis that nitrogen-containing metabolites should be common in macroalgae from nitrogen-replete, carbon-limited environments such as the coastal waters of Antarctica. Macroalgal acid extracts targeting nitrogenous secondary metabolites were subjected to thin-layer chromatography analysis; no such compounds were detected.

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The present study surveyed the palatability of all sponge species that could be collected in sufficient quantities in a shallow-water area along the western Antarctic Peninsula. Of 27 species assayed, 78% had outermost tissues that were significantly unpalatable to the sympatric, omnivorous sea star Odontaster validus. Of those species with unpalatable outer tissues, 62% had inner tissues that were also unpalatable to the sea stars. Sea stars have often been considered as the primary predators of sponges in other regions of Antarctica, and their extra-oral mode of feeding threatens only the outermost sponge tissues. The observation that many of the sponges allocate defenses to inner tissues suggests the possibility that biting predators such as mesograzers, which could access inner sponge layers, may also be important in communities along the Antarctic Peninsula. In feeding bioassays with extracts from 12 of the unpalatable species in artificial foods, either lipophilic or hydrophilic extracts were deterrent in each species. These data indicate an overall level of chemical defenses in these Antarctic sponges that is comparable to, and slightly greater than, that found in a previous survey of tropical species.

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