The chemical ecology of Antarctic sponges

Sponges were collected from shallow waters along the western Antarctic Peninsula near Palmer Station on Anvers Island. The majority (78%) of the sponges were found to have outer tissues defended against the omnivorous, Antarctic sea star Odontaster validus. Of the species that had outer tissues defended, 62% of them also had internal tissues that inhibited feeding by O. validus. Lipophilic or hydrophilic extracts coated on artificial food pellets were found to be unpalatable for all of the sponge species tested. These data provide evidence that defenses are common survival strategies in sessile macroinvertebrates from Antarctica but that the allocation of the defenses do not follow predictions made by the Optimal Defense Theory in an environment where sea stars drive the evolution of defenses. While it cannot be determined whether all of the unpalatability is due to secondary metabolites in these sponges, a large proportion of the defenses can be explained in those organisms that were analyzed.;The secondary metabolites present in the sponges did not inhibit the majority of bacteria isolated from the same system from which the sponges were collected. These chemical compounds did cause significant diatom mortality when presented to a diatom species collected from the same area ( Syndroposis sp.). It appears that bacterial pathogens are not a substantial threat to sponge survival in these waters due to the sponges not producing compounds that would prevent bacterial growth in their presence. However, fouling by diatoms does appear to be a threat which these sponges try to prevent.;No solid conclusions can be made concerning the ability of a sponge to have defenses induced by the constant presence of predators. The sponges are commonly found to have defensives present in all body tissues although some compounds have been seen to be isolated in external tissues. These sequestered compounds are not always feeding deterrents. There are many questions that remain relating to predator-prey relationships in coastal Antarctic Peninsula waters.;Keywords: Chemical ecology, Antarctica, Porifera, Antimicrobial, Optimal Defense Theory, Inducible defenses.