An examination of secondary metabolites and inorganic acids as chemical defenses against predation and fouling in antarctic and sub-tropical ascidians.

摘要

Palatability of fresh outer tissues of 12 species of ascidians from the Western Antarctic Peninsula was evaluated using the sympatric, omnivorous fish Notothenia coriiceps and sea star Odontaster validus as model predators. All ascidians were unpalatable to fish, while 58% were unpalatable to sea stars. Lipophilic and hydrophilic extracts of 11 ascidian species were incorporated into food pellets and tested in fish and sea star bioassays. Only the lipophilic extract from Distaplia colligans caused feeding deterrence in either predator. Organic extracts were also examined in food pellets using the sympatric, omnivorous amphipod Gondogeneia antarctica. Only the lipophilic extract of Distaplia cylindrica was deterrent. Five species of ascidians had acidic tunics. Acidified food pellets were deterrent against sea stars but not fish.;The secondary metabolites from a similar suite of Antarctic ascidian species were tested against sympatric marine bacteria and diatoms from the Western Antarctic Peninsula. All ascidians had lipophilic and hydrophilic extracts assayed against twenty bacterial strains and against a sympatric diatom (Syndroposis sp.). Only the lipophilic extract of D. colligans showed broad-spectrum antimicrobial activity. At least one extract from all but one ascidian taxa caused significant diatom mortality.;The palatability of five species of ascidians commonly found in sub-tropical seagrass habitats were evaluated using the sympatric, omnivorous pinfish Lagodon rhomboides as a model predator. Fresh outer tissues of three of the ascidian species were unpalatable to fish. Food pellets containing organic extracts of these species did not deter feeding by the fish. The toughness of the tunic of all five ascidian species was evaluated using a penetrometer. Tunic toughness is likely to explain the lack of palatability of two of the three species. Acidity is unlikely to explain deterrence in the third species as fish consumed acidified food pellets.;Despite their lack of palatability to sea stars and fish, organic chemical defenses against predation are uncommon in both Antarctic and Sub-tropical ascidians. Toughness and inorganic chemicals appear more important. Despite the lack of antibacterial defenses in Antarctic ascidians, secondary metabolites appear to play a potential role in preventing fouling by diatoms. Keywords: Antarctica seagrass chemical defense ascidian fouling