Palatability and chemical anti-predatory defenses in common ascidians from the Antarctic Peninsula

Abstract

Palatability of outer tissues of a suite (12 species) of Antarctic ascidians was evaluated using omnivorous fish and sea star predators. Tissues of 100% of those tested were unpalatable to fish, while 58% were unpalatable to sea stars. Lipophilic and hydrophilic extracts of 11 species were incorporated into pellets and tested in fish and sea star bioassays. Only the lipophilic extract from Distaplia colligans caused fish feeding deterrence. Organic extracts from 10 ascidian species were also examined in food pellet assays using an omnivorous amphipod. Only the lipophilic extract of D. cylindrica was a deterrent. Five of the ascidians possessed acidified outer tunics (pH < 3). We tested the ability of acidified krill pellets (pH 2 to 7) to deter fish and sea star predators and found that, while fish readily ingested acidified food pellets (pH 2), sea stars were deterred at pH 5 or less. Thus either organic or inorganic chemical defenses explain defense in 5 of the 7 ascidian species found unpalatable to sea stars. In contrast, chemical defenses only explain 1 of 12 species found unpalatable to fish, and only 1 of 10 ascidians tested against an amphipod predator. This predator-specific pattern of chemical defense may reflect greater predation pressure on ascidians from Antarctic sea stars. Alternatively, Antarctic ascidians may rely on other factors such as the toughness of their tunic or sequestration of heavy metals such as vanadium to inhibit feeding by Antarctic fish, a taxonomic group known to lack strong jaws.

Feeding deterrency in Antarctic marine organisms: bioassays with the omnivore amphipod Cheirimedon femoratus

“…This contrasted with other Antarctic surveys (e.g. Koplovitz et al 2009). Our samples pertained to common Antarctic colonial species (Varela 2007), some reported to harbor bioactive products (see Avila et al 2008, Blunt et al 2011, and previous reviews).…”

Cytotoxic Activity of Antarctic Benthic Organisms Against the Common Sea Urchin Sterechinus neumayeri

“…Although the Antarctic solitary tunicate Cnemidocarpa verrucosa is known to be fouled by different invertebrates, it seems to be well defended in our study against both the development and the sperm of S. neumayeri. In recent studies, the tissues of C. verrucosa proved to be unpalatable to O. validus [35,73]. Conversely, in another study this species did not show antifeeding activity against the same sea star [33].…”

“…Although this group is also an important source of natural products with pharmaceutical properties [23], few studies have focused on the chemical interactions that these compounds play in the ecosystems [19,30,33,64,72,73]. In particular, some colonial and solitary ascidians possess chemical defenses to protect them from predators [20,64,72].…”

Chemical Interactions in Antarctic Marine Benthic Ecosystems

“…Another species used as a model in previous Antarctic bioassays is the sympatric omnivorous amphipod Gondogeneia antarctica. This species, however, has repeatedly shown the problem of finding artificial foods too attractive, causing a phagostimulatory effect on the crustacean, and hence making the results obtained hard to interpret (Amsler et al, 2005, 2009a andb;Iken et al, 2009;Koplovitz, et al, 2009). Instead, the amphipod C. femoratus seems to possess a quite discriminatory potential to detect unpalatabilities, as observed in our results.…”