(2011a) observed that amphipod densities on D. menziesii significantly decreased, while densities on I. cordata significantly increased at night compared to day such that the densities (per unit wet weight) on the two species were not significantly different in the dark. Overall amphipod density on P. decipiens also increased at night, but the trend was not statistically significant. However, densities of the amphipod G. antarctica, selleck inhibitor which, as noted previously, consumes P. decipiens as fresh thallus, did significantly increase on it during the night. Aumack et al. (2011a) suggested that the amphipods were leaving their chemically defended
macroalgal refuges during the night so as to forage on diatoms, other palatable microalgae and macroalgae, and other potential food sources, while their predators were less LY294002 ic50 successful at foraging on them because of the darkness. We have come to describe this association between macroalgae and amphipods as a community-wide mutualism because, as detailed previously in this mini-review, chemically defended macroalgae are the dominant structural components of the community, amphipods, and to
a lesser extent also gastropods, appear to be the major second trophic level consumers in the community, and organisms at both trophic levels appear to gain substantial benefits from their association. These interactions are summarized graphically in Figure 1. Most macroalgae in the community are chemically defended from consumption, but benefit the dense assemblage of macroalgal-associated amphipods by providing an associational refuge from fish predation. The amphipods benefit the macroalgae by keeping them relatively clean of diatoms and other epiphytes. However, the dense amphipod assemblage appears to have selected for filamentous algae with an ability to grow as endophytes within Exoribonuclease the chemically defended macroalgae, which then provide them with a refuge from amphipod grazing. The endophytes can be pathogenic to their hosts, but evidence to date suggests that this is not always true and even when it is, the effects can be relatively mild.
This community-wide mutualism can be considered at least somewhat analogous to the relationships on tropical reefs between herbivorous fish and corals where the fish reduce macroalgal cover, benefiting the corals, and the fish benefit in turn because the increased structural complexity provided by corals increases fish recruitment (Mumby and Steneck 2008, Hughes et al. 2010). Our description of this interaction as a mutualism assumes that the macroalgae in nature are indeed benefiting from the presence of the dense associated amphipods and somewhat less abundant gastropods. Since light is considered to be the main growth-limiting environmental variable for Antarctic macroalgae (Wiencke et al. 2007, Zacher et al. 2009, Wiencke and Amsler 2012), it is reasonable to expect that the removal of light-blocking epiphytes would indeed be beneficial.