Biology and phenotypic plasticity of the Antarctic nototheniid fish Trematomus newnesi in McMurdo Sound

Trematomus newnesi inhabited inshore (<20 m) subzero waters in McMurdo Sound where it fed in the water column on Euphausia crystallorophias and fishes. This sample included the largest reported specimens of this species. The length-weight relationship was Weight = 3.17 x 10(-6)(Standard Length)(3.34), n = 67, r(2) = 0.95. The population was phenotypically plastic, with two distinct morphs easily separated by visual inspection - the typical morph and a large mouth/broad headed morph comprising 28% of the sample. The large mouth morph had a wider and blunter head, longer upper jaw, wider gape, more heavily ossified jaws and darker colouration. To document this morphology, four views of the head are illustrated. Inference from morphology and measurements suggested that the large mouth morph was more benthic than the typical semipelagic morph. Museum specimens from Cape Adare confirmed the presence of the large mouth morph 700 km north of McMurdo Sound. This is the first clear example of phenotypic plasticity in any species of marine fish and its discovery extends the bounds of the nototheniid adaptive radiation to the population level. This finding suggests ecological and evolutionary parallels between the inshore waters of the high Antarctic shelf and the low diversity ichthyofaunas of Arctic, boreal and some temperate lakes. Although there is no data on genetic diversification, an alternate interpretation of the phenotypic plasticity is that the large mouth morph is a cryptic or sibling species.