Morphological differences in Lissarca notorcadensis Melvill and Standen, 1907 from the Scotia, Weddell and Ross Seas

The isolated Southern Ocean has given rise to specially adapted, endemic species. The uniformity of physical conditions within the Southern Ocean south of the Polar Front is thought to limit latitudinal differences in shell size and growth of these species. The small bivalve Lissarca notorcadensis is one of the most successful colonisers with a wide circum-Antarctic distribution. We collected specimens from three areas of the Southern Ocean: the Scotia arc, the Weddell Sea and the Ross Sea and examined their shell morphometrics. The regression lines of the shell length/height relationships were similar. However, whilst specimens from the Weddell and Ross Sea were almost identical, the Scotia arc specimens had consistently shorter shell heights at similar shell length. The average body size (shell length/height) per sample location plotted against latitude showed a decrease over the latitudinal range between 53°′S and 73°S. The differences observed between the three geographically separate areas, concurrent with this latitudinal cline, were examined further using ANOVA. Significant differences were found between populations of the Scotia arc and the Weddell Sea as well as between populations of the Scotia arc and the Ross Sea, but there were no significant differences between the populations of the Weddell and Ross Seas. The Scotia arc further divided into two significantly different groups, the Shag Rocks populations and the populations from islands of the southern arc.

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