LAUSR

Because sister species share a phenotypic axis, they hold morphological and ecological traits in common and, when sympatric, are ideal subjects for examining the extent of morphological divergence associated with the non-shared ecological parameters of the niche. Trematomus lepidorhinus and T. loennbergii have overlapping depth ranges of > 1000 m and occupy the deep shelf-upper slope niche in the waters of East Antarctica, an area where glacial troughs and their 1000–1500 m-deep landward basins (innershelf depressions) encompass 40% of the sea floor. The sense organ and brain divergence between the two species at the gross anatomical and histological levels involves differences in: number of olfactory lamellae, composition of the photoreceptor array and the number of retinal cells, size of the cephalic lateral line pores, and overall size of the brain and degree of development of different brain regions. Size and shape of the sagittal otoliths also differ. The morphology is unequivocally differentiating for habitat depths and conclusively documents a shift toward reliance on non-visual senses in T. loennbergii, consistent with collection data indicating this species lives at greater depths than T. lepidorhinus. Depths of peak abundances—200–500 m for T. lepidorhinus and 600–800 m for T. loennbergii—are the major differentiating parameter, with the ≈300 m differential between the peaks providing considerable, although not absolute, habitat separation. This is also reflected in the greater absolute abundances of the shallower-living T. lepidorhinus in trawl catches. Only T. loennbergii is found near the floors of 1000–1500 m-deep innershelf depressions, a distinctive microhabitat on the high latitude shelf.