Neuromorphological disparity in deep-living sister species of the Antarctic fish genus Trematomus

Abstract

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.

The Axes of Divergence for the Evolutionary Radiation of Notothenioid Fishes in Antarctica

“…The sister species T. lepidorhinus and T. loennbergii occupy the deep shelf-upper slope niche and exhibit differences in the size of brain regions, and divergence in all sensory systems. 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 that this species has a greater depth of peak abundance than T. lepidorhinus (Eastman and La Mesa, 2021) [60]. Pleuragramma antarcticum exemplifies an interesting evolutionary phenomenon at play in the neural divergence of «cryonotothenioids».…”

“…Trematomus has a circum-Antarctic distribution in both the seasonal and permanent pack ice zones. Most of the 15 species live at 400-700 m, although T. loennbergii (65-1243 m) and T. lepidorhinus (99-1658 m) have more extensive depth ranges and exhibit neuromorphological disparity (Eastman & La Mesa 2021) [60]. Depth ranges for artedidraconids, bathydraconids, and channichthyids also include one to a few deep-living (≥1500 m) genera in each family.…”

“…However, ecomorphological measurements and statistical analyses do reveal divergence of body and fin shapes and proportions associated with locomotion for foraging in the water column, even among closely related species that are demersal based on measurements of their %B. For example, Trematomus lepidorhinus (Figure 9f) is more streamlined and presumably more active than its sister species T. loennbergii (Klingenberg and Ekau, 1996) [67], and this is also reflected in their neuromorphology (Eastman and La Mesa, 2021) [60].…”

“…Detailed gross morphological and histological descriptions are available for the brains of 15 species representing six families, including all «cryonotothenioid» families with the exception of the Harpagiferidae. There is also information on sense organ morphology and intraocular vasculature for many additional species (Montgomery and Macdonald, 1987;Eastman, 1988b;Montgomery et al, 1988Montgomery et al, , 1994Montgomery, 1989, 1990;Janssen et al, 1993;Eastman and Lannoo, 1995, 2003a,b, 2004Janssen, 1996 [60,[231][232][233][234][235][236][237][238][239][240][241][242][243][244][245][246][247][248]. This section provides an overview of neuromorphological disparity.…”

The Arrangement of the Peripheral Olfactory System of Pleuragramma antarcticum: A Well-Exploited Small Sensor, an Aided Water Flow, and a Prominent Effort in Primary Signal Elaboration

“…Notothenioidei is the only fish sub-order that has adapted to the coldest ocean on the Earth: the Antarctic Ocean. The morphology of at least some parts of the olfactory system of notothenioids has been investigated over the years [ 16 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. The Antarctic silverfish Pleuragramma antarcticum is the dominant pelagic fish in coastal Antarctica and a keystone species in local ecosystems.…”