Adaptive radiation at a low taxonomic level: ­divergence in buoyancy of the ecologically similar Antarctic fish Notothenia coriiceps and N. rossii 

Abstract

We studied the buoyancy of 263 specimens of the sympatric sister species Notothenia coriiceps and N. rossii captured at King George Island. In these species without a swim bladder, we expressed measurements of buoyancy as percentage buoyancy (%B = weight in water/weight in air × 100), with smaller numbers reflecting more buoyant (i.e. less dense) species. Mean %B of N. coriiceps (4.34%) was significantly greater than that of N. rossii (3.82%), reflecting the lower density and more active swimming, migratory, and feeding behavior of N. rossii compared to N. coriiceps. Skeletal weight as a percentage of body weight was also significantly greater in N. coriiceps (2.46%) than N. rossii (1.65%). Paradoxically, the weight of the vertebral column as a percentage of skeletal weight was significantly greater in N. rossii (28.01%) than in N. coriiceps (24.29%); however, this is intelligible in light of the bone anatomy and histology. Our sample encompassed a wide size and age range, and trend lines for scatter plots of %B against standard length were curvilinear for both species, peaking at 140 to 160 mm, a phenomenon not previously seen in studies of notothenioid buoyancy. We conclude that the ontogenetic increase in %B (reduced buoyancy) in fingerlings/young juveniles and the ontogenetic decrease in %B (increased buoyancy) in older juveniles/immature adults reflects disproportionate growth of skeletal and visceral tissues, respectively, and is not causally related to concurrent pelagic-demersal habitat shifts. KEY WORDS: Notothenioid · Ontogenetic variation · Skeletal anatomy and histology · South Shetland IslandsResale or republication not permitted without written consent of the publisher

Mitochondrial Function in Antarctic Nototheniids with ND6 Translocation

“…N. coriiceps is a heavy rugged fish, and field observations with underwater cameras showed that it is a rather inactive sit-and-wait predator [55]. In comparison to N. rossii , N. coriiceps has a greater weight per unit length that is associated with a thicker body; skeletal weight as a percentage of body weight was also significantly greater in N. coriiceps (2.46%) than N. rossii (1.65%; t  = 5.611, P <0.03) [73]. The more active N. rossii is more gracile with a laterally compressed and streamlined body.…”

“…3.1), relate to the morphological and physiological differences consistent with the differential adaptations to inhabit the water column of the benthic N. coriiceps and the bentho-pelagic N. rossii . This becomes evident by a greater density, expressed as mean percentage buoyancy of N. coriiceps (4.34%) over N. rossii (3.82%) [73]. N. coriiceps is a heavy rugged fish, and field observations with underwater cameras showed that it is a rather inactive sit-and-wait predator [55].…”

“…The more active N. rossii is more gracile with a laterally compressed and streamlined body. Its vertebrae morphology consists of bone that is more spongy or porous and of neural arches thinner than those of N. coriiceps [73] . N. rossii occurs in Potter Cove from 0 to 6–7 years exclusively in the juvenile stage [74] , [75] , after which they migrate offshore to join and spawn with the adult population [76] .…”

Impact of Climate Change on Fishes in Complex Antarctic Ecosystems

“…However, the success of notothenioid species in the South Polar Sea ecosystem is not only based on physiological adaptations, but also on morphological modifications related to buoyancy (Eastman, 2005;Eastman et al, 2011). Notothenioids lack a swim bladder and, without this organ, the exploitation of bentho-pelagic or pelagic food sources is extremely energy consuming.…”

“…To compensate for the lack of a swim bladder, some notothenioid species developed modifications in body structure, which allow them to inhabit and to exploit the pelagic realm without an energetic disadvantage. In these species, mineralization of skeleton and scales is reduced, and the skeleton contains a high proportion of cartilage, which is less dense than bone (DeVries and Eastman, 1978;DeVries, 1981, 1982;Eastman et al, 2011). P. antarcticum has a persistent notochord and large amounts of lipids (accounting for $39% of muscle dry mass) are stored in subcutaneous and intramuscular lipid sacs which provide static lift (DeVries and Eastman, 1978).…”