We addressed hypotheses that anguillid eels use mud as a substrate refuge only in the absence of substrate cavities, and that the winter distribution of eels in coastal bay and estuarine habitat is limited to waters warmer than the freezing point of fish tissue (~–0.7°C). In the seasonally ice-covered southern Gulf of St Lawrence, Canada, locations of summer fyke and winter spear fisheries indicate that American eels (Anguilla rostrata) are widely distributed in both summer and winter in shallow soft-bottomed bay and estuarine habitat. Captive eels in fresh water preferred mud substrates during summer, during pre-winter cooling and during post-winter warming periods, but in winter chose mud and cobble substrates at approximately equal frequencies. Plasma antifreeze was not detected in blood sampled from eels speared in mud under winter ice. Winter bottom water temperatures in an eel wintering site were below the approximate freezing point of fish tissue 29.9% of the time. Mud of eel wintering grounds is warmer than overlying water and appears to serve as a thermal sanctuary that allows eels to safely overwinter under ice-covered waters. American eels in the southern Gulf of St Lawrence spend ~67% of their annual cycle within the substrate.
During winter, the coastal waters of Newfoundland can be considered a 'freeze risk ecozone' for teleost fishes, where the shallower habitats pose a high (and the deeper habitats a low) risk of freezing. Atlantic (Anarhichas lupus) and spotted (Anarhichas minor) wolffish, which inhabit these waters, reside at opposite ends of this ecozone, with the Atlantic wolffish being the species facing the greatest risk, because of its shallower niche. In order to resist freezing, this species secretes five times the level of antifreeze protein (AFP) activity into the plasma than does the spotted wolffish. The main basis for this interspecific difference in AFP levels is gene dosage, as the Atlantic wolffish has approximately three times as many AFP gene copies as the spotted wolffish. In addition, AFP transcript levels in liver (the primary source of circulating AFPs) are several times higher in the Atlantic wolffish. One explanation for the difference in gene dosage and transcript levels is the presence of tandemly arrayed repeats in the latter, which make up two-thirds of its AFP gene pool. Such repeats are not present in the spotted wolffish. The available evidence indicates that the two species diverged from a common ancestor at a time when the ebb and flow of northern glaciations would have resulted in the emergence of shallow water 'freeze risk ecozones'. The results of this study suggest that the duplication ⁄ amplification of AFP genes in a subpopulation of ancestral wolffish would have facilitated the exploitation of this high-risk habitat, resulting in the divergence and evolution of modern-day Atlantic and spotted wolffish species.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.