14We report analyses of a dataset spanning 38 years of near-annual fishing for Antarctic toothfish 15 Dissostichus mawsoni, using a vertical setline through the fast ice of McMurdo Sound, 16

Nesting sites of the naked dragonfish Gymnodraco acuticeps have been identified in 15-35 m water under fast ice adjacent to McMurdo Station, making it possible to examine embryonic development and early larval growth. Egg-laying (predominantly in October) is preceded by a distinctive whirling behavioural pattern driven by the male prodding the side of the female's abdomen. The eggs (3.42 ± 0.19 mm in diameter) are laid on rocks as a single adherent layer (c. 2500 per patch). Development is unusually protracted, the first cleavage occurring after about 24 hr at about -1.9ºC. Hatching occurs about 10 months post-fertilization, beginning soon after the sun rises above the horizon. During this period one of the parents may act as a guard in an attempt to keep predators at bay. Upon hatching, the larvae (12.09 ± 0.36 mm long) swim towards the surface ice where they presumably seek refuge. Yolk absorption is complete in about 15 days. Larvae (grown in aquaria at a density of 0.7 larvae l -1 ) display an average daily growth rate of 0.42% over nine weeks. Hatching in aquaria can occur up to 100 days in advance of that seen in the field, suggesting that under natural conditions hatching may be delayed until an appropriate stimulus (such as the return of the sun) is received.

Antifreeze proteins (AFPs) of polar marine teleost fishes are widely recognized as an evolutionary innovation of vast adaptive value in that, by adsorbing to and inhibiting the growth of internalized environmental ice crystals, they prevent death by inoculative freezing. Paradoxically, systemic accumulation of AFP-stabilized ice could also be lethal. Whether or how fishes eliminate internal ice is unknown. To investigate if ice inside high-latitude Antarctic notothenioid fishes could melt seasonally, we measured its melting point and obtained a decadal temperature record from a shallow benthic fish habitat in McMurdo Sound, Antarctica. We found that AFP-stabilized ice resists melting at temperatures above the expected equilibrium freezing/melting point (eqFMP), both in vitro and in vivo. Superheated ice was directly observed in notothenioid serum samples and in solutions of purified AFPs, and ice was found to persist inside live fishes at temperatures more than 1°C above their eqFMP for at least 24 h, and at a lower temperature for at least several days. Field experiments confirmed that superheated ice occurs naturally inside wild fishes. Over the long-term record (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012), seawater temperature surpassed the fish eqFMP in most summers, but never exceeded the highest temperature at which ice persisted inside experimental fishes. Thus, because of the effects of AFP-induced melting inhibition, summer warming may not reliably eliminate internal ice. Our results expose a potentially antagonistic pleiotropic effect of AFPs: beneficial freezing avoidance is accompanied by melting inhibition that may contribute to lifelong accumulation of detrimental internal ice crystals.antagonistic pleiotropy | antifreeze glycoprotein | antifreeze potentiating protein | McMurdo Sound temperature | melting hysteresis

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Phylogenetically diverse polar and subpolar marine teleost fishes have evolved antifreeze proteins (AFPs) or antifreeze glycoproteins (AFGPs) to avoid inoculative freezing by internalized ice. For over three decades since the first fish antifreeze (AF) protein was discovered, many studies of teleost freezing avoidance showed hepatic AF synthesis and distribution within the circulation as pivotal in preventing the blood, and therefore the fish, from freezing. We have uncovered an important twist to this long-held paradigm: the complete absence of liver synthesis of AFGPs in any life stage of the Antarctic notothenioids, indicating that the liver plays no role in the freezing avoidance in these fishes. Instead, we found the exocrine pancreas to be the major site of AFGP synthesis and secretion in all life stages, and that pancreatic AFGPs enter the intestinal lumen via the pancreatic duct to prevent ingested ice from nucleating the hyposmotic intestinal fluids. AFGPs appear to remain undegraded in the intestinal milieu, and the composition and relative abundance of intestinal AFGP isoforms are nearly identical to serum AFGPs. Thus, the reabsorption of intact pancreas-derived intestinal AFGPs, and not the liver, is the likely source of circulatory AFGPs in notothenioid fishes. We examined diverse northern fish taxa and Antarctic eelpouts with hepatic synthesis of bloodborne AF and found that they also express secreted pancreatic AF of their respective types. The evolutionary convergence of this functional physiology underscores the hitherto largely unrecognized importance of intestinal freezing prevention in polar teleost freezing avoidance, especially in the chronically icy Antarctic waters.antifreeze glycoprotein-null liver ͉ antifreeze paradigm shift ͉ evolutionary adaptation ͉ intestinal freeze avoidance ͉ functional convergence