The Antarctic Toothfish – life and adaptations of one of the most chilled fish in the sea
Given that temperatures in the antarctic can drop to below -30°C , it is not easy to comprehend the number of marine mammals and birds that have colonised the environment. However wrapped in layers of feathers and blubber, these organisms have some protection from the extremities aiding their evolutionary success. Unfortunately this cannot be said for the Antarctic Toothfish (Dissostichus mawsoni), which has to rely on a number of molecular adaptations in order to survive and thrive in the sub-zero waters.
D. mawsoni pictured to the right is a species completely endemic to the Antarctic sea where it has an uninterrupted circumpolar distribution stretching north to around 60°S. This species is commonly found in the benthic community between depths of 279-2,210 m, but has been known to venture up into the pelagic environment of shallower depths. It is a dioecious species, spawning every 1 to 2 years, with sexual maturity reached at an average age of 16.6 years and a length of 133.2 cm. Details of the reproductive biology are limited, however spawning is believed to take place between June-November each year, with individuals releasing a minimum of 500,000 eggs. These eggs then hatch into pelagic larvae where they develop into pelagic juveniles, remaining in the water column for up to 2 years after they’ve hatched before settling out on the sea floor to be part of the benthic community.
Challenges Faced in the Antarctic Marine Environment:
Waters of the antarctic are warmer than the terrestrial environment at around -2°C. However this still causes problems for species inhabiting these waters as most cellular activity is temperature dependent, with rates of reaction decreasing rapidly as surrounding temperatures decrease. Therefore species have to be adapted to fueling metabolic reactions and cellular processes with the lower amounts of thermal energy available. In sub-zero conditions species face the risk of ice forming in their bodily fluids, freezing them from the inside out; a problem not aided by the ectothermy of most teleost fish. Freezing temperatures don’t only affect the internal systems of antarctic species; they also impact the eyes causing cataracts. If the impacts of freezing temperature weren’t detrimental enough, D. mawsoni also faces death by predation from species such as Sperm whales, Killer whales, and Weddell seals, most likely as a result of its casual ventures away from the benthic and into the pelagic environment. To deal with these stressors, D. mawsoni has developed a number of adaptations.
Membrane Fluidity and Rates of Reaction:
It is vital for all species that cell membranes maintain fluidity in order to allow easy transport of ions and proteins in order to ensure success of metabolic reactions. For most mesophilic organisms low temperatures inhibit cell function by causing rigidity of the cell membranes, lowering the rate of cell transport and potentially ceasing it altogether. Thankfully for the Antarctic Toothfish, it is a psychrophilic organism. This means it has evolved cells membranes designed to function under low temperatures. This species in particular has specific amino acids, which increase overall cell membrane fluidity at low temperatures, allowing for successful cross-membrane transport. In addition to this, the species has an array of cold-active enzymes working to lower the activation energy needed for different reactions allowing them to take place in the freezing environment.
Internal ice forms when external temperatures drop to below freezing in the form of crystals rendering cells useless as they shrink and dehydrate. D. mawsoni is adapted to deal with this by having blood with a freezing point of -2.1°C, which is 0.2°C lower than that of seawater at -1.9°C. Its blood is full of NaCl, which lowers the freezing point of a liquid, along with Antifreeze Glycoproteins (AFGPs). AFGPs aren’t just restricted to the blood of an organism, they can be found across all bodily fluids where they prevent ice crystal formation. They work non-colligatively by adsorbing to the surface of newly formed ice crystals via hydrogen bonding, preventing them from further growth as can be seen in the opposing image. Attachment to the crystals forces them to change their shape resulting in ice crystals needing a lower temperature in order to continue with their growth. AFGP’s in D. mawsoni are made up of 8 individual glycopeptides (GP) each named according to their size; GP 1 being the largest and GP 8 being the smallest. Effectiveness of these molecules is size dependent with the larger molecules (ie – GP 1) being the most effective and the smaller molecules (ie – GP 8) being the least effective.
The eyes of tropical fish species will form cataracts impairing their vision from temperatures 7°C and below. D. mawsoni however is able to live in temperatures down to -12°C without cataract formation. The physical structure of the eyes and retina is identical between D. mawsoni and tropical species. The eye-saving adaptation lies within the proteins that make up the cornea; in particular Crystallin proteins. Crystallin proteins can be divided into 3 groups; γ crystallins, α /βH crystallins, and β crystallins. A study by Kiss et al (2004) compared the structure and thermal stability of these proteins from D. mawsoni with those from a tuna and a cow, and found that it was the γ crystallins of the toothfish that grant its eyes protection from cold-induced cataracts.
Over the years commercial importance of D. mawsoni has increased due to the decrease in cod population, with landings in the Antarctic exceeding 3,000 tonnes per year and some stocks having completely collapsed. Given that the average age needed to reach maturity is 16.6 years, and spawning only occurs once per year this species is highly vulnerable to over-exploitation. Much research has been undertaken to study the cold-adaptations of this species in terms of its AFGPs, but little has been undertaken to understand the full life history of the organism. Therefore remaining stocks of this species cannot be accurately assessed to determine levels of sustainable landings which may result in the complete collapse of this population. This could have drastic impacts on the species reliant on these fish as a food source, potentially resulting in trophic cascades. As stated in the above video, currently the Conservation of Antarctic Marine Living Resources (CCAMLR) regulates the areas D. mawsoni can be fished from in the Antarctic, but there needs to be further measures implemented and more research undertaken to protect this species.