Crocodile Icefish, Channichtyidae, are highly specialised Perciform Fish found in the waters around Antarctica. They are sometimes known as White-blooded Icefish, as they lack haemoglobin; the cold waters around Antarctica being so saturated in oxygen that these Fish no not need a specialised means to transport it to their tissues (water is able to retain more free oxygen at lower temperatures). These Fish are known to nest in colonies, with a few tens of Fish typically building their nests together in a favoured spot.
In a paper published in the journal Current Biology on 13 January 2022, Autun Purser and Laura Hehemann of the Alfred Wegener Institute at the Helmholtz Centre for Polar and Marine Research, Lilian Boehringer, also of the Alfred Wegener Institute, and of Universität Bremen, Sandra Tippenhauer again of the Alfred Wegener Institute, Mia Wege of the Alfred Wegener Institute and the Mammal Research Institute at the University of Pretoria, Horst Bornemann, Santiago Pineda-Metz, Clara Flintrop, Florian Koch, and Hartmut Hellmer, again of the Alfred Wegener Institute, Patricia Burkhardt-Holm of the Programme Man-Society-Environment at the University of Basel, Markus Janout, again of the Alfred Wegener Institute, Ellen Werner of the HafenCity University Hamburg, Barbara Glemser of Universität Bremen and the Max Planck Institute for Marine Microbiology, Jenna Balaguer, also of the Alfred Wegener Institute, Andreas Rogge of the Alfred Wegener Institute and the Institute for Ecosystem Research at Kiel University, Moritz Holtappels, once again of the Alfred Wegener Institute, and Frank Wenzhoefer of the Alfred Wegener Institute, the Max Planck Institute for Marine Microbiology, and the Department of Biology at the University of Southern Denmark, describe a vast colony of Channichtyida Icefish numbering tens of thousands of nests, discovered on the eastern flank of the Filchner Trough within the Weddell Sea.
The colony is of Jonah's Icefish, Neopagetopsis ionah, a benthopelagic species (species that lives just above thes seafloor) known from the Weddell Sea, Kapp Norvegica, Halley Bay, Vahsel Bay, the Antarctic Peninsula, and the Ross Sea, with a pelagic juvenile stage (juvenile stage that lives in the water column), which has been reported from the Weddell Sea, South Shetland Islands, and McMurdo Sound. Purser et al. report a colony of about 16 160 Icefish covering an area of about 45 600 m², which was discovered by the Ocean Floor Observation and Bathymetry System, towed camera platform deployed by the RV Polarstern.
The deepest parts of the colony were at a depth of 535 m, the shallowest 420 m. Nests were of a fairly uniform size, about 75 cm wide and 15 cm deep, and were a minimum of 25 cm from their neighbours, even in the most densely populated parts of the colony. Of the total 16 160 nests directly imaged by camera, 12 020 (79%) were currently occupied (defined by the presence of either a Fish and eggs or just eggs). Another 15% of the nests were empty, 9% contained at least one dead Fish, and 2% contained Fish but not eggs.
The nests were bowl shaped and comprised a ring of stones, with a base of fine-grained material. The Icefish kept these areas free of any debris, as well as guarding the eggs against predators and fanning them to ensure a good supply of oxygen. The outer ring of stones may serve to prevent the eggs being blown away by this fanning action.
In addition to the occupied area, further, more widely spaced nests could be observed in the area around the colony, all empty and all less than 100 m deeper than the deepest colony nests or less than 100 m shallower than the shallowest colony nests, though the edge of the colony is quite abrupt in both directions, suggesting that the edge of the environmentally suitable zone was also abrupt, but had moved in the past, presumably in response to climate variability.
Throughout the period of the study the seafloor temperature remained between -1.0°C and 0°C. This is typical of the modified Warm Deep Water current, which flows upward onto the Weddell Shelf, through the Filchner Trough and other similar troughs. These waters have an oxygen saturation of 65-75%, which is lower than the surrounding waters, which have an average oxygen saturation of 80%, and a temperature of -2.0°C to -1.5°C, supporting the idea that the eggs need very specific environmental conditions to survive.
Chlorophyll a levels and primary production appeared to be higher above the colony area, and areas with unoccupied nests, than the surrounding waters, with the highest concentrations of particles around the modified Warm Deep Water current-High-Salinity Shelf Water interface. The majority of these particles were below 300 μm in equivalent spherical diameter, with the density of particles increasing at night, which is probably indicative of zooplankton migrating into the photic zone to feed at night. This would suggest a potential food source, both for the nesting Icefish and for their larvae, known to migrate into overlying waters following hatching.
The benthic invertebrate community around the colony was both low density and low diversity; dominated by Brittle Stars and Star Fish, and with some conspicuously large Pycnogonids ('Sea Spiders'), which were often in excess of 15 cm in diameter, and were often seen near eggs or egg husks outside of Fish nests, potentially washed out by currents or Fish movements. This is a distinctive fauna, and suggests that the Icefish are modifying the environment sufficiently to shape the local invertebrate community.
The carcasses of the Icefish appeared to provide an important food source for invertebrates. As many as four dead Fish were seen in a single nest, which since no more than two live Fish were ever seen in association with a single nest, suggest that the dead Fish, which are close to neutrally bouyant, can accumulate within nests. Brittle Stars, Starfish, Octopus, and various Fish species opportunistically feeding were observed around dead Icefish. It is likely that the breeding season takes a high toll on Icefish, as several months of tending eggs is apparently exhausting.
The area around the colony is also known to be home to a group of Weddell Seals, Leptonychotes weddelli, some of which have been tracked by satellite for long periods of time. These Seals are known to dive deeply looking for food, and to take Jonah's Icefish, suggesting that the colony may be serving as a regular food source for them. Other Seal species, such as Elephant Seals, Mirounga leonina, have also been seen in the area.
Abandoned nests outside the colony also appeared to play a significant ecological role, being colonised by sessile organisms, such as tube dwelling Polychaetes, colonial bryozoans, and Sponges. These sites also appeared to be hydrodynamically trapping phytodetritus, which would make them an excellent site for sessile organisms.
Because these observations were based upon a single observing season, a number of important questions about the breeding behaviour of the Jonah's Icefish remain unanswered. It is unclear how often the Fish build new nests; do they reuse them each year, or construct new ones? Do the Fish remain with the eggs from when they are laid till when they hatch, or do they leave to forage? How do the predators of both the Fish and their eggs behave around the colony? How do the hatchling Fish behave when they emerge? How do the Fish behave during the mating and spawning seasons? To what extent is the colony a food source for Weddell Seals, and how do they exploit it? In order to address these problems two LED light-and-camera systems were positioned 3 m above the seafloor and left collecting data, with the plan being to return and collect them in 2023 or 2024.
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