Growth and ecophysiology of two Antarctic benthic predators; Isotealia antarctica and Urticinopsis antarctica

There is a dearth of basic life history and physiological data from Southern Ocean species, particularly from benthic vagile predators. This is an important data gap because species inhabiting the Southern Ocean live in a more temperature stable but seasonally varying environment than temperate and tropical counterparts. For many species living below 0 °C for a significant proportion of the year, bodily functions are slowed to disproportionately lower rates than would be predicted by temperature alone. Certain life history and physiological processes are often aligned with the short summer season of productivity. However, predators may behave differently because they are decoupled from the phytoplankton bloom and some have been shown to exhibit less seasonal physiological change. To further our understanding of Antarctic predator growth and seasonal ecophysiology, field growth rates were measured for two soft-bodied Antarctic anemone benthic predators, Isotealia antarctica and Urticinopsis antarctica, using in situ sampling of anemones on uniquely marked tiles. Ex situ measurements of oxygen consumption and seven-day faecal output were obtained from recently collected specimens in aquaria and compared between summer and winter. Winter physiological data for Antarctic species are rare, and we tested the hypothesis that generalist feeders or predators continue to feed during the winter. Growth rates differed between species and between years. I. antarctica and U. antarctica both exhibited overall positive field growth rates across a 15 month period between 2020 and 2021; with U. antarctica increasing 199.80% (± SE 25.8) in mass compared to a 16.85% (± SE 8.9) increase in I. antarctica. There was no significant difference in I. antarctica's growth between 15 and 25 months field deployment. After 25 months, I. antarctica showed an average 7.96% (± SE 8.05) increase in buoyant weight. Ex situ oxygen consumption and faecal egestion did not differ seasonally, which, demonstrates that anemones fed at similar rates during the winter and summer. In contrast to some members of the Antarctica benthos, I. antarctica and U. antarctica actively feed all year round, whereas several other species have been reported to enter a state of torpor in winter.