Fishbase 2017: Joseph T Eastman
Channel: Fishbase Sweden
Date: 2017-10-30

Joseph T. Eastman, Ohio University, USA, gave a lecture titled "THE NATURE OF ANTARCTIC FISH DIVERSITY"

Abstract: The interaction between geological and biological processes has resulted in much of what we recognize as the modern fauna of the world. This interplay has frequently given rise to distinctive faunas in areas that became isolated during geologic history. Antarctica is such a place. Formerly part of Gondwana, Antarctica is now surrounded by the cold Southern Ocean, and its distinctive fauna inhabits the sea rather than the ice-covered landmass. 

This talk will utilize notothenioid fishes, and their antecedents — the Eocene La Meseta fossil fishes from Seymour Island—as an example of faunal change, ecological opportunity and radiation over the past 50 million years. During this time Antarctica waters cooled, the ice sheet developed, inshore habitats were reduced, and food webs were disrupted thereby creating an opportunity for faunal turnover. The outcome was that a taxonomically diverse and cosmopolitan Eocene fauna was succeeded by radiations of notothenioids that yielded a taxonomically restricted, highly endemic group of modern species that dominates abundance and biomass on the high latitude shelves. 

The radiation of notothenioids includes both adaptive and non-adaptive components. The greatest morphological and ecological disparity, involving just a few species, appeared early in the radiation and was uncoupled from the 61% of species diversity produced by three subsequent pulses separated by millions of years. The axes of the radiation were buoyancy (although notothenioids lack a swim bladder), habitat depth and body size. The 2-meter long, intermittently buoyant piscivore (_Dissostichus mawsoni_) and the 25-cm long neutrally buoyant zooplanktivore (_Pleuragramma antarctica_) exemplify the adaptive 
component, while the 27 similar species of _Pogonophryne_ are a non-adaptive element. I will discuss the different buoyancy mechanisms in _D. mawsoni_ and _Pleuragramma_. When _D. mawsoni_ encounters oligotrophic conditions during its life cycle, its lipid-based neutral buoyancy is relinquished for gonadal maturation. Both species have sufficient physiological and ecological plasticity to have persisted recurrent habitat instability caused by advances of the ice sheet on the shelf. 

Liparids (snailfishes) and zoarcids (eelpouts) also radiated in the Antarctic and, although speciose, they are not ecologically and morphologically diverse when compared to notothenioids. I will suggest possible explanations for this.