The Axes of Divergence for the Evolutionary Radiation of Notothenioid Fishes in Antarctica

Abstract

Notothenioid fishes, a perciform group, radiated in the cold shelf waters around the Antarctic continent and the 110 species dominate fish diversity, abundance, and biomass at levels of ≈77%, 92%, and 91%, respectively. This occurred in a locality with frequent glaciomarine cycles that fragmented and obliterated habitats, disrupted ecosystems, and made parts of the high latitude shelves periodically uninhabitable. The notothenioid radiation encompasses three stages and 10 axes: for the habitat stage, divergence in (1) depth and (2) biotope, meaning subdivisions within the pelagic and benthic realms; for the morphology stage, divergence in (3) body size, (4) body density based on proportions of skeletal and adipose tissues, (5) body shape, (6) trophic morphology, specifically head morphology related to feeding (jaws, teeth, head size, and pharyngeal gape), and (7) neuromorphology (brain and sensory systems); and for the communication stage, divergence in (8) fecundity and egg size, (9) sexual dichromatism and dimorphism in body and fins, and (10) parental care behavior associated with nesting. There was an antecedent Eocene fossil fauna that did not include notothenioids and that has minimal taxonomic representation in the modern fauna. Liparids (snailfishes) and zoarcids (eelpouts) also diversified in Antarctic waters but with minimal divergence. Hypotheses are offered as to why these radiations played out the way they did and why notothenioids are dominant.