Aspects of the morphology of phyletically basal bovichtid fishes of the Antarctic suborder Notothenioidei (Perciformes)

In studying the soft tissue anatomy and histology of notothenioids, especially the bovichtids Bovichtus diacanthus and Cottoperca gobio, I evaluated the structure and phyletic distribution of two characters identified by Balushkin (2000) and a new ocular character complex first recognized here. Histology indicates that Balushkin’s antesupracleithral organ is the thymus, a lymphoid organ that involutes with age in notothenioids. Given the universal phyletic distribution of the thymus in gnathostomes, the variation introduced by ontogenetic regression, and its predilection to preservation artifacts, neither its presence nor the appearance of its epidermis are reliable systematic characters in notothenioids. Balushkin’s hypoglossal gland, found in Bovichtus and Cottoperca, is a projection of the mucosa of the oral cavity lateral to the tongue. Histology reveals that it is not a multicellular gland and that its composition does not differ from that of the oral musosa in general—stratified squamous epithelium containing unicellular mucous glands and a few taste buds. While an elaboration of a mucosal fold present in some other notothenioids, the hypoglossal gland is nevertheless sufficiently different and distinct in Bovichtus and Cottoperca that it is a valid synapomorphy for bovichtids. Study of ocular vascular morphology reveals that bovichtids, but not other notothenioids, have a persistent choroid fissure and a low falciform process with a “Dreiecke” (triangle of Virchow). A lentiform body is also present in these two genera but is seen in Pseudaphritis urvillii and Eleginops maclovinus as well. A choroid fissure, falciform process and lentiform body have not been previously noted in notothenioids. The ocular character complex reinforces the phyletically basal position of bovichtids since a choroid fissure and falciform process are widely distributed among perciform outgroups but lost in non-bovichtid notothenioids. Non-traditional morphological characters provide useful information, but preservation can be problematic and museum specimens may not suffice when the structures are truly soft tissue such as the thymus and interior of the eye.