Antarctica can easily be characterized as the harshest and most punishing continent. The Southern Ocean encircles and envelops the fringes of the Antarctic continent. This too is an area of extremes. It is the windiest and waviest ocean on earth with the coldest temperatures exhibited by seawater (-1.9°C, equivalent to the freezing point) with more moderate temperatures (e.g., a balmy 1°C) found in lower latitude regions off the western shores of the Antarctic Peninsula.
The Southern Ocean is delineated by the Antarctic Polar Front, which lies between 50°S
and 60°S depending on season and ocean sector. The Antarctic Polar Front represents a
boundary in which cold Antarctic Surface Water meets and sinks beneath Subantarctic Surface water. It is a zone with steep physical and chemical gradients (e.g., temperature and salinity), along with changes in abundance and distribution of phytoplankton and zooplankton. Although only 10% of the world’s oceans
comprise the Southern Ocean, the Antarctic Circumpolar Current, which moves
more than 130 million cubic meters/second (by contrast all the world’s rivers
combined have a flow of about one million cubic meters/second), is the largest
volume current on the planet.
Despite shifting conditions on the surface of Antarctica, the pelagic and benthic environments of the Southern Ocean have been relatively stable for millions of years. Annual temperatures in the Southern Ocean range negligibly (e.g., -1.9°C to -1.7°C), or only by a few degrees depending on the locale, and temperatures throughout the water column are relatively constant. The Southern Ocean and many of its biota are effectively isolated from warmer waters found northward by circumpolar currents, and an abrupt temperature shift at the Antarctic Polar Front.
Underneath the surface of the Southern Ocean in the deep and dark waters of the continental shelf, and in the upper reaches of the continental slope of Antarctica, lives a magnificent fauna consisting of more than 320 known species of fishes (Eastman, 2005). The modern fish fauna of the Southern Ocean is dominated by the Notothenioidei, which comprise 50% and 90% of the Antarctic finfish diversity and biomass, respectively. The notothenioids represent a suborder of perch-like fishes (Order Perciformes), which are mostly benthic and highly endemic (97% endemism) to the Antarctic region. The Notothenioidei comprise eight families, which have radiated from an ancestral stock during the last 20 million years to fill a variety of niches. Among these are: the Bovichtidae (or thornfishes, most species of which actually reside north of the Antarctic Polar Front), the Nototheniidae (the most diverse and speciose family of the notothenioids), the Bathydraconidae (the dragonfishes, so named because of their especially long and narrow body forms), and finally the Channichthyidae (the icefishes, known for their colorless blood).
Antarctica has not always been cold. Prior to the separation of the Antarctic continent from the rest of Gondwana, the fishes associated with the continent during the early Eocene epoch were exceptionally diverse, and included many fish groups (e.g., chondrichthyan species such as sharks and rays), most of which are no longer represented in the modern fauna of the Southern Ocean (Eastman, 1993; Eastman, 2005). As Antarctica drifted southward 20-25 million years ago, and new patterns of oceanic circulation were established (e.g., the Antarctic Circumpolar Current and Antarctic Polar Front) (Eastman, 1993; Kennett, 1982; Near et al, 2004), a crash in species diversity occurred. Warm water refuges to the north were now no longer accessible to many species. Those fishes able to survive in cooling Antarctic waters gained a series of biological innovations that are essential for life in the cold. The large number of closely related, highly endemic species that have undergone rapid evolution in a discrete locale permits the characterization of the notothenioids as a rare, marine species flock. For more information on Antarctic fishes, see Dr. Joseph Eastman's website and his outstanding book Antarctic fish Biology: Evolution in a Unique Environment.