animals with the habitat. It remains intriguing, and persists in being hard to
understand fully, that deep-sea sediments sustain hundreds of species in modest areas
with little apparent diversity in the habitat.
Benthic Biogeography
(^) This topic is of obvious importance, although its study is hampered by the isolation of
the seafloor beneath a thick mantle of water. Moreover, distributions are not
necessarily moved about by currents to provide genetic exchange across whole gyres
as is the case with epipelagic plankton. Thus, a sufficient spatial density of samples to
reveal patterns securely is difficult both to acquire and to work up in good systematic
detail. Russian workers have given oceanic, abyssal distributions the most attention,
having developed an extensive data-set in their worldwide cruise work of the 1950s to
1980s. Vinogradova (1997) has summarized the results. In agreement with
impressions from the early global expeditions (named after the ships: Challenger,
Valdivia, Albatross, Galathea, ...), the Russian results showed very widespread, in
many cases cosmopolitan distributions of macrofaunal genera and all higher taxa. Of
course, genera are subjectively delimited by taxonomists, but at least they are in
nearly all cases closely related species. On the other hand, species distributions are
much more restricted, with 85% of >1000 species in Vinogradova’s analysis occurring
in one ocean only, and only 4% were cosmopolitan. Most of the latter have very wide
depth ranges, and presumably can extend their distributions across ridges. Of course,
these proportions, while typical, vary among taxa. Thus, species endemism is high,
which corresponds to individual movement being mostly short range and to isolation
occurring readily between basins separated by ridges. Many regional studies agree.
For example, Menzies (1965) found that only 22 of 158 isopod species in 22 genera
collected from the Argentine and Cape basins of the SW Atlantic are present in both.
In contrast, all of those 22 genera are also found in the Pacific. Vinogradova found
that Pacific and North Indian faunas are more closely related at the generic and higher
levels than to either Atlantic or Antarctic faunas. Both the Pacific and Atlantic faunas
have distinctive eastern, western, and northern subdivisions. In the Atlantic, the
division line follows the Mid-Atlantic Ridge. The Antarctic fauna is partly distinct
between the Atlantic, Pacific, and Indian sectors. Trenches showed very high levels of
endemism, half or more of the species in a given trench are found only there.
(^) For abyssal sea bottoms deeper than 3000 m, Vinogradova (1997; basic pattern
originally published in 1959) proposed that patterns of faunal similarity (shared
species, shared proportions of genera, consideration of many taxonomic groups)
separate the world seafloor into three main regions (Fig. 13.21): (1) Pacific and North
Indian; (2) Atlantic; and (3) Southern Ocean out to the subtropical convergence.