Biogeography of Fishes

Introduction

Fishes live in virtually every watery habitat found on earth. The world’s deepest living fish (Abyssobrotula galatheae) was found in the Puerto Rican Trench at a depth of 8372 meters while the Tibetan stoneloach (Triplophysa stoliczkai) lives at altitudes over 5200 meters in the Himalaya. The habitats of fishes vary greatly not only in physical features, such as pH, salinity, temperature, oxygen content, and light level, but also differ immensely in the space available. Some fishes have extraordinarily (and dangerously) restricted distributions, such as the relict populations of 15–20 species of desert pupfishes (Cyprinodon spp.) endemic to isolated small spring systems in the desert regions of southwest USA and Mexico (Echelle and Echelle, 1993; some species can withstand temperatures up to 44.6°C or salinities greater than 100 ppt), or the equally isolated populations of cave fishes, such as Lucifuga in Cuba and the Bahamas.

In the marine environment, living spaces are usually greater, although, even in the marine environment, some fish may be restricted to the reefs around single atolls, or, like the bythitid vent fish (Thermichthys hollisi), to thermal vents such as those of the Galapagos rift at 2400 m (Cohen et al., 1990; Nielsen and Cohen, 2005). By contrast, other marine fishes, such as the pelagic blue shark (Prionace), range over all the oceans, while the minnow-sized bathypelagic stomiatoids, Cyclothone microdon and C. acclinidens, are found worldwide from 100 m to below 2000 m, and must comprise many billions of individuals.

Biogeography

Biogeography (or when restricted to animals, zoogeography) is the study of the distributions of organisms in space and time. Although not the first biogeographer, Charles Darwin was one of the first whose ideas on biogeography were widely disseminated. While some of his conclusions, such as that continental land masses are barriers to the distribution of many species of shallow-water marine fishes, may seem obvious, others, such as that large expanses of open ocean can also be an effective barrier, may have seemed less obvious to readers in his time.

Early biogeographers believed that patterns of distribution result primarily from the dispersal of organisms from “centers of origin” across barriers. Such dispersals often required the existence of connections (land bridges, which might contain freshwater lakes and streams) or mechanisms (fish eggs adhering to the feet of birds) that are highly improbable or scientifically unsupported.

However, in the 1960s, as evidence of sea floor spreading and altered continental arrangements began to accumulate, a new mechanism for explaining biogeographic patterns was proposed. Under the vicariance hypothesis, it’s the land masses, not the organisms that have changed and modern patterns are simply the result of the distribution patterns of species, or their ancestral forms. Organisms may disperse within a range, but the subsequent creation of barriers (vicariant events) divides the populations and permits the development of new and different species on either side of the barrier. As with many opposing theories, each school of thought had its adherents and opponents and many refused to accept the possibility that biogeographic patterns could result from the alternative mechanism. There is no good reason to assume the two theories are mutually exclusive, and, as with most such dichotomies, both mechanisms often need to be evoked to explain fully the distributions of modern groups of fishes. Recent molecular studies on the genetics of fish populations have shown that both mechanisms are required to explain adequately the distributions of Atlantic needlefish (garfish) species, or populations of bonito (Sarda), and species of the family Sparidae in the eastern Atlantic and Mediterranean.

The patterns of distribution of fishes across the 4000–7000 km expanse of deep water separating the eastern Pacific from the central Pacific, which many biogeographers from Darwin to the present have described as “impassable” have also recently been reexamined using mtDNA with the result that we now know that, although infrequent, there is continued genetic interchange between some species across even this vast expanse (Lessios et al., 1998, 2006).