particularly from mesopelagic and deeper levels, while others, including euphausiids
and chaetognaths, appear to be almost completely catalogued and named. The total for
all mesozooplankton is <5000, and microplanktonic heterotrophs are not notably
diverse. We have done a great deal of sampling and describing of species, and while
there may yet be much lumping and splitting and some completely new finds, for the
upper ocean we know most of the species. The Census of Marine Life program of
recent years is adding somewhat to the species counts of deep-sea groups, but it is too
soon to say what the new totals will be. There are only about 2000 species of all
pelagic vertebrates (fish, mammals, etc.) and other nekton (squid, large shrimp, etc.).
There are certainly fewer than 6000 described species of phytoplankton. Thus, in
pelagic habitats there are only a few more than 10^4 specifically distinct organisms
(apart from microbes, for which species definitions are different and a count would
still be premature). The numbers for the land are surely greater than 2 × 10^6 ; there are
∼300,000 species of beetles alone, and described vascular plants exceed 300,000.
Recently, claims have been made that there may be >10^7 species. While we are
frequently regaled with the notion of the marvelous diversity of marine life, and it is
diverse at high systematic levels (phyla, classes), there are actually relatively few
kinds of specifically distinct organisms, especially in pelagic habitats. Keep that in
mind and wonder why.
Global Patterns
(^) We will examine patterns for epipelagic zooplankton because they are the best studied
and good maps are available. All other pelagic groups, from phytoplankton to
cetaceans, exhibit very similar patterns. A few species, like the blue whale, are more
wide-ranging than the zooplankton patterns, migrating seasonally between feeding
areas in antarctic or subarctic waters to calving and mating areas in the tropics.
Mesopelagic patterns remain obscure, although few species are cosmopolitan despite
similar deep-water conditions globally. Most of the zooplankton samples were
collected by oblique tows with ring-nets from about 300 m to the surface and filtering
500 to 1000 m^3 . Examples of most of the basic distribution types are provided by
Bruce Frost’s (1969) worldwide study of Clausocalanus, a temperate–tropical genus
of small, epipelagic copepods. The 13 species of this genus fall into three
morphologically defined groups. Frost analyzed over 800 globally distributed samples
to determine distributions. His results are presented in seven maps (Figs. 10.1 &
10.2).
Fig. 10.1 Distribution of plankton net tows used by Frost (1969) in determining the
distributions of the 13 species of the copepod genus Clausocalanus.
(^) (Kindly provided by B.W. Frost.)