The Useful Results from 150 Years of Marine Biogeography
29
Ekman commented that the global distribution of benthos reflects the geological
history of the oceans more strongly than does the biogeography of plankton. The benthos
of the warm-water shelf in the western Atlantic has considerable affinities with that of
the eastern Pacific; although the Isthmus of Panama in our days is a barrier (or was until
the Canal was constructed) between Atlantic and Pacific, in the not-so-distant Pliocene
past there was a sea-level connection, testified to by the fact that many genera and species
of invertebrates and fish are common to both oceans, but occur nowhere else.
Nor can ecological consequences of events at even longer geological time scales, such as
the relatively late opening of the Atlantic itself, be ignored: the entire Atlantic warm-water
fauna of benthos and demersal fish is significantly less diverse than in the Indo-Pacific.
Consider the tropical coral reefs of the Atlantic and of the Indo-West Pacific, as discussed
by Wells (1957): although some of the dominant coral species are identical in the two
oceans, the Atlantic reefs are less diverse and comprise only 35 species of 26 coral genera.
The Indo-West Pacific supports 700 species of 80 genera. Acropora and Porites each
comprise only 3 Atlantic species, but include 150 and 30, respectively, in the Indo-West
Pacific. Important structural differences in the coral communities occur so that, for
instance, there are relatively very few Atlantic calcareous algae or coral gall crabs, while
there is a relative lack of alcyonarians on Pacific reefs.
More generally, regional characteristics of the benthic fauna are very largely dictated
by the nature of the sediments: the animals living in muddy sediments must differ from
those of, say, a shelly-sand ground. This was the basis of C. J. Petersen’s dictum in his
classical 1918 paper, that it is the nature of the physical biotope that determines the
nature of the benthic biocoenosis: what follows from this is of much greater importance
to ecological geography, and to the task at hand, than is the biogeography of Ekman and
later students.
The importance of Petersen’s contribution was that it led him to propose that
structurally similar and recognizable assemblages of benthic macroorganisms—mollusks,
echinoderms, polychaetes, and so on—were associated with each characteristic type of
sediment on continental shelves around the world at all latitudes. This concept has
startling implications, not often recognized; it means that organic material originating in
phytoplankton carbon fixation or in terrestrial runoff is processed in the same manner, by
similar benthic organisms inhabiting each characteristic sediment type, no matter where
these occur. The simple observation that similar genera occur in the benthic communities
of both polar and tropical seas has yet to be properly assimilated by marine ecologists.
Only recently I read (in a paper that I shall not cite) that the benthos of the Gulf of
Guinea is not “truly tropical” because many “nontropical” genera occur there.
Petersen’s synthesis, of course, led to bitter arguments concerning the nature of these
communities, for it was pointed out—quite correctly—that it had yet to be demonstrated
that they react collectively to changes in the environment. His approach to benthic
community ecology has also been heavily criticized on statistical grounds, and because
interaction has not been demonstrated to be essential to the formation of each species
association: but, as Mills pointed out in 1989, this has been a sterile and emotionally
charged debate.
Fortunately, Gunnar Thorson and others continued to exploit the concept, and out of
their geographically more extensive work was erected the very useful structure of parallel
benthic isocommunities occurring on all continental shelves. This global classification
of benthic communities is frank natural history, but no worse for that, and it works
observationally. I shall very frequently have recourse to it in my descriptions of regional
benthic ecology: Thorson’s chapter in Joel Hedgepeth’s 1957 Marine Ecology is not the
only gem in that volume, but it is the one to which I refer back most frequently.
These isocommunities form a very useful preliminary description of the benthic community found in sandy, muddy, and sandy-mud deposits. Thorson’s system of about a
29
Ekman commented that the global distribution of benthos reflects the geological
history of the oceans more strongly than does the biogeography of plankton. The benthos
of the warm-water shelf in the western Atlantic has considerable affinities with that of
the eastern Pacific; although the Isthmus of Panama in our days is a barrier (or was until
the Canal was constructed) between Atlantic and Pacific, in the not-so-distant Pliocene
past there was a sea-level connection, testified to by the fact that many genera and species
of invertebrates and fish are common to both oceans, but occur nowhere else.
Nor can ecological consequences of events at even longer geological time scales, such as
the relatively late opening of the Atlantic itself, be ignored: the entire Atlantic warm-water
fauna of benthos and demersal fish is significantly less diverse than in the Indo-Pacific.
Consider the tropical coral reefs of the Atlantic and of the Indo-West Pacific, as discussed
by Wells (1957): although some of the dominant coral species are identical in the two
oceans, the Atlantic reefs are less diverse and comprise only 35 species of 26 coral genera.
The Indo-West Pacific supports 700 species of 80 genera. Acropora and Porites each
comprise only 3 Atlantic species, but include 150 and 30, respectively, in the Indo-West
Pacific. Important structural differences in the coral communities occur so that, for
instance, there are relatively very few Atlantic calcareous algae or coral gall crabs, while
there is a relative lack of alcyonarians on Pacific reefs.
More generally, regional characteristics of the benthic fauna are very largely dictated
by the nature of the sediments: the animals living in muddy sediments must differ from
those of, say, a shelly-sand ground. This was the basis of C. J. Petersen’s dictum in his
classical 1918 paper, that it is the nature of the physical biotope that determines the
nature of the benthic biocoenosis: what follows from this is of much greater importance
to ecological geography, and to the task at hand, than is the biogeography of Ekman and
later students.
The importance of Petersen’s contribution was that it led him to propose that
structurally similar and recognizable assemblages of benthic macroorganisms—mollusks,
echinoderms, polychaetes, and so on—were associated with each characteristic type of
sediment on continental shelves around the world at all latitudes. This concept has
startling implications, not often recognized; it means that organic material originating in
phytoplankton carbon fixation or in terrestrial runoff is processed in the same manner, by
similar benthic organisms inhabiting each characteristic sediment type, no matter where
these occur. The simple observation that similar genera occur in the benthic communities
of both polar and tropical seas has yet to be properly assimilated by marine ecologists.
Only recently I read (in a paper that I shall not cite) that the benthos of the Gulf of
Guinea is not “truly tropical” because many “nontropical” genera occur there.
Petersen’s synthesis, of course, led to bitter arguments concerning the nature of these
communities, for it was pointed out—quite correctly—that it had yet to be demonstrated
that they react collectively to changes in the environment. His approach to benthic
community ecology has also been heavily criticized on statistical grounds, and because
interaction has not been demonstrated to be essential to the formation of each species
association: but, as Mills pointed out in 1989, this has been a sterile and emotionally
charged debate.
Fortunately, Gunnar Thorson and others continued to exploit the concept, and out of
their geographically more extensive work was erected the very useful structure of parallel
benthic isocommunities occurring on all continental shelves. This global classification
of benthic communities is frank natural history, but no worse for that, and it works
observationally. I shall very frequently have recourse to it in my descriptions of regional
benthic ecology: Thorson’s chapter in Joel Hedgepeth’s 1957 Marine Ecology is not the
only gem in that volume, but it is the one to which I refer back most frequently.
These isocommunities form a very useful preliminary description of the benthic community found in sandy, muddy, and sandy-mud deposits. Thorson’s system of about a