30
Chapter 2: Biogeographic Partition of the Ocean
dozen series of parallel communities has stood the test of time (Rosenberg, 2001): these
are grouped into Macoma communities on inshore soft, muddy deposits; Amphiura,
Amphipoda, or Amphioplus communities in slightly deeper muddy sands; Tellina communities in clean sands in shallow water; and Venus communities in shelly sand deeper
on the continental shelf. Each, you will note, is characterized by one or several dominant
genera and each forms the dominant species association in deposits of a certain geological
grade and organic content.
Thorson recognized Venus isocommunities in the White Sea and Scoresby Sound,
from the Baltic to the Adriatic, in the Persian Gulf and off Madras, and also off New
England. I found that clean shell-sand grounds on the continental shelf off Sierra Leone
8N in the Gulf of Guinea were occupied by a species grouping that closely matched
those that had been described elsewhere as Venus isocommunities. During a multistation
benthos survey of this community a total of 47 bivalve mollusks were taken, while a Venus
community sampled with similar equipment in the Celtic Sea comprised 57 macrobenthic
species, many congeneric with those I took off Sierra Leone (Longhurst, 1958; Warwick
et al., 1978). In fact, the Sierra Leone genera would be familiar to any North Sea biologist:
Abra, Cardium, Nucula, Venus, Macoma, Mactra, Spisula, Tellina, Pecten, Ensis, and so on.
Macoma isocommunities were recognized by Thorson on muddy, estuarine-influenced
deposits in the White Sea, off East Greenland, along the Danish Baltic coast, off
Vancouver-Seattle, on the Japanese coast, and in the English Channel. One was subsequently located in the Bristol Channel and I found yet another example off Sierra Leone.
These observations were later supported by a formal study of k-dominance in three
counterpart benthic communities on the continental shelves of northern Java and western
Britain (Warwick and Ruswahyuni, 1987). Diversity indices were computed to represent benthic communities occupying similar habitat and composed of similar (or even
the same) genera in the two regions. Off Java, three isocommunities were identified
on appropriate deposits: an Amphioplus/Lovenia, a Tellinoides, and a Lacionella community. These parallel, respectively, European Amphiura/Brissopsis, a Tellina, and a Macoma
community. Strict comparison of the first two pairs of isocommunities from Java and
Britain showed that within-habitat diversity is similar in the two regions. In each comparison, k-dominance was similar even though total community biomass differed. The
third comparison was less satisfactory because at their shallower depth of occurrence,
physical disturbance was found to be maintaining the communities in various subclimax
stages.
I am confident that these results may be generalized, although you will find many
contrary statements in the literature, such as those of Gray (2002), who demonstrated,
using data from 6 intertidal, 13 shelf, and 7 deep-sea locations, that species richness
increases into deep water and toward the tropics. But the sites were selected only for
their representation of depth zones, so I am not surprised at the result. The significant
comparison is between sites carefully chosen to represent similar habitats, differing only
in the latitudes at which they occur.
Perhaps equally interesting is the generalization, also attributable to Thorson, that the
larval development of benthic invertebrates in cold and warm seas is quite different. He
remarked that in benthic species at very high latitudes, the planktonic larva is abandoned
in favor of viviparity, or of yolk-supported external development. Thorson’s results were
based largely on data from prosobranch mollusks in which up to 90% of species in tropical
seas have planktonic larvae, compared with 35–65% in temperate seas and 5–27% in cold
seas. Later compilations showed essentially the same thing for lamellibranch mollusks,
echinoderms, and some decapod crustaceans. Subsequent investigations (Mileikovsky,
1971) supported these findings but emphasized that it is necessary to distinguish several
kinds of planktonic larval existence: planktotrophic, lecicotrophic, and those larvae that
do not venture more than a few centimeters above the deposits.
Chapter 2: Biogeographic Partition of the Ocean
dozen series of parallel communities has stood the test of time (Rosenberg, 2001): these
are grouped into Macoma communities on inshore soft, muddy deposits; Amphiura,
Amphipoda, or Amphioplus communities in slightly deeper muddy sands; Tellina communities in clean sands in shallow water; and Venus communities in shelly sand deeper
on the continental shelf. Each, you will note, is characterized by one or several dominant
genera and each forms the dominant species association in deposits of a certain geological
grade and organic content.
Thorson recognized Venus isocommunities in the White Sea and Scoresby Sound,
from the Baltic to the Adriatic, in the Persian Gulf and off Madras, and also off New
England. I found that clean shell-sand grounds on the continental shelf off Sierra Leone
8N in the Gulf of Guinea were occupied by a species grouping that closely matched
those that had been described elsewhere as Venus isocommunities. During a multistation
benthos survey of this community a total of 47 bivalve mollusks were taken, while a Venus
community sampled with similar equipment in the Celtic Sea comprised 57 macrobenthic
species, many congeneric with those I took off Sierra Leone (Longhurst, 1958; Warwick
et al., 1978). In fact, the Sierra Leone genera would be familiar to any North Sea biologist:
Abra, Cardium, Nucula, Venus, Macoma, Mactra, Spisula, Tellina, Pecten, Ensis, and so on.
Macoma isocommunities were recognized by Thorson on muddy, estuarine-influenced
deposits in the White Sea, off East Greenland, along the Danish Baltic coast, off
Vancouver-Seattle, on the Japanese coast, and in the English Channel. One was subsequently located in the Bristol Channel and I found yet another example off Sierra Leone.
These observations were later supported by a formal study of k-dominance in three
counterpart benthic communities on the continental shelves of northern Java and western
Britain (Warwick and Ruswahyuni, 1987). Diversity indices were computed to represent benthic communities occupying similar habitat and composed of similar (or even
the same) genera in the two regions. Off Java, three isocommunities were identified
on appropriate deposits: an Amphioplus/Lovenia, a Tellinoides, and a Lacionella community. These parallel, respectively, European Amphiura/Brissopsis, a Tellina, and a Macoma
community. Strict comparison of the first two pairs of isocommunities from Java and
Britain showed that within-habitat diversity is similar in the two regions. In each comparison, k-dominance was similar even though total community biomass differed. The
third comparison was less satisfactory because at their shallower depth of occurrence,
physical disturbance was found to be maintaining the communities in various subclimax
stages.
I am confident that these results may be generalized, although you will find many
contrary statements in the literature, such as those of Gray (2002), who demonstrated,
using data from 6 intertidal, 13 shelf, and 7 deep-sea locations, that species richness
increases into deep water and toward the tropics. But the sites were selected only for
their representation of depth zones, so I am not surprised at the result. The significant
comparison is between sites carefully chosen to represent similar habitats, differing only
in the latitudes at which they occur.
Perhaps equally interesting is the generalization, also attributable to Thorson, that the
larval development of benthic invertebrates in cold and warm seas is quite different. He
remarked that in benthic species at very high latitudes, the planktonic larva is abandoned
in favor of viviparity, or of yolk-supported external development. Thorson’s results were
based largely on data from prosobranch mollusks in which up to 90% of species in tropical
seas have planktonic larvae, compared with 35–65% in temperate seas and 5–27% in cold
seas. Later compilations showed essentially the same thing for lamellibranch mollusks,
echinoderms, and some decapod crustaceans. Subsequent investigations (Mileikovsky,
1971) supported these findings but emphasized that it is necessary to distinguish several
kinds of planktonic larval existence: planktotrophic, lecicotrophic, and those larvae that
do not venture more than a few centimeters above the deposits.