THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
61
maintain the assemblages. Some knowledge of how
ocean basins and their water masses developed over
geological time is a basic need for understanding how
the current composition of pelagic assemblages came
into existence. Imprints of the circulation patterns in
ancient oceans (Fig. 3.1) are still detectable in the
zoogeographical distributions of some of the more
ancient lineages. Superimposed on these patterns are
the effects of one-off geophysical or vicariance events,
such as the opening and closing of the Panama Isthmus,
the Messinian salinity crisis in the Mediterranean, and
to a lesser extent, the impacts of fluctuating sea levels
during the glacial cycles. All these events led either
to the creation or to the removal of land barriers
to the dispersion of the species. If, as the result
of such an event, a population of a species became
fragmented into two or more isolated metapopulations,
then over time each metapopulation will have tended
to diverge as a result of genetic drift and adaptation to
the local environmental conditions. In time they may
have diverged sufficiently to have evolved into separate
species. If these barriers are subsequently removed,
once again restoring the potential for gene flow between
the metapopulations, there are several possible outcomes. The simplest is if the extent of differentiation
has been only slight, and the metapopulations remerge
into being a single, albeit rather variable, species.
However, if interbreeding is no longer possible because
of either genetic or behavioural shifts, then the two
types may directly compete with one another as they
intermix. If one population proves to be competitively
more successful throughout the total range, it may
replace the other driving it to extinction. Alternatively
if each population is competitively more successful
in different areas or zones within the ranges, they
may co-exist, partitioning the available resources (and
habitats) either in time or in space, and emerging as
distinct, albeit closely related species. This process
appears to have been the way in which the richly
diverse communities of inshore copepods found in the
waters around the East Indies evolved. In this case,
the succession of Quaternary glacial and inter-glacial
eras resulted in sea-levels fluctuating by nearly 100 m,
so that during the glacial episodes some of the deep
basins became isolated, but were re-connected during
the subsequent interglacial period (Fleminger, 1986).
The seas around the East Indies are a hot spot for
species richness in a wide range of shallow-living
marine groups.
A similar process may currently be taking place in
the Mediterranean, where during the last glaciation
some temperate/boreal species gained access from the
North Atlantic and have persisted as “glacial relicts”,
although they are now experiencing very much warmer
water temperatures particularly below 300 m. These
stocks are now isolated from their parental stocks
in the North Atlantic. The marked contrast between
the environmental conditions inside the Mediterranean
and outside in the North Atlantic will probably be
imposing contrasting selective pressures. Thus, the
relict populations of the euphausiid Meganyctiphanes
norvegica and the myctophid fish Benthosema glaciale
can be expected to have been diverging from their
original genetic stocks and from the other isolated
populations of these species which occur in the cool
waters in the coastal upwelling region off Northwest
Africa. Whether the period of 18 000 to 20 000 years
that has elapsed since the end of the last glaciation
has been long enough for them to have become
separate species is yet to be examined. Studies on
the biodiversity of pelagic marine communities have
focused predominantly on the processes that are
currently maintaining the community structure, rather
than on those that led to their origins (White, 1994;
Angel, 1997).
Speciation rates vary. An interesting trait that has
emerged from the study of fossil molluscs is that
taxa that are highly specialized speciate more readily,
but are also more susceptible to extinction than more
generalist, adaptable taxa (Taylor, 1997). However,
the unchanging physical characteristics of the oceanic
water column over evolutionary time appears to have
favoured the evolution of those highly specialized
organisms whose biology has become very finely tuned
to conditions in the water column. In the absence
of change, these specialized taxa appear to be able
to out-compete new, less highly adapted, immigrants
(Maynard Smith, 1989).
There are two distinct aspects to biodiversity, which
are confusing and often confused. Moreover, both
aspects give different values of diversity according
to the time and spatial scales of sampling. The first
and simplest aspect to understand is species richness.
This is the number of species in an inventory, which
of course varies from global, to regional, to local,
and even between replicated samples. Estimating the
numbers of species in an assemblage has to be based
on sampling, but each individual sample contains only
a subset (and often a small one) of the total inventory.
Hence the larger the sample taken and the more
61
maintain the assemblages. Some knowledge of how
ocean basins and their water masses developed over
geological time is a basic need for understanding how
the current composition of pelagic assemblages came
into existence. Imprints of the circulation patterns in
ancient oceans (Fig. 3.1) are still detectable in the
zoogeographical distributions of some of the more
ancient lineages. Superimposed on these patterns are
the effects of one-off geophysical or vicariance events,
such as the opening and closing of the Panama Isthmus,
the Messinian salinity crisis in the Mediterranean, and
to a lesser extent, the impacts of fluctuating sea levels
during the glacial cycles. All these events led either
to the creation or to the removal of land barriers
to the dispersion of the species. If, as the result
of such an event, a population of a species became
fragmented into two or more isolated metapopulations,
then over time each metapopulation will have tended
to diverge as a result of genetic drift and adaptation to
the local environmental conditions. In time they may
have diverged sufficiently to have evolved into separate
species. If these barriers are subsequently removed,
once again restoring the potential for gene flow between
the metapopulations, there are several possible outcomes. The simplest is if the extent of differentiation
has been only slight, and the metapopulations remerge
into being a single, albeit rather variable, species.
However, if interbreeding is no longer possible because
of either genetic or behavioural shifts, then the two
types may directly compete with one another as they
intermix. If one population proves to be competitively
more successful throughout the total range, it may
replace the other driving it to extinction. Alternatively
if each population is competitively more successful
in different areas or zones within the ranges, they
may co-exist, partitioning the available resources (and
habitats) either in time or in space, and emerging as
distinct, albeit closely related species. This process
appears to have been the way in which the richly
diverse communities of inshore copepods found in the
waters around the East Indies evolved. In this case,
the succession of Quaternary glacial and inter-glacial
eras resulted in sea-levels fluctuating by nearly 100 m,
so that during the glacial episodes some of the deep
basins became isolated, but were re-connected during
the subsequent interglacial period (Fleminger, 1986).
The seas around the East Indies are a hot spot for
species richness in a wide range of shallow-living
marine groups.
A similar process may currently be taking place in
the Mediterranean, where during the last glaciation
some temperate/boreal species gained access from the
North Atlantic and have persisted as “glacial relicts”,
although they are now experiencing very much warmer
water temperatures particularly below 300 m. These
stocks are now isolated from their parental stocks
in the North Atlantic. The marked contrast between
the environmental conditions inside the Mediterranean
and outside in the North Atlantic will probably be
imposing contrasting selective pressures. Thus, the
relict populations of the euphausiid Meganyctiphanes
norvegica and the myctophid fish Benthosema glaciale
can be expected to have been diverging from their
original genetic stocks and from the other isolated
populations of these species which occur in the cool
waters in the coastal upwelling region off Northwest
Africa. Whether the period of 18 000 to 20 000 years
that has elapsed since the end of the last glaciation
has been long enough for them to have become
separate species is yet to be examined. Studies on
the biodiversity of pelagic marine communities have
focused predominantly on the processes that are
currently maintaining the community structure, rather
than on those that led to their origins (White, 1994;
Angel, 1997).
Speciation rates vary. An interesting trait that has
emerged from the study of fossil molluscs is that
taxa that are highly specialized speciate more readily,
but are also more susceptible to extinction than more
generalist, adaptable taxa (Taylor, 1997). However,
the unchanging physical characteristics of the oceanic
water column over evolutionary time appears to have
favoured the evolution of those highly specialized
organisms whose biology has become very finely tuned
to conditions in the water column. In the absence
of change, these specialized taxa appear to be able
to out-compete new, less highly adapted, immigrants
(Maynard Smith, 1989).
There are two distinct aspects to biodiversity, which
are confusing and often confused. Moreover, both
aspects give different values of diversity according
to the time and spatial scales of sampling. The first
and simplest aspect to understand is species richness.
This is the number of species in an inventory, which
of course varies from global, to regional, to local,
and even between replicated samples. Estimating the
numbers of species in an assemblage has to be based
on sampling, but each individual sample contains only
a subset (and often a small one) of the total inventory.
Hence the larger the sample taken and the more