Studies on the biology of three species of sea urchin (Echinodermata : Echnoidea), on the South African east coast.
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Date
1993
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Abstract
Ten species of shallow water echinoid are found on the subtropical
east coast of South Africa. Although their distributions are
patchy, the most common species, Echinometra mathaei, stomopneustes
variolaris and Diadema savignyi, are nontheless conspicuous
components of intertidal communities on this coast. As little was
known about these three species, the overall intention of this
study was to provide some fundemental information on their biology
and ecology. For the purposes of achieving this aim a life history
approach was adopted, where the relative investments by each
species in growth, maintenance and reproduction were investigated
and compared. These patterns of investment were then related to
the habitat occupied by each species, in an attempt to identify the
selective forces which may have been implicated in shaping their
life histories.
It was apparent from the results of investigations conducted
between January 1991 and June 1993 that there were distinct
differences in the patterns of investment in growth, maintenance
and reproduction between the three species. The life history of
S. variolaris, which occupied exposed habitats in the lower
intertidal, was characterised by a large investment in maintenance,
lower reproductive output, slower growth and a longer lifespan,
relative to the other two species. In contrast, Q. savignyi, which
inhabited less exposed mid-shore pools, had a relatively higher
reproductive output, more rapid growth, a smaller investment in
maintenance and a shorter lifespan. While selection ln S.
variolaris and Q. savignyi appears to favour survival and
reproduction respectively, the life history of E. mathaei, a species which also occupies mid-shore pools, was balanced between
these two extremes, allocating sufficient resources to maintenance
to permit tolerance of harsh physical conditions while still making
a moderate investment in reproduction over a lifespan of
intermediate duration.
The predictions generated by the r-K selection and "bet hedging"
theories of life history evolution, were applied in the process of
speculating on the selective forces which may have shaped these
life histories. However, it was found that neither set of
predictions and associated selective forces could adequately
explain the observed life histories. Rather, it seemed that the
life histories of the three species represented evolved responses
to the direct and indirect effects of exposure to wave action and
sand movements which dominate the intertidal environment on the
South African east coast. In the exposed lower intertidal,
unpredictable recruitment, drag and impact forces associated with
wave action, which impose limits to body size and necessitate a
large investment in maintenance to ensure survival, select for slow
growth, low reproductive output and high longevity. In contrast
reproduction and growth of species occupying the more sheltered
mid-shore pool habitats would be less effected by the demands of
maintenance investment or limits to body size. In addition
predictable recruitment in the mid-shore, would obviate the need
for long life in order to ensure a contribution to future
generations.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1993.
Keywords
Sea urchins--South Africa., Echinometra mathaei., Stomopneustes variolaris., Diadema savignyi., Theses--Zoology.