The physiological ecology and life history strategies of the nudibranch molluscs 'Adalaria proxima' (Alder & Hancock) and 'Onchidoris muricata' (Müller) (Gastropoda: Opisthobranchia)
Abstract
This
study investigated the physiological ecology, larval biology
and population
genetics of the nudibranch molluscs Adalaria proxima
(A & H)
and Onchidoris muricata
(Müller). These two species are annual, simultaneous hermaphrodites and are ecologically
very similar with the exception that A. proxima reproduces by
means of pelagic
lecithotrophic larvae whereas Omuricata has long-term planktotrophic larvae. The aim of
the study was therefore to determine the selective pressures which resulted in the evolution of
different larval types in these two species, and to ascertain the ecological and population
genetic consequences thereof.
Simple
energy budgets comprising the major components (consumption, growth,
respiration and reproduction) were constructed for laboratory populations of each species. In
both A. proxima and O. muricata,
feeding rate displayed an asymptotic increase with
body
size.
Mean feeding rates of
A. proxima were greater than those of comparable O. muricata
individuals,
and overall assimilation efficiency was higher in A. proxima than in O. muricata.
This difference was reflected in the somatic growth rates which were correspondingly greater
in A. proxima than in O. muricata.
Net
growth efficiencies were broadly comparable between
the two species, however, growth of
A. proxima was approximately linear
over' time whilst
that of
O. muricata
displayed a curvilinear, almost exponential, pattern.
This is interpreted as
demonstrating that some form
of constraint (possibly feeding rate) operated on the growth
rates of
A. proxima
but
not on those of
O. muricata.
Respiration rates were found to be relatively constant within given animals, but
significant differences were found between individuals. The
allometry of respiration rate
was not constant; Omuricata demonstrated a more rapid
increase in
respiration rate with
increasing body
size than did A. proxima.
Individual
variations
in
respiration rate did
not
reflect variations in the energy partitioned to either growth or reproduction.
Reproductive patterns in the two species were dissimilar. A. proxima
laid fewer
spawn masses containing fewer, larger ova than those laid by O. muricata
individuals. In
addition, the spawning period of
A. proxima was shorter than that of
O. muricata
(60 days
and
105 days respectively). Both
species exhibited a similar
(proportional) degree of somatic
catabolism over these periods. The
consequently more rapid
"degrowth"
of
A. proxima
is
interpreted as the necessary utilization of an energy resource (i. e. the soma) caused by
an
inability to meet the energy demands of reproduction through feeding alone.
This
was not
the case in Oanuricata individuals
which exhibited a much smaller maximum body size and
were able to feed at a sufficiently rapid rate to maintain reproduction.
In the latter case, the
longer reproductive period served to maximise the total reproductive output.
Several different
measures of
"Reproductive Effort" (RE)
were calculated.
These
generally indicated that the RE
of
Omuricata
was considerably greater than that of
A. proxima.
Although
such differences have been used in the literature to classify the
respective costs of
different larval types or
"reproductive strategies", the variability of the
RE's obtained from
the different
measures used here has led to the suggestion that the
general lack
of association between RE
and reproductive strategy which
has been reported
elsewhere may
(partially) be attributable to the different
measures of
RE
employed in
different
studies.
Studies of the embryonic and larval
period showed that the egg-to-juvenile period of
O. muricata was approximately 50% longer than that of
A. proxima.
This difference was
primarily attributable to the extended pelagic development of
O. muricata
larvae. Estimates
of the degree of
dispersal, and hence gene-flow, between populations of these species were
tested by investigating the biochemical genetics of such populations. No data were available
for O. muricata,
but A. proxima populations proved to be more genetically heterogeneous
than had been expected. It is therefore concluded that actual pelagic dispersal may be
considerably abbreviated over that expected on the basis of
larval
culture
data alone.
A
model is developed to explain the possible consequences of
different
egg-to-juvenile periods (which
accrue from different larval types) on
both
the ecology of the
benthic adult, and on overall energy partitioning to reproduction.
However,
although
(probable) proximate causes and effects of the different reproductive traits exhibited by
A. proxima and Oanuricata
are shown, it has not
been possible to determine the exact
selective pressures which caused A. proxima to diverge from the ancestral "O. muricata"
stock through the evolution of a pelagic lecithotrophic larva.
Type
Thesis, PhD Doctor of Philosophy
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