412
Craig M. YOUNG
Starr et al. (1994) demonstrated that phytodetritus
can stimulate larval release in animals living in
relatively shallow water but below the euphotic zone;
but, to date, natural spawning cues have only been
documented for a single deep-sea animal: a vesicomyid
clam living at cold seeps in Sagami Bay, Japan
(Fujiwara et al., 1998). In this species, spawning was
correlated with small natural temperature rises in the
environment, and was induced experimentally in an
elegant experiment wherein seawater was artificially
heated in situ.
Fecundity and egg size
Background
The relationships among parental investment, egg
size, and fecundity in marine invertebrates have received much attention. It is assumed that an animal of
any particular body size should have a fixed amount
of energy available for reproduction and that this
energy allotment may be divided into many small
eggs with little energy invested per egg, or into fewer
large eggs with more energy per egg. A portion of
the available energy may also be used for parental
protection of broods, for auxiliary structures such as
egg capsules, or for assuring successful fertilization.
Among shallow-water free-spawning invertebrates, the
former strategy is most common, especially among
species with relatively large body sizes (e.g., Menge,
1975). Broadcasting many small eggs with no parental
protection and little yolk requires high fecundity to
compensate for high mortality during a long planktonic
life (Thorson, 1950; Chia, 1974). These tradeoffs
were first formalized for marine invertebrates by
Vance (1973), whose classic model was immediately
challenged (Underwood, 1974; Strathmann, 1974) and
has been updated many times since (Christensen and
Fenchel, 1979; Havenhand, 1993). Recently, there has
been discussion about the potential role of fertilization
processes in driving the evolution of egg size (Levitan,
1995), though this idea has not yet been accepted as
fully as Vance’s initial premise that egg size evolves
because of limitations in fecundity and mortality during
the larval stage.
Vance’s model and most subsequent formulations
predict that the very large and small egg sizes are
evolutionarily stable, and that few eggs of intermediate
size will be found. Thus, species tend to have either
very large yolky eggs and lecithotrophic development,
or very small eggs with planktotrophic larval development. In all of these models, it is assumed that a
species has a fixed amount of energy to allocate to
reproduction, that an animal with small body size will
have less energy for reproduction than one with large
body size, and that mortality in the plankton will be
greater for small eggs than large eggs.
Predictions
What egg sizes are to be expected in the deep sea?
With the dramatic exceptions of a number of very large
amphipods, isopods and pycnogonids, deep-sea animals
tend to be smaller on average than related animals in
coastal and littoral environments, a pattern that may
be related to energy limitation in deep water (Rex and
Etter, 1998). Small animals are expected to produce
small numbers of large eggs not only because they have
inadequate energy to invest in large fecundity, but also
because space for brooding embryos may be limited
(Strathmann and Strathmann, 1982). Thus, in the
deep sea, models of reproductive partitioning predict
large egg size, short or non-existent larval periods,
and significant investment in parental care. Even in
megafauna with large body sizes, limited energy in the
environment might reduce the amount of energy that
can be devoted to reproduction, thus driving the species
toward decreased fecundity and large egg size. On the
other hand, if mortality in the plankton is less important
in the deep sea than in shallow water, then animals with
small body size could perhaps reproduce successfully
with low fecundities and smaller egg sizes than their
shallow-water counterparts.
Reproductive output, which is sometimes but not
always reflected in fecundity, is predicted to decrease
with depth because of food limitation in deep water.
Evidence
The relationships among egg size, fecundity and
depth have been examined for galatheid crabs (Van
Dover and Williams, 1991), caridean shrimps (Ramirez
Llodra et al., 2000), protobranch bivalves (Scheltema,
1994), and echinoderms (Pearse, 1994). Van Dover and
Williams plotted the mean egg volume against depth
for 52 species of galatheids ranging from intertidal
to abyssal depths (Fig. 12.7). There was a general
positive correlation between egg volume and depth.
Statistical analysis within genera showed that this trend
was significant in Galathea spp. and Munidopsis spp.,
but not in Munida spp. Apparent depth-related trends
in fecundity in other groups of crustaceans are often
caused by body-size differences or by a shift to larger
Craig M. YOUNG
Starr et al. (1994) demonstrated that phytodetritus
can stimulate larval release in animals living in
relatively shallow water but below the euphotic zone;
but, to date, natural spawning cues have only been
documented for a single deep-sea animal: a vesicomyid
clam living at cold seeps in Sagami Bay, Japan
(Fujiwara et al., 1998). In this species, spawning was
correlated with small natural temperature rises in the
environment, and was induced experimentally in an
elegant experiment wherein seawater was artificially
heated in situ.
Fecundity and egg size
Background
The relationships among parental investment, egg
size, and fecundity in marine invertebrates have received much attention. It is assumed that an animal of
any particular body size should have a fixed amount
of energy available for reproduction and that this
energy allotment may be divided into many small
eggs with little energy invested per egg, or into fewer
large eggs with more energy per egg. A portion of
the available energy may also be used for parental
protection of broods, for auxiliary structures such as
egg capsules, or for assuring successful fertilization.
Among shallow-water free-spawning invertebrates, the
former strategy is most common, especially among
species with relatively large body sizes (e.g., Menge,
1975). Broadcasting many small eggs with no parental
protection and little yolk requires high fecundity to
compensate for high mortality during a long planktonic
life (Thorson, 1950; Chia, 1974). These tradeoffs
were first formalized for marine invertebrates by
Vance (1973), whose classic model was immediately
challenged (Underwood, 1974; Strathmann, 1974) and
has been updated many times since (Christensen and
Fenchel, 1979; Havenhand, 1993). Recently, there has
been discussion about the potential role of fertilization
processes in driving the evolution of egg size (Levitan,
1995), though this idea has not yet been accepted as
fully as Vance’s initial premise that egg size evolves
because of limitations in fecundity and mortality during
the larval stage.
Vance’s model and most subsequent formulations
predict that the very large and small egg sizes are
evolutionarily stable, and that few eggs of intermediate
size will be found. Thus, species tend to have either
very large yolky eggs and lecithotrophic development,
or very small eggs with planktotrophic larval development. In all of these models, it is assumed that a
species has a fixed amount of energy to allocate to
reproduction, that an animal with small body size will
have less energy for reproduction than one with large
body size, and that mortality in the plankton will be
greater for small eggs than large eggs.
Predictions
What egg sizes are to be expected in the deep sea?
With the dramatic exceptions of a number of very large
amphipods, isopods and pycnogonids, deep-sea animals
tend to be smaller on average than related animals in
coastal and littoral environments, a pattern that may
be related to energy limitation in deep water (Rex and
Etter, 1998). Small animals are expected to produce
small numbers of large eggs not only because they have
inadequate energy to invest in large fecundity, but also
because space for brooding embryos may be limited
(Strathmann and Strathmann, 1982). Thus, in the
deep sea, models of reproductive partitioning predict
large egg size, short or non-existent larval periods,
and significant investment in parental care. Even in
megafauna with large body sizes, limited energy in the
environment might reduce the amount of energy that
can be devoted to reproduction, thus driving the species
toward decreased fecundity and large egg size. On the
other hand, if mortality in the plankton is less important
in the deep sea than in shallow water, then animals with
small body size could perhaps reproduce successfully
with low fecundities and smaller egg sizes than their
shallow-water counterparts.
Reproductive output, which is sometimes but not
always reflected in fecundity, is predicted to decrease
with depth because of food limitation in deep water.
Evidence
The relationships among egg size, fecundity and
depth have been examined for galatheid crabs (Van
Dover and Williams, 1991), caridean shrimps (Ramirez
Llodra et al., 2000), protobranch bivalves (Scheltema,
1994), and echinoderms (Pearse, 1994). Van Dover and
Williams plotted the mean egg volume against depth
for 52 species of galatheids ranging from intertidal
to abyssal depths (Fig. 12.7). There was a general
positive correlation between egg volume and depth.
Statistical analysis within genera showed that this trend
was significant in Galathea spp. and Munidopsis spp.,
but not in Munida spp. Apparent depth-related trends
in fecundity in other groups of crustaceans are often
caused by body-size differences or by a shift to larger