398
Craig M. YOUNG
limited dispersal, but genetics did not bear out this
prediction. However, Etter and Rex (1990) showed
that genetic differentiation decreases concomitantly
with a shift away from lecithotrophy along a depth
gradient. Some of this confusion probably results
from an inability to distinguish pelagic lecithotrophs
from direct developers and some is probably because
lecithotrophs living at low temperatures (and therefore
having low metabolic rates) can disperse substantial
distances without the benefit of feeding. Just as mode
of development is not a good predictor of geographic
range in deep-sea ascidians and echinoderms (Young
et al., 1997), it may not predict dispersal potential or
genetic differentiation in deep-sea molluscs.
Echinodermata
From the standpoint of reproduction, echinoderms are
the best-studied group of deep-sea animals. Table 12.6
gives a summary of some reproductive parameters
for all species studied. Most deep-sea echinoderms
have separate sexes, though hermaphroditism is known
among the holothurians (Tyler et al., 1992b) and
occasional hermaphroditic echinoids have been observed (e.g., Allocentrotus fragilis, Lytechinus euerces:
Young, personal observations). With few exceptions,
the reproductive mechanisms and patterns found in
shallow-water echinoderms are also represented among
their deep-water relatives. Echinoderm reproduction
has been thoroughly reviewed elsewhere (see Giese
et al., 1991 as a convenient portal to the literature).
Gonads, gametogenesis and reproductive
periodicity
Echinoderms were among the first deep-sea animals
shown conclusively to have seasonal reproduction
(Lightfoot et al., 1979; Tyler et al., 1982a) and
recruitment (Schoener, 1968), though it is now known
that seasonal breeding is the exception in this group
rather than the rule. In the Rockall Trough, where
numerous species have been examined in multiple
seasons, only a small proportion of species breed
seasonally (Table 12.6). However, at shallower bathyal
depths on the Bahamian Slope, the majority of species
are seasonal breeders. All known deep-sea species
with seasonal breeding produce planktotrophic larvae,
and all known species of continuous breeders produce
non-planktotrophic larvae (Tyler and Young, 1993).
Gonadal development is essentially the same as that
of other echinoderms, proceeding through sequential
stages of gamete proliferation, vitellogenesis and gamete storage prior to spawning. However, in semicontinuous breeders such as Echinus affinis, the gonad
contains multiple cohorts of gametes, and the cycle
may take more than one year to complete (Tyler and
Gage, 1984a).
Eckelbarger (1994b) has listed 34 species of bathyal
and abyssal echinoderms whose ovaries have been
examined by electron microscopy; but modified ovaries
have been found only in four species of deep-sea
holothuroids. In Bathyplotes natans, Hansenothuria
benti, Holothuria occidentalis and Mesothuria sp.,
the inner epithelial cells of the oocytes resembled
podocytes, cells which are generally associated with excretion in other invertebrates (Eckelbarger and Young,
1992; Tyler et al., 1994). In the case of sea-cucumber
ovaries, it has been suggested that podocytes may
function in exchange of nutrients between the genital
haemal sinus and the ovary during vitellogenesis
(Eckelbarger and Young, 1992).
Fecundity
Ramirez Llodra (2002) has reviewed the literature
on fecundity in marine animals, including echinoderms
in the deep sea. Fecundity has been estimated for
many of the echinoid, asteroid and holothuroid species
listed in Table 12.6. In general, deep-sea species
follow the expected pattern (Vance, 1973) in which
lecithotrophic species with large eggs have a smaller
number of eggs than planktotrophic species. Thus, for
example, the planktotrophic species Plutonaster bifrons
produces about one million eggs (Tyler and Pain,
1982a), whereas co-occurring lecithotrophic starfish
such as Bathybiaster vexillifer (Tyler et al., 1982b) and
Benthopecten simplex (Pain et al., 1982a) produce only
a small number of mature oocytes at any given time.
Body size, gonad structure and depth all covary with
fecundity in echinoderms. This is illustrated well in
the forcipulate starfish of the Rockall Trough (Tyler
et al., 1984). The brisingid starfish Brisingella coronata
has an egg size (1250 mm) identical with that of the
confamilial Freyella spinosa, yet the former has a
fecundity of about 60 000 eggs and the latter produces
only 2500 eggs. Freyella lives deeper (below 4000 m)
than Brisingella (750–2450 m).
Gamete structure and fertilization
Eckelbarger et al. (1989c) and Eckelbarger (1994b)
have reviewed the sperm morphology of deep-sea
echinoderms. The majority have sperm of the primitive (Franzen, 1956) or ect-aquasperm (Rouse and
Craig M. YOUNG
limited dispersal, but genetics did not bear out this
prediction. However, Etter and Rex (1990) showed
that genetic differentiation decreases concomitantly
with a shift away from lecithotrophy along a depth
gradient. Some of this confusion probably results
from an inability to distinguish pelagic lecithotrophs
from direct developers and some is probably because
lecithotrophs living at low temperatures (and therefore
having low metabolic rates) can disperse substantial
distances without the benefit of feeding. Just as mode
of development is not a good predictor of geographic
range in deep-sea ascidians and echinoderms (Young
et al., 1997), it may not predict dispersal potential or
genetic differentiation in deep-sea molluscs.
Echinodermata
From the standpoint of reproduction, echinoderms are
the best-studied group of deep-sea animals. Table 12.6
gives a summary of some reproductive parameters
for all species studied. Most deep-sea echinoderms
have separate sexes, though hermaphroditism is known
among the holothurians (Tyler et al., 1992b) and
occasional hermaphroditic echinoids have been observed (e.g., Allocentrotus fragilis, Lytechinus euerces:
Young, personal observations). With few exceptions,
the reproductive mechanisms and patterns found in
shallow-water echinoderms are also represented among
their deep-water relatives. Echinoderm reproduction
has been thoroughly reviewed elsewhere (see Giese
et al., 1991 as a convenient portal to the literature).
Gonads, gametogenesis and reproductive
periodicity
Echinoderms were among the first deep-sea animals
shown conclusively to have seasonal reproduction
(Lightfoot et al., 1979; Tyler et al., 1982a) and
recruitment (Schoener, 1968), though it is now known
that seasonal breeding is the exception in this group
rather than the rule. In the Rockall Trough, where
numerous species have been examined in multiple
seasons, only a small proportion of species breed
seasonally (Table 12.6). However, at shallower bathyal
depths on the Bahamian Slope, the majority of species
are seasonal breeders. All known deep-sea species
with seasonal breeding produce planktotrophic larvae,
and all known species of continuous breeders produce
non-planktotrophic larvae (Tyler and Young, 1993).
Gonadal development is essentially the same as that
of other echinoderms, proceeding through sequential
stages of gamete proliferation, vitellogenesis and gamete storage prior to spawning. However, in semicontinuous breeders such as Echinus affinis, the gonad
contains multiple cohorts of gametes, and the cycle
may take more than one year to complete (Tyler and
Gage, 1984a).
Eckelbarger (1994b) has listed 34 species of bathyal
and abyssal echinoderms whose ovaries have been
examined by electron microscopy; but modified ovaries
have been found only in four species of deep-sea
holothuroids. In Bathyplotes natans, Hansenothuria
benti, Holothuria occidentalis and Mesothuria sp.,
the inner epithelial cells of the oocytes resembled
podocytes, cells which are generally associated with excretion in other invertebrates (Eckelbarger and Young,
1992; Tyler et al., 1994). In the case of sea-cucumber
ovaries, it has been suggested that podocytes may
function in exchange of nutrients between the genital
haemal sinus and the ovary during vitellogenesis
(Eckelbarger and Young, 1992).
Fecundity
Ramirez Llodra (2002) has reviewed the literature
on fecundity in marine animals, including echinoderms
in the deep sea. Fecundity has been estimated for
many of the echinoid, asteroid and holothuroid species
listed in Table 12.6. In general, deep-sea species
follow the expected pattern (Vance, 1973) in which
lecithotrophic species with large eggs have a smaller
number of eggs than planktotrophic species. Thus, for
example, the planktotrophic species Plutonaster bifrons
produces about one million eggs (Tyler and Pain,
1982a), whereas co-occurring lecithotrophic starfish
such as Bathybiaster vexillifer (Tyler et al., 1982b) and
Benthopecten simplex (Pain et al., 1982a) produce only
a small number of mature oocytes at any given time.
Body size, gonad structure and depth all covary with
fecundity in echinoderms. This is illustrated well in
the forcipulate starfish of the Rockall Trough (Tyler
et al., 1984). The brisingid starfish Brisingella coronata
has an egg size (1250 mm) identical with that of the
confamilial Freyella spinosa, yet the former has a
fecundity of about 60 000 eggs and the latter produces
only 2500 eggs. Freyella lives deeper (below 4000 m)
than Brisingella (750–2450 m).
Gamete structure and fertilization
Eckelbarger et al. (1989c) and Eckelbarger (1994b)
have reviewed the sperm morphology of deep-sea
echinoderms. The majority have sperm of the primitive (Franzen, 1956) or ect-aquasperm (Rouse and