REPRODUCTION, DEVELOPMENT AND LIFE-HISTORY TRAITS
395
epizoites on other organisms such as sea urchins are
also common.
Bivalves are represented mostly by protobranchs
(Allen, 1979; Zardus, 2002), though xylophagid bivalves are commonly found in waterlogged wood
(Turner, 1973), and a number of other orders also live
as infauna in the soft sediment. Deep-sea mussels and
clams are among the most abundant animals living at
cold seeps and hydrothermal vents (Van Dover, 2000).
Gonads, gametogenesis and reproductive
periodicity
Prosobranch gastropods are mostly dioecious, though
hermaphroditic species, especially protandric hermaphrodites, are known. The gonad is a discrete organ
located on the upper part of the body whorl and
connected to the outside by a gonoduct, which passes
through the pericardium. In the more advanced forms,
the gonoduct may be divided into pallial and cardiac portions, which in turn may be elaborated into
various kinds of glands for the secretion of fluids,
mucus or capsular material to protect the embryos.
Scaphopods are all dioecious (Reynolds, 2002), as are
aplacophorans and, with very few exceptions, bivalves.
The gonad in mussels and clams is found dorsally, often
in close association with the digestive gland.
Gametogenesis has been described at the ultrastructural level in only a few species of deep-sea molluscs,
all from chemosynthetic ecosystems (Le Pennec and
Beninger, 1997; Eckelbarger and Young, 1999). Eckelbarger and Young (1999) described oogenesis and
spermatogenesis in the methanotrophic mussel Bathymodiolus childressi; the gametogenic mechanisms were
shown to be similar to those of seasonally breeding
shallow-water mussels. Spermatogenesis has also been
described for the gastropod Bathynerita naticoidea
from the same habitat (Hodgson et al., 1998). Ongoing
studies of gametogenesis in limpets from hydrothermal
vents show that some species have rapid mechanisms
of vitellogenesis virtually identical to those found
in certain intertidal limpets (Eckelbarger, unpublished
data).
Reproductive periodicity has been inferred from
gonad histology in a number of deep-sea molluscs
(Table 12.4), most of which have asynchronous gametogenesis indicative of continuous breeding. Seasonal
breeders are found not only at relatively shallow slope
depths (Rokop, 1977a; Eckelbarger and Young, 1999),
but also at depths greater than 2000 m on the lower
slope (Tyler et al., 1992a) and at 3480 m in a hydrothermal vent system on the Mid-Atlantic Ridge (Le Pennec
and Beninger, 1997). It is somewhat surprising that
several species of mussels breed synchronously at vents
and seeps, since these animals are presumably exposed
to a continuous source of chemical energy that can be
allocated to gonadal development.
Gamete structure and fertilization
Sperm have been described at the ultrastructural
level for deep-sea gastropods from several families
(Healy, 1988, 1989b, 1990; Hodgson et al., 1998) as
well as for a midwater cephalopod, Vampyroteuthis
infernalis (Healy, 1989a), and some chemosynthetic
mussels (Eckelbarger and Young, 1999). In every case,
the sperm structure was similar to that of shallow-water
members of the same groups.
Transfer of spermatophores has been described in a
deep-sea octopus (Lutz and Voight, 1994). Although no
other specific studies of fertilization have been undertaken in deep-sea molluscs, it is presumed that deepsea gastropods follow the strongly conservative patterns
known for shallow-water taxa. Thus, archaeogastropods
(with the exception of the Neritidae) spawn their
gametes and fertilize externally, while other groups all
fertilize internally by copulation.
Fecundity
Scheltema (1994) has reviewed the literature on fecundity in deep-sea molluscs. Instantaneous fecundity
in bivalves ranges from two eggs in two species of
pristoglomid protobranchs on the continental slope
(Sanders and Allen, 1973) to more than 30 000 eggs in
opportunistic xylophagids that colonize wood (Turner,
1973). There are almost no reliable data on the
fecundity of deep-sea gastropods despite reported
counts of embryos in egg capsules (Gustafson et al.,
1991), since the number of capsules produced by a
single female is generally impossible to determine.
Embryogenesis and larval development
Egg masses from deep-sea snails have been found
and described several times (Gustafson et al., 1991;
Bouchet and War´ en, 1994), but there are no descriptions of early embryology, nor has a complete sequence
of larval development been reported for any deep-sea
mollusc. The known patterns of larval development
have been reviewed by Bouchet and War´ en (1994)
and Scheltema (1994). Most of what is known about
larval development has been inferred from the apex of
the adult shell, where the larval protoconch (in gastropods) or prodissoconch (in bivalves) is retained as a
395
epizoites on other organisms such as sea urchins are
also common.
Bivalves are represented mostly by protobranchs
(Allen, 1979; Zardus, 2002), though xylophagid bivalves are commonly found in waterlogged wood
(Turner, 1973), and a number of other orders also live
as infauna in the soft sediment. Deep-sea mussels and
clams are among the most abundant animals living at
cold seeps and hydrothermal vents (Van Dover, 2000).
Gonads, gametogenesis and reproductive
periodicity
Prosobranch gastropods are mostly dioecious, though
hermaphroditic species, especially protandric hermaphrodites, are known. The gonad is a discrete organ
located on the upper part of the body whorl and
connected to the outside by a gonoduct, which passes
through the pericardium. In the more advanced forms,
the gonoduct may be divided into pallial and cardiac portions, which in turn may be elaborated into
various kinds of glands for the secretion of fluids,
mucus or capsular material to protect the embryos.
Scaphopods are all dioecious (Reynolds, 2002), as are
aplacophorans and, with very few exceptions, bivalves.
The gonad in mussels and clams is found dorsally, often
in close association with the digestive gland.
Gametogenesis has been described at the ultrastructural level in only a few species of deep-sea molluscs,
all from chemosynthetic ecosystems (Le Pennec and
Beninger, 1997; Eckelbarger and Young, 1999). Eckelbarger and Young (1999) described oogenesis and
spermatogenesis in the methanotrophic mussel Bathymodiolus childressi; the gametogenic mechanisms were
shown to be similar to those of seasonally breeding
shallow-water mussels. Spermatogenesis has also been
described for the gastropod Bathynerita naticoidea
from the same habitat (Hodgson et al., 1998). Ongoing
studies of gametogenesis in limpets from hydrothermal
vents show that some species have rapid mechanisms
of vitellogenesis virtually identical to those found
in certain intertidal limpets (Eckelbarger, unpublished
data).
Reproductive periodicity has been inferred from
gonad histology in a number of deep-sea molluscs
(Table 12.4), most of which have asynchronous gametogenesis indicative of continuous breeding. Seasonal
breeders are found not only at relatively shallow slope
depths (Rokop, 1977a; Eckelbarger and Young, 1999),
but also at depths greater than 2000 m on the lower
slope (Tyler et al., 1992a) and at 3480 m in a hydrothermal vent system on the Mid-Atlantic Ridge (Le Pennec
and Beninger, 1997). It is somewhat surprising that
several species of mussels breed synchronously at vents
and seeps, since these animals are presumably exposed
to a continuous source of chemical energy that can be
allocated to gonadal development.
Gamete structure and fertilization
Sperm have been described at the ultrastructural
level for deep-sea gastropods from several families
(Healy, 1988, 1989b, 1990; Hodgson et al., 1998) as
well as for a midwater cephalopod, Vampyroteuthis
infernalis (Healy, 1989a), and some chemosynthetic
mussels (Eckelbarger and Young, 1999). In every case,
the sperm structure was similar to that of shallow-water
members of the same groups.
Transfer of spermatophores has been described in a
deep-sea octopus (Lutz and Voight, 1994). Although no
other specific studies of fertilization have been undertaken in deep-sea molluscs, it is presumed that deepsea gastropods follow the strongly conservative patterns
known for shallow-water taxa. Thus, archaeogastropods
(with the exception of the Neritidae) spawn their
gametes and fertilize externally, while other groups all
fertilize internally by copulation.
Fecundity
Scheltema (1994) has reviewed the literature on fecundity in deep-sea molluscs. Instantaneous fecundity
in bivalves ranges from two eggs in two species of
pristoglomid protobranchs on the continental slope
(Sanders and Allen, 1973) to more than 30 000 eggs in
opportunistic xylophagids that colonize wood (Turner,
1973). There are almost no reliable data on the
fecundity of deep-sea gastropods despite reported
counts of embryos in egg capsules (Gustafson et al.,
1991), since the number of capsules produced by a
single female is generally impossible to determine.
Embryogenesis and larval development
Egg masses from deep-sea snails have been found
and described several times (Gustafson et al., 1991;
Bouchet and War´ en, 1994), but there are no descriptions of early embryology, nor has a complete sequence
of larval development been reported for any deep-sea
mollusc. The known patterns of larval development
have been reviewed by Bouchet and War´ en (1994)
and Scheltema (1994). Most of what is known about
larval development has been inferred from the apex of
the adult shell, where the larval protoconch (in gastropods) or prodissoconch (in bivalves) is retained as a