REPRODUCTION, DEVELOPMENT AND LIFE-HISTORY TRAITS
385
that fertilization may take place either internally or
just outside the gonopores (Southward and Coates,
1985). Upon breaking out of the masses, individual
sperm of this species undergo a transformation in
which the acrosome migrates from its extreme terminal
position to surround and cap the tip of the nucleus.
Although sperm have been reported from the terminal
part of the oviduct in Riftia pachyptila (Gardiner and
Jones, 1985), there is growing evidence that these
and other siboglinids are free spawners. Apparent
spawning events have been observed repeatedly in
Riftia pachyptila (Van Dover, 1994; L. Mullineaux,
T. Shanks, R. Lutz, J. Childress, personal communications; C. Young, personal observations) as well as
the cold-seep siboglinid Lamellibrachia sp. (C. Young,
unpublished observations). In these putative spawning
events, gametes are expelled in a puff as the animal
withdraws partially or completely into its tube. It
is not known whether all of these observations are
of sperm bundles or if eggs are also released in
puffs. However, it seems unlikely that the sperm of
other species are transferred to females in the same
manner as occurs in Ridgeia, as the sperm bundles
of these other species separate into individual sperm
(Fig. 12.1) shortly after they are diluted in seawater
(Cary et al., 1989; Young et al., 1996b). The heads
of siboglinid sperm are extremely elongate and consist
of a helical mitochondrion wrapped around a helical
nucleus (Gardiner and Jones, 1985). The terminal
acrosome, which is columnar rather than conical in
shape, seems to hold the bundles together on the apical
ends, while the posterior end is held together by the
individual flagellae, which beat as a unit until the
bundle breaks apart. Upon dilution in seawater, the
acrosome changes shape and is lost before fertilization
(Young, personal observations). The individual sperm,
as well as the sperm bundles, are motile in seawater.
Spawning has recently been observed in the so-called
“ice worm”, the hesionid Hesiocaeca methanicola that
lives on exposed lumps of methane hydrate near cold
seeps in the Gulf of Mexico (Fisher et al., 2000;
Eckelbarger et al., 2001). Females apparently release
eggs through nephridiopores, whereas males spawn
their sperm through the cloaca, which is connected to
the coelom by means of a ciliated duct (Eckelbarger
et al., 2001). Fertilization in this species is indicated
by a slight elevation of the fertilization membrane
followed by production of unusually large polar bodies
as the egg completes its maturation divisions.
Fecundity
Instantaneous fecundity has been estimated for only
two species of non-vent deep-sea polychaetes, Pholoe
anoculata and the orbinid Microrbinia lineata, both
from the slope off North Carolina (Blake, 1993). The
former has a fecundity of about 160 eggs and the
latter bears only two large eggs in a single segment
(Blake, 1993). These values are substantially lower
than fecundities that have been reported for various
polychaetes from hydrothermal vents, which range
from less than 4000 in Paralvinella grasslei (Zal et al.,
1995) through 230 000 in the much larger alvinellid
polychaete Alvinella pompejana (Chevaldonn´ e et al.,
1997). The large hydrothermal-vent tube worm Riftia
pachyptila holds as many as 700 000 ripe eggs in
its ovisac, suggesting that as many eggs as this
could be spawned at a time; but egg traps deployed
in situ always yielded much smaller numbers (Young,
unpublished data), suggesting that not all of these
eggs are released at once. The smaller tube worm
Tevnia jerichonana, also from the East Pacific Rise,
is often the first colonist arriving at a newly formed
vent habitat. Paradoxically, its fecundity is at least an
order of magnitude lower than that of Riftia pachyptila,
which generally arrives much later. Annual fecundities
and lifetime fecundities cannot be estimated for any
deep-sea polychaete because of inadequate information
about longevity, spawning frequency, and the speed
of the gametogenic cycle. It would not be surprising,
however, if the annual fecundity of the continuously
reproducting giant tube worm Riftia pachyptila greatly
exceeds the fecundity of any lecithotrophic polychaete
from shallow water.
Embryogenesis and larval development
Although the developmental modes of most deep-sea
polychaetes remain completely unknown, a number of
species are known to brood their young. Levin et al.
(1994) listed eleven species of known brooders on
the slopes of the Volcano 7 seamount in the eastern
Pacific (Table 12.2). The mechanism of brooding
varies among species, some brooding larvae in the
tubes, others employing specialized brood pouches, and
still others having the larvae attached directly to the
segments of the mother, either laterally or dorsally
(Levin et al., 1994). Because it is much easier to
document brooding than other forms of reproduction,
it is still not known whether brooding predominates
among deep-sea polychaetes, or whether a majority
of species have indirect development. Levin et al.
385
that fertilization may take place either internally or
just outside the gonopores (Southward and Coates,
1985). Upon breaking out of the masses, individual
sperm of this species undergo a transformation in
which the acrosome migrates from its extreme terminal
position to surround and cap the tip of the nucleus.
Although sperm have been reported from the terminal
part of the oviduct in Riftia pachyptila (Gardiner and
Jones, 1985), there is growing evidence that these
and other siboglinids are free spawners. Apparent
spawning events have been observed repeatedly in
Riftia pachyptila (Van Dover, 1994; L. Mullineaux,
T. Shanks, R. Lutz, J. Childress, personal communications; C. Young, personal observations) as well as
the cold-seep siboglinid Lamellibrachia sp. (C. Young,
unpublished observations). In these putative spawning
events, gametes are expelled in a puff as the animal
withdraws partially or completely into its tube. It
is not known whether all of these observations are
of sperm bundles or if eggs are also released in
puffs. However, it seems unlikely that the sperm of
other species are transferred to females in the same
manner as occurs in Ridgeia, as the sperm bundles
of these other species separate into individual sperm
(Fig. 12.1) shortly after they are diluted in seawater
(Cary et al., 1989; Young et al., 1996b). The heads
of siboglinid sperm are extremely elongate and consist
of a helical mitochondrion wrapped around a helical
nucleus (Gardiner and Jones, 1985). The terminal
acrosome, which is columnar rather than conical in
shape, seems to hold the bundles together on the apical
ends, while the posterior end is held together by the
individual flagellae, which beat as a unit until the
bundle breaks apart. Upon dilution in seawater, the
acrosome changes shape and is lost before fertilization
(Young, personal observations). The individual sperm,
as well as the sperm bundles, are motile in seawater.
Spawning has recently been observed in the so-called
“ice worm”, the hesionid Hesiocaeca methanicola that
lives on exposed lumps of methane hydrate near cold
seeps in the Gulf of Mexico (Fisher et al., 2000;
Eckelbarger et al., 2001). Females apparently release
eggs through nephridiopores, whereas males spawn
their sperm through the cloaca, which is connected to
the coelom by means of a ciliated duct (Eckelbarger
et al., 2001). Fertilization in this species is indicated
by a slight elevation of the fertilization membrane
followed by production of unusually large polar bodies
as the egg completes its maturation divisions.
Fecundity
Instantaneous fecundity has been estimated for only
two species of non-vent deep-sea polychaetes, Pholoe
anoculata and the orbinid Microrbinia lineata, both
from the slope off North Carolina (Blake, 1993). The
former has a fecundity of about 160 eggs and the
latter bears only two large eggs in a single segment
(Blake, 1993). These values are substantially lower
than fecundities that have been reported for various
polychaetes from hydrothermal vents, which range
from less than 4000 in Paralvinella grasslei (Zal et al.,
1995) through 230 000 in the much larger alvinellid
polychaete Alvinella pompejana (Chevaldonn´ e et al.,
1997). The large hydrothermal-vent tube worm Riftia
pachyptila holds as many as 700 000 ripe eggs in
its ovisac, suggesting that as many eggs as this
could be spawned at a time; but egg traps deployed
in situ always yielded much smaller numbers (Young,
unpublished data), suggesting that not all of these
eggs are released at once. The smaller tube worm
Tevnia jerichonana, also from the East Pacific Rise,
is often the first colonist arriving at a newly formed
vent habitat. Paradoxically, its fecundity is at least an
order of magnitude lower than that of Riftia pachyptila,
which generally arrives much later. Annual fecundities
and lifetime fecundities cannot be estimated for any
deep-sea polychaete because of inadequate information
about longevity, spawning frequency, and the speed
of the gametogenic cycle. It would not be surprising,
however, if the annual fecundity of the continuously
reproducting giant tube worm Riftia pachyptila greatly
exceeds the fecundity of any lecithotrophic polychaete
from shallow water.
Embryogenesis and larval development
Although the developmental modes of most deep-sea
polychaetes remain completely unknown, a number of
species are known to brood their young. Levin et al.
(1994) listed eleven species of known brooders on
the slopes of the Volcano 7 seamount in the eastern
Pacific (Table 12.2). The mechanism of brooding
varies among species, some brooding larvae in the
tubes, others employing specialized brood pouches, and
still others having the larvae attached directly to the
segments of the mother, either laterally or dorsally
(Levin et al., 1994). Because it is much easier to
document brooding than other forms of reproduction,
it is still not known whether brooding predominates
among deep-sea polychaetes, or whether a majority
of species have indirect development. Levin et al.