404
Craig M. YOUNG
sumhi dredged from a depth of 2451 m off Nova Scotia.
This observation suggests that this species reproduces
in the same way as its shallow-water congeners, which
produce larvae that settle preferentially on the female
worm.
Bryozoa
On the basis of skeletal morphology, it appears
that some deep-sea bryozoans brood embryos and
release coronate larvae similar to those of shallowwater ascophorans (Davidson, 1880). Although there
has been considerable taxonomic work on deep-water
bryozoans in recent years, beginning with the bryozoan
volume of the Galathea reports (Hayward, 1981),
there have been no studies specifically directed at
reproduction.
Brachiopoda
Larvae of the deep-sea inarticulate brachiopod
Pelagodiscus atlanticus, which lives at depths ranging
from 365 m to 4435 m, were collected from shallow
water by Ashworth (1915). Other inarticulate larvae
have been taken in plankton samples between 1000
and 3000 m (Simroth, 1897; Eichler, 1911). There have
been no recent studies of reproduction in deep-sea
brachiopods.
Hemichordata
Enteropneust worms are relatively common in the
deep sea, though reproduction has only been examined
in a single species. The very elongate “spaghetti
worm,” Saxipendium coronatum, lives draped over
rocks in areas of diffuse hydrothermal flow at about
2500 m depth on the East Pacific Rise. Most individuals
that have been collected are male, suggesting a strongly
biased sex ratio. The testes, which are easily visible
through the transparent body wall, occur in series
along most of the trunk of the worm, giving the
worm the appearance of a double pearl necklace. The
sperm ultrastructure, as described by Franzen et al.
(1985), reveals mushroom-shaped primitive sperm suggestive of external fertilization. Franzen et al. (1985)
also reported small eggs, from which they inferred
planktotrophic development. However, a recent reexamination of this species (Young, unpublished data)
reveals very large floating eggs suggestive of either
lecithotrophic larval development (a developmental
mode unknown in the enteropneusts) or pelagic direct
development. Giant larvae (up to 22 mm diameter)of
an enteropneust known as Planktosphaera pelagica
have been collected from oceanic waters in both the
Atlantic and Pacific (Spengel, 1932; Hadfield and
Young, 1983). Although the adults remain unknown,
it is generally thought that these are the larvae of an
unknown enteropneust from the deep sea.
Chordata
Colonial ascidians collected from deep water have
often been found to contain brooded larvae, like their
shallow-water relatives (e.g., Herdman, 1886). A few
deep-water solitary ascidians, including Hypobythius
calycodes (Moseley, 1876) and some species of carnivorous octacnemid ascidians (Young and E. V´ azquez,
unpublished data) produce eggs much larger than those
of any known shallow-water solitary ascidian. It is not
known whether these gigantic eggs (nearly 1 mm in
diameter) develop directly, bypassing the larval stage,
or if they develop by means of lecithotrophic tadpoles
like the majority of shallow-water solitary ascidians.
Larval development has been described for only a
single bathyal species, Bathypera feminalba (Young and
V´ azquez, 1995). Tadpoles of this species were very
similar to those of shallow-water species in the same
family.
LIFE-HISTORY TRAITS IN THE DEEP SEA:
PREDICTIONS AND EVIDENCE
Life-history theory predicts traits that maximize fitness
of an organism in the particular environment where
it lives. These include brood size, size of young, age
distribution of reproductive effort, the interaction of
reproductive effort with adult mortality, and variation
in all of these traits among the progeny of an individual
(Stearns, 1976). In the context of marine organisms,
“brood size” is generally called “fecundity” and “size
of young” may be equated with egg size, which
often determines the mode of development. Most
marine animals reproduce until they die, so the “age
distribution of reproductive effort” is often equivalent
to age at first reproduction. To these one may add
a number of traits that are especially germane for
marine animals, including developmental mode, degree
of parental protection, and various traits that assure
successful fertilization (the “breeding strategy”: Young,
1999), including mating systems, spawning behaviors,
sperm chemotaxis and pheromonal communication.
An extensive literature on life-history traits considers
differences that are expected between species or populations occupying stable vs. unstable environments. Although there is considerable habitat variation at bathyal
and abyssal depths, extensive regions (e.g., abyssal
Craig M. YOUNG
sumhi dredged from a depth of 2451 m off Nova Scotia.
This observation suggests that this species reproduces
in the same way as its shallow-water congeners, which
produce larvae that settle preferentially on the female
worm.
Bryozoa
On the basis of skeletal morphology, it appears
that some deep-sea bryozoans brood embryos and
release coronate larvae similar to those of shallowwater ascophorans (Davidson, 1880). Although there
has been considerable taxonomic work on deep-water
bryozoans in recent years, beginning with the bryozoan
volume of the Galathea reports (Hayward, 1981),
there have been no studies specifically directed at
reproduction.
Brachiopoda
Larvae of the deep-sea inarticulate brachiopod
Pelagodiscus atlanticus, which lives at depths ranging
from 365 m to 4435 m, were collected from shallow
water by Ashworth (1915). Other inarticulate larvae
have been taken in plankton samples between 1000
and 3000 m (Simroth, 1897; Eichler, 1911). There have
been no recent studies of reproduction in deep-sea
brachiopods.
Hemichordata
Enteropneust worms are relatively common in the
deep sea, though reproduction has only been examined
in a single species. The very elongate “spaghetti
worm,” Saxipendium coronatum, lives draped over
rocks in areas of diffuse hydrothermal flow at about
2500 m depth on the East Pacific Rise. Most individuals
that have been collected are male, suggesting a strongly
biased sex ratio. The testes, which are easily visible
through the transparent body wall, occur in series
along most of the trunk of the worm, giving the
worm the appearance of a double pearl necklace. The
sperm ultrastructure, as described by Franzen et al.
(1985), reveals mushroom-shaped primitive sperm suggestive of external fertilization. Franzen et al. (1985)
also reported small eggs, from which they inferred
planktotrophic development. However, a recent reexamination of this species (Young, unpublished data)
reveals very large floating eggs suggestive of either
lecithotrophic larval development (a developmental
mode unknown in the enteropneusts) or pelagic direct
development. Giant larvae (up to 22 mm diameter)of
an enteropneust known as Planktosphaera pelagica
have been collected from oceanic waters in both the
Atlantic and Pacific (Spengel, 1932; Hadfield and
Young, 1983). Although the adults remain unknown,
it is generally thought that these are the larvae of an
unknown enteropneust from the deep sea.
Chordata
Colonial ascidians collected from deep water have
often been found to contain brooded larvae, like their
shallow-water relatives (e.g., Herdman, 1886). A few
deep-water solitary ascidians, including Hypobythius
calycodes (Moseley, 1876) and some species of carnivorous octacnemid ascidians (Young and E. V´ azquez,
unpublished data) produce eggs much larger than those
of any known shallow-water solitary ascidian. It is not
known whether these gigantic eggs (nearly 1 mm in
diameter) develop directly, bypassing the larval stage,
or if they develop by means of lecithotrophic tadpoles
like the majority of shallow-water solitary ascidians.
Larval development has been described for only a
single bathyal species, Bathypera feminalba (Young and
V´ azquez, 1995). Tadpoles of this species were very
similar to those of shallow-water species in the same
family.
LIFE-HISTORY TRAITS IN THE DEEP SEA:
PREDICTIONS AND EVIDENCE
Life-history theory predicts traits that maximize fitness
of an organism in the particular environment where
it lives. These include brood size, size of young, age
distribution of reproductive effort, the interaction of
reproductive effort with adult mortality, and variation
in all of these traits among the progeny of an individual
(Stearns, 1976). In the context of marine organisms,
“brood size” is generally called “fecundity” and “size
of young” may be equated with egg size, which
often determines the mode of development. Most
marine animals reproduce until they die, so the “age
distribution of reproductive effort” is often equivalent
to age at first reproduction. To these one may add
a number of traits that are especially germane for
marine animals, including developmental mode, degree
of parental protection, and various traits that assure
successful fertilization (the “breeding strategy”: Young,
1999), including mating systems, spawning behaviors,
sperm chemotaxis and pheromonal communication.
An extensive literature on life-history traits considers
differences that are expected between species or populations occupying stable vs. unstable environments. Although there is considerable habitat variation at bathyal
and abyssal depths, extensive regions (e.g., abyssal