402
Craig M. YOUNG
Embryogenesis and larval development
Early predictions notwithstanding, only two deepsea echinoderm species, the holothurian Oneirophanta
mutabilis (Hansen, 1968) and the concentricycloid
Xyloplax medusiformis (Rowe et al., 1988), are known
to brood their young internally, though it is likely
that many ophiuroids and a few asteroids will also
eventually be found to brood. Although a number of
shallow-water or eurybathic species from Antarctica
are known to be brooders (Thomson, 1878; Agassiz,
1881; Sladen, 1889), all other species of strictly bathyal
or abyssal echinoderms, including many which were
once assumed on the basis of egg size to be brooders (Agassiz, 1881; Thorson, 1946; Madsen, 1961)
reproduce with either pelagic lecithotrophic larvae or
planktotrophic larvae (Pearse, 1994; Young, 1994a).
The lecithotrophs, exemplified by the echinothuriid
echinoids (Tyler and Gage, 1984a), elasipod holothurians (Tyler and Billett, 1987) and porcellanasterid sea
stars (Madsen, 1961; Ramirez Llodra et al., 2002), have
yolky eggs approaching or exceeding 1 mm in diameter.
Although it is clear from morphology and observation
that these large eggs float (Young and Cameron, 1987;
Cameron et al., 1988) and that these species do not
retain their large eggs, lecithotrophic larvae have not
been described for any deep-sea echinoderm. Indeed,
Tyler and Billett (1987) have suggested that the large
eggs of elasipod holothurians are direct developers
which bypass a larval stage and develop directly into
swimming juveniles of the sort commonly found in
mid-water trawls (Gebruk et al., 1997).
Planktotrophic larvae have been reared for a number
of echinoderm species living at bathyal depths, including the ophiuroid Ophiocten gracilis (Mortensen, 1898;
Tyler and Gage, 1982) and the echinoid Cidaris cidaris
from European waters (Prouho, 1888), the echinoid
Laganum diploporum from Japan (Mortensen, 1921),
and nine species of echinoids from the Bahamian
slope: Archaeopneustes hystrix (Young et al., 1996c),
Aspidodiadema jacobyi (Young et al., 1989; Young
and George, 2000), Cidaris blakei (Young, unpublished), Coelopleurus floridanus (Young, unpublished),
Conolampas sigsbei (C.M. Young and P.A. Tyler,
unpublished), Linopneustes longispinus (Young and
Cameron, 1989), Lytechinus euerces (Young et al.,
1996c). Palaeobrissus hilgardi (C.M. Young and
J.L. Cameron, unpublished) and Stylocidaris lineata
(Young et al., 1996c, 1997). With the exception of
Aspidodiadema jacobyi (Young and George, 2000), all
of the larvae described are morphologically similar
to shallow-water members of the taxa to which they
belong.
The larvae of Aspidodiadema jacobyi are unusual in
several ways. Like typical planktotrophs, they develop
from a small egg (90 mm) but, unlike other species,
development of a mouth is delayed for the first
21 days, as the blastocoel becomes filled with yolky
cells from the vegetal plate (Young et al., 1989).
The yolky cells permit dispersal and perhaps vertical
migration (Young et al., 1996c) for up to two months
before planktonic food is required (Young et al.,
1989). Eventually, the larva becomes a very large
and complicated echinopluteus having 12 arms, a long
posterior process and convoluted ciliary lobes (Young
and George, 2000).
Cnidaria
In the deep sea, benthic cnidarians including various octocorals, scleractinian corals and actinian and
zoanthic sea anemones, are particularly common on
hard substrata such as seamounts and boulders, and on
the volcanic rocks at some hydrothermal vents. Many
species that require a firm surface have adopted an
epizoic lifestyle, often becoming specialized for life on
a single species of echinoderm, sponge or gorgonian.
The most common cnidarians on soft bottoms are
pennatulids (sea pens), which anchor their rachises in
the sediment like a root.
Gonads, gametogenesis and reproductive
periodicity
Aspects of reproduction have recently been described
for several deep-sea pennatulid octocorals. Kophobelemnon stelliferum, a sea pen from the Porcupine
Seabight in the northeast Atlantic, produces large eggs
(800 mm), has separate sexes and apparently breeds
continuously (Rice et al., 1992). Members of the
cosmopolitan pennatulid genus Umbellula have eggs
of a similar size which originate on the mesenterial
filament, then complete oogenesis while floating freely
in the gastrovascular cavity (Tyler et al., 1995).
Ultrastructural details of gametogenesis have been
described for Pennatula aculeata, a species that lives
in relatively shallow (110 m) water and to a depth of
at least 1500 m (Eckelbarger et al., 1998). Females
of this species contained oocytes of all sizes and at
all stages of oogenesis, suggesting that breeding is
aperiodic. Sperm were packaged in sperm cysts, each
of which contained spermatocytes at a comparable
stage of development. The sperm remain packaged
Craig M. YOUNG
Embryogenesis and larval development
Early predictions notwithstanding, only two deepsea echinoderm species, the holothurian Oneirophanta
mutabilis (Hansen, 1968) and the concentricycloid
Xyloplax medusiformis (Rowe et al., 1988), are known
to brood their young internally, though it is likely
that many ophiuroids and a few asteroids will also
eventually be found to brood. Although a number of
shallow-water or eurybathic species from Antarctica
are known to be brooders (Thomson, 1878; Agassiz,
1881; Sladen, 1889), all other species of strictly bathyal
or abyssal echinoderms, including many which were
once assumed on the basis of egg size to be brooders (Agassiz, 1881; Thorson, 1946; Madsen, 1961)
reproduce with either pelagic lecithotrophic larvae or
planktotrophic larvae (Pearse, 1994; Young, 1994a).
The lecithotrophs, exemplified by the echinothuriid
echinoids (Tyler and Gage, 1984a), elasipod holothurians (Tyler and Billett, 1987) and porcellanasterid sea
stars (Madsen, 1961; Ramirez Llodra et al., 2002), have
yolky eggs approaching or exceeding 1 mm in diameter.
Although it is clear from morphology and observation
that these large eggs float (Young and Cameron, 1987;
Cameron et al., 1988) and that these species do not
retain their large eggs, lecithotrophic larvae have not
been described for any deep-sea echinoderm. Indeed,
Tyler and Billett (1987) have suggested that the large
eggs of elasipod holothurians are direct developers
which bypass a larval stage and develop directly into
swimming juveniles of the sort commonly found in
mid-water trawls (Gebruk et al., 1997).
Planktotrophic larvae have been reared for a number
of echinoderm species living at bathyal depths, including the ophiuroid Ophiocten gracilis (Mortensen, 1898;
Tyler and Gage, 1982) and the echinoid Cidaris cidaris
from European waters (Prouho, 1888), the echinoid
Laganum diploporum from Japan (Mortensen, 1921),
and nine species of echinoids from the Bahamian
slope: Archaeopneustes hystrix (Young et al., 1996c),
Aspidodiadema jacobyi (Young et al., 1989; Young
and George, 2000), Cidaris blakei (Young, unpublished), Coelopleurus floridanus (Young, unpublished),
Conolampas sigsbei (C.M. Young and P.A. Tyler,
unpublished), Linopneustes longispinus (Young and
Cameron, 1989), Lytechinus euerces (Young et al.,
1996c). Palaeobrissus hilgardi (C.M. Young and
J.L. Cameron, unpublished) and Stylocidaris lineata
(Young et al., 1996c, 1997). With the exception of
Aspidodiadema jacobyi (Young and George, 2000), all
of the larvae described are morphologically similar
to shallow-water members of the taxa to which they
belong.
The larvae of Aspidodiadema jacobyi are unusual in
several ways. Like typical planktotrophs, they develop
from a small egg (90 mm) but, unlike other species,
development of a mouth is delayed for the first
21 days, as the blastocoel becomes filled with yolky
cells from the vegetal plate (Young et al., 1989).
The yolky cells permit dispersal and perhaps vertical
migration (Young et al., 1996c) for up to two months
before planktonic food is required (Young et al.,
1989). Eventually, the larva becomes a very large
and complicated echinopluteus having 12 arms, a long
posterior process and convoluted ciliary lobes (Young
and George, 2000).
Cnidaria
In the deep sea, benthic cnidarians including various octocorals, scleractinian corals and actinian and
zoanthic sea anemones, are particularly common on
hard substrata such as seamounts and boulders, and on
the volcanic rocks at some hydrothermal vents. Many
species that require a firm surface have adopted an
epizoic lifestyle, often becoming specialized for life on
a single species of echinoderm, sponge or gorgonian.
The most common cnidarians on soft bottoms are
pennatulids (sea pens), which anchor their rachises in
the sediment like a root.
Gonads, gametogenesis and reproductive
periodicity
Aspects of reproduction have recently been described
for several deep-sea pennatulid octocorals. Kophobelemnon stelliferum, a sea pen from the Porcupine
Seabight in the northeast Atlantic, produces large eggs
(800 mm), has separate sexes and apparently breeds
continuously (Rice et al., 1992). Members of the
cosmopolitan pennatulid genus Umbellula have eggs
of a similar size which originate on the mesenterial
filament, then complete oogenesis while floating freely
in the gastrovascular cavity (Tyler et al., 1995).
Ultrastructural details of gametogenesis have been
described for Pennatula aculeata, a species that lives
in relatively shallow (110 m) water and to a depth of
at least 1500 m (Eckelbarger et al., 1998). Females
of this species contained oocytes of all sizes and at
all stages of oogenesis, suggesting that breeding is
aperiodic. Sperm were packaged in sperm cysts, each
of which contained spermatocytes at a comparable
stage of development. The sperm remain packaged