414
Craig M. YOUNG
Sewell et al., 1998). Planktotrophic development is
found in a number of north Atlantic echinoderms,
including the sea stars Plutonaster bifrons and Psilaster
andromeda (Tyler and Pain, 1982a), the echinoids
Cidaris cidaris (Prouho, 1888) and Echinus affinis
(Tyler and Gage, 1984a), and the ophiuroids Ophiocten
gracilis (Sumida et al., 2000) and Ophiura ljungmani
(Tyler and Gage, 1980). However, these represent
only a small fraction of the species studied; the
remainder have pelagic lecithotrophic development.
In tropical seas, planktotrophic development is very
common among echinoids (e.g., Young and Cameron,
1989; Young and George, 2000), though pelagic
lecithotrophic development remains the most common
mode of development in asteroids and holothuroids.
Lecithotrophic echinoderm eggs that have been studied
are buoyant and probably move great distances up into
the water column. For example, the eggs of soft-bodied
echinothuriid echinoids, which are more than 1 mm in
diameter, float at a rate that should carry them to the
surface from 1000 m depth in about 4 days (Young and
Cameron, 1987; Cameron et al., 1988).
Many gastropods in the deep sea lay egg masses
(Table 12.5), but some hatch in the capsule and
emerge as crawl-away juveniles (Bouchet and War´ en,
1994); the proportion of species that hatch from
the egg mass to disperse in the water column as
either lecithotrophic or planktotrophic pelagic larvae
is unknown. Most of what is known concerning the
development of deep-sea molluscs has been inferred
from the dimensions of the protoconch (gastropods) or
prodissoconch (bivalves) at the apex of the adult shell
(Rex and War´ en, 1982; Scheltema, 1994; Gustafson
and Lutz, 1994), although also some deep-sea larvae
have been captured in near-surface plankton hauls
(Bouchet, 1976b). Xylophagid bivalves in the deep
sea have very small eggs, planktotrophic larvae, and
high fecundity, as might be expected for weedy species
that rapidly colonize isolated and ephemeral substrata
(Turner, 1973). Most vent and seep molluscs, including
a myriad of tiny limpets, are apparently lecithotrophic,
though some of the large-bodied mussels probably
produce planktotrophic larvae (Gustafson and Lutz,
1994).
One could argue that brooding is the most typical
reproductive mode in the deep sea, since peracarid
crustaceans brood and are also the most speciose group
in the deep sea. However, from an evolutionary standpoint, one cannot argue that there has been selection
for brooding in the deep sea, since without exception
shallow water peracarids also brood their young. Thus,
it appears that these animals are constrained phylogenetically to brooding as a developmental strategy.
Decapod crustaceans, including brachyuran crabs and
caridean shrimps, generally produce planktonic larvae,
many of which are probably planktotrophic. The larva
of an abyssal eryonid crab was described at the time
of the Challenger expedition (von Willemoes-Suhm,
1876). The abundant vent shrimp on the Mid-Atlantic
Ridge, Rimicaris exoculata, is known to produce
larvae, some of which have been captured in plankton
trawls many hundreds of meters above the bottom
(Herring and Dixon, 1998).
Developmental modes of deep-sea infaunal polychaetes have been inferred from egg size in a number
of studies (Table 12.1). Planktotrophy, lecithotrophy
and brooding strategies are all present in these species,
though lecithotrophic development again predominates
and phylogenetic patterns of developmental mode are
generally conserved. At hydrothermal vents and cold
seeps, a number of polychaetes have been studied, most
of which probably have larval development. Deep-sea
perviate pogonophorans, now considered polychaetes,
brood their embryos in their tubes, eventually releasing
larval forms that are probably short-lived and demersal
(Baake, 1976), but the related siboglinid polychaetes
from vents and seeps produce small yolky eggs (100–
115 mm) which develop into lecithotrophic trochophore
larvae (Young et al., 1996b; Marsh et al., 2001). Wax
esters confer buoyancy on these eggs despite their small
size (Marsh et al., 2001).
Larval dispersal and migration
Background
Both the role of dispersal in evolution and the functions of dispersal in the life cycles of individual animals
remain somewhat controversial, but most arguments
revolve around the genetic advantages of outcrossing
and the spread of risk among spatially and temporally heterogeneous environments (Strathmann, 1974;
Palmer and Strathmann, 1981). Dispersal could also
be important for reducing competition when resources
are limiting locally. In the marine environment, most
species disperse by means of microscopic larvae. Larvae may disperse for very long distances or they may
have short lives and limited dispersal. With few known
exceptions, those shallow-water species that disperse
for long distances have feeding (planktotrophic) larvae,
whereas most philopatric dispersers have lecithotrophic
Craig M. YOUNG
Sewell et al., 1998). Planktotrophic development is
found in a number of north Atlantic echinoderms,
including the sea stars Plutonaster bifrons and Psilaster
andromeda (Tyler and Pain, 1982a), the echinoids
Cidaris cidaris (Prouho, 1888) and Echinus affinis
(Tyler and Gage, 1984a), and the ophiuroids Ophiocten
gracilis (Sumida et al., 2000) and Ophiura ljungmani
(Tyler and Gage, 1980). However, these represent
only a small fraction of the species studied; the
remainder have pelagic lecithotrophic development.
In tropical seas, planktotrophic development is very
common among echinoids (e.g., Young and Cameron,
1989; Young and George, 2000), though pelagic
lecithotrophic development remains the most common
mode of development in asteroids and holothuroids.
Lecithotrophic echinoderm eggs that have been studied
are buoyant and probably move great distances up into
the water column. For example, the eggs of soft-bodied
echinothuriid echinoids, which are more than 1 mm in
diameter, float at a rate that should carry them to the
surface from 1000 m depth in about 4 days (Young and
Cameron, 1987; Cameron et al., 1988).
Many gastropods in the deep sea lay egg masses
(Table 12.5), but some hatch in the capsule and
emerge as crawl-away juveniles (Bouchet and War´ en,
1994); the proportion of species that hatch from
the egg mass to disperse in the water column as
either lecithotrophic or planktotrophic pelagic larvae
is unknown. Most of what is known concerning the
development of deep-sea molluscs has been inferred
from the dimensions of the protoconch (gastropods) or
prodissoconch (bivalves) at the apex of the adult shell
(Rex and War´ en, 1982; Scheltema, 1994; Gustafson
and Lutz, 1994), although also some deep-sea larvae
have been captured in near-surface plankton hauls
(Bouchet, 1976b). Xylophagid bivalves in the deep
sea have very small eggs, planktotrophic larvae, and
high fecundity, as might be expected for weedy species
that rapidly colonize isolated and ephemeral substrata
(Turner, 1973). Most vent and seep molluscs, including
a myriad of tiny limpets, are apparently lecithotrophic,
though some of the large-bodied mussels probably
produce planktotrophic larvae (Gustafson and Lutz,
1994).
One could argue that brooding is the most typical
reproductive mode in the deep sea, since peracarid
crustaceans brood and are also the most speciose group
in the deep sea. However, from an evolutionary standpoint, one cannot argue that there has been selection
for brooding in the deep sea, since without exception
shallow water peracarids also brood their young. Thus,
it appears that these animals are constrained phylogenetically to brooding as a developmental strategy.
Decapod crustaceans, including brachyuran crabs and
caridean shrimps, generally produce planktonic larvae,
many of which are probably planktotrophic. The larva
of an abyssal eryonid crab was described at the time
of the Challenger expedition (von Willemoes-Suhm,
1876). The abundant vent shrimp on the Mid-Atlantic
Ridge, Rimicaris exoculata, is known to produce
larvae, some of which have been captured in plankton
trawls many hundreds of meters above the bottom
(Herring and Dixon, 1998).
Developmental modes of deep-sea infaunal polychaetes have been inferred from egg size in a number
of studies (Table 12.1). Planktotrophy, lecithotrophy
and brooding strategies are all present in these species,
though lecithotrophic development again predominates
and phylogenetic patterns of developmental mode are
generally conserved. At hydrothermal vents and cold
seeps, a number of polychaetes have been studied, most
of which probably have larval development. Deep-sea
perviate pogonophorans, now considered polychaetes,
brood their embryos in their tubes, eventually releasing
larval forms that are probably short-lived and demersal
(Baake, 1976), but the related siboglinid polychaetes
from vents and seeps produce small yolky eggs (100–
115 mm) which develop into lecithotrophic trochophore
larvae (Young et al., 1996b; Marsh et al., 2001). Wax
esters confer buoyancy on these eggs despite their small
size (Marsh et al., 2001).
Larval dispersal and migration
Background
Both the role of dispersal in evolution and the functions of dispersal in the life cycles of individual animals
remain somewhat controversial, but most arguments
revolve around the genetic advantages of outcrossing
and the spread of risk among spatially and temporally heterogeneous environments (Strathmann, 1974;
Palmer and Strathmann, 1981). Dispersal could also
be important for reducing competition when resources
are limiting locally. In the marine environment, most
species disperse by means of microscopic larvae. Larvae may disperse for very long distances or they may
have short lives and limited dispersal. With few known
exceptions, those shallow-water species that disperse
for long distances have feeding (planktotrophic) larvae,
whereas most philopatric dispersers have lecithotrophic