REPRODUCTION, DEVELOPMENT AND LIFE-HISTORY TRAITS
407
shelf/slope break where the environment becomes more
stable. Slope snails seem to allocate more energy to
growth and less to reproduction than conspecifics on
the shelf. A similar analysis within the protobranch
bivalve genus Nucula was conducted by Scheltema
(1972) over a depth gradient from the shelf to the
abyss. He showed that species from the abyss allocated
much less energy to reproduction than did their
shallow congeners, and also that abyssal species tended
to have lower fecundities and larger eggs. All of
these observations are consistent with the predicted
shift from r- to K-selection with increasing depth
and environmental stability. However, the opposite
may hold true for brachyuran crabs. Hines (1988)
noted that deep-sea brachyurans in the genus Chaceon
(formerly Geryon) have higher reproductive outputs
than shallow-water crabs of comparable sizes, but that
their fecundities were lower because of larger egg
sizes.
The r–K selection theory predicts that animals
should grow slowly, mature late, and live to a greater
age in more stable environments. The most dramatic
and oft-cited example of this is in the deep-sea
protobranch bivalve Tindaria callistiformis from 3.8 km
depth, which, on the basis of radioisotopic dating and
shell sculpturing are estimated to attain reproductive
maturity in about 50 years and to live for more
than a century (Turekian et al., 1975). Gage and
Tyler (1991) have discussed potential sources of error
in this life span estimate, which has a confidence
interval of 76 years. Similar longevities have been
inferred for shallow-water bivalves using a variety of
techniques (Breen and Shields, 1983; Jones et al.,
1978), so long-lived clams are not unique to the very
stable habitats of the abyss. Moreover, analysis of
isotopes in deep-sea clams on the Galapagos Rise
reveals very fast growth rates and shorter life spans
(Turekian et al., 1979) Analysis of size-frequency
data of protobranch bivalves in the Rockall Trough
indicate that growth rates are fairly rapid, and that
clams there live for only about eight years (Gage,
1985), which is similar to the expected longevity of
a small clam from inshore waters (Gage and Tyler,
1991). Grassle (1977) found reproductively mature
protobranchs (Deminucula cancellata) in colonization
trays that had only been on the sea floor for about two
years. Similar data from sediment trays indicate that the
aplacophoran mollusc Prochaetoderma yongei attains
full adult size in only two months and reproductive
maturity in less than a year (Scheltema, 1987). Taken
together, these data do not support the contention that
deep-sea bivalves live longer, mature later, or grow
slower than shallow-water species.
Gage and Tyler (1985) have analysed growth and
longevity in the sea urchin Echinus affinis from
a depth of 2200 meters on the Hebridean Slope.
This species lives for up to 28 years and attains
reproductive maturity after about five years. A shallowwater congener, Echinus esculentus, lives for only
up to 12 years and attains maturity after about 1.5–
2.5 years (Nichols et al., 1985). Other echinoids from
various shallow temperate and tropical seas are likewise
shorter-lived and attain reproductive maturity much
faster (Ebert, 1982) than E. affinis. The limited data on
echinoids are consistent with a shift toward K-strategy
in the deep sea. However, other predicted attributes of
a K-adapted species do not apply to Echinus affinis.
This species has high fecundity, small egg size (Tyler
and Gage, 1984a), produces a pelagic larva (Young
and Tyler, 1993) and sometimes recruits in very large
numbers (Gage and Tyler, 1985).
There have been no explicit analyses comparing
“r-selected” and “K-selected” traits of deep-sea animals
from vents and seeps with animals that rely on
allochthonous food in the “normal” deep sea. The
predictions would be ambiguous in any case, since the
food supply of vent/seep animals is more predictable
and constant, yet the environment itself may remain
stable for much less time.
“Bet hedging” models are currently difficult to
evaluate for deep-sea habitats because there are no data
whatsoever on juvenile vs. adult survival probabilities
in the deep sea. It would seem reasonable to assume
that juvenile survival is less variable in the stable deep
sea than in temporally more variable shallow systems.
However, Jumars and Gallagher (1983) have argued
that, in the virtual absence of physical disturbance,
predation becomes the most important source of
mortality and that predation pressure should be more
intense on younger animals because more mouth sizes
would be able to ingest them. An extreme case of
heavy juvenile mortality has been documented in the
ophiuroid Ophiocten gracilis (Gage and Tyler, 1981).
Also, Gage et al. (1980) and Gage and Tyler (1981)
have invoked temporally variable juvenile mortality as
a possible explanation for apparent seasonal recruitment in species that reproduce continuously.
In summary, there is evidence for a depth-related
increase in certain K−selected traits within species and
families of a few taxa, but there is also abundant
407
shelf/slope break where the environment becomes more
stable. Slope snails seem to allocate more energy to
growth and less to reproduction than conspecifics on
the shelf. A similar analysis within the protobranch
bivalve genus Nucula was conducted by Scheltema
(1972) over a depth gradient from the shelf to the
abyss. He showed that species from the abyss allocated
much less energy to reproduction than did their
shallow congeners, and also that abyssal species tended
to have lower fecundities and larger eggs. All of
these observations are consistent with the predicted
shift from r- to K-selection with increasing depth
and environmental stability. However, the opposite
may hold true for brachyuran crabs. Hines (1988)
noted that deep-sea brachyurans in the genus Chaceon
(formerly Geryon) have higher reproductive outputs
than shallow-water crabs of comparable sizes, but that
their fecundities were lower because of larger egg
sizes.
The r–K selection theory predicts that animals
should grow slowly, mature late, and live to a greater
age in more stable environments. The most dramatic
and oft-cited example of this is in the deep-sea
protobranch bivalve Tindaria callistiformis from 3.8 km
depth, which, on the basis of radioisotopic dating and
shell sculpturing are estimated to attain reproductive
maturity in about 50 years and to live for more
than a century (Turekian et al., 1975). Gage and
Tyler (1991) have discussed potential sources of error
in this life span estimate, which has a confidence
interval of 76 years. Similar longevities have been
inferred for shallow-water bivalves using a variety of
techniques (Breen and Shields, 1983; Jones et al.,
1978), so long-lived clams are not unique to the very
stable habitats of the abyss. Moreover, analysis of
isotopes in deep-sea clams on the Galapagos Rise
reveals very fast growth rates and shorter life spans
(Turekian et al., 1979) Analysis of size-frequency
data of protobranch bivalves in the Rockall Trough
indicate that growth rates are fairly rapid, and that
clams there live for only about eight years (Gage,
1985), which is similar to the expected longevity of
a small clam from inshore waters (Gage and Tyler,
1991). Grassle (1977) found reproductively mature
protobranchs (Deminucula cancellata) in colonization
trays that had only been on the sea floor for about two
years. Similar data from sediment trays indicate that the
aplacophoran mollusc Prochaetoderma yongei attains
full adult size in only two months and reproductive
maturity in less than a year (Scheltema, 1987). Taken
together, these data do not support the contention that
deep-sea bivalves live longer, mature later, or grow
slower than shallow-water species.
Gage and Tyler (1985) have analysed growth and
longevity in the sea urchin Echinus affinis from
a depth of 2200 meters on the Hebridean Slope.
This species lives for up to 28 years and attains
reproductive maturity after about five years. A shallowwater congener, Echinus esculentus, lives for only
up to 12 years and attains maturity after about 1.5–
2.5 years (Nichols et al., 1985). Other echinoids from
various shallow temperate and tropical seas are likewise
shorter-lived and attain reproductive maturity much
faster (Ebert, 1982) than E. affinis. The limited data on
echinoids are consistent with a shift toward K-strategy
in the deep sea. However, other predicted attributes of
a K-adapted species do not apply to Echinus affinis.
This species has high fecundity, small egg size (Tyler
and Gage, 1984a), produces a pelagic larva (Young
and Tyler, 1993) and sometimes recruits in very large
numbers (Gage and Tyler, 1985).
There have been no explicit analyses comparing
“r-selected” and “K-selected” traits of deep-sea animals
from vents and seeps with animals that rely on
allochthonous food in the “normal” deep sea. The
predictions would be ambiguous in any case, since the
food supply of vent/seep animals is more predictable
and constant, yet the environment itself may remain
stable for much less time.
“Bet hedging” models are currently difficult to
evaluate for deep-sea habitats because there are no data
whatsoever on juvenile vs. adult survival probabilities
in the deep sea. It would seem reasonable to assume
that juvenile survival is less variable in the stable deep
sea than in temporally more variable shallow systems.
However, Jumars and Gallagher (1983) have argued
that, in the virtual absence of physical disturbance,
predation becomes the most important source of
mortality and that predation pressure should be more
intense on younger animals because more mouth sizes
would be able to ingest them. An extreme case of
heavy juvenile mortality has been documented in the
ophiuroid Ophiocten gracilis (Gage and Tyler, 1981).
Also, Gage et al. (1980) and Gage and Tyler (1981)
have invoked temporally variable juvenile mortality as
a possible explanation for apparent seasonal recruitment in species that reproduce continuously.
In summary, there is evidence for a depth-related
increase in certain K−selected traits within species and
families of a few taxa, but there is also abundant