The Impact of Planktivory on the Life Histories of Estuarine Crabs
The Impact of Planktivory on the Life Histories of Estuarine Crabs
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Date
1986
Authors
Morgan, Steven G.
Advisor
Reaka, Marjorie L.
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Abstract
Estuarine crabs commonly display two larval dispersal
patterns in which larvae are either exported from or
retained within estuaries. Explanations for the disparate
dispersal patterns are that larvae differ in their susceptibility
to predation, which is greater within the estuary
than offshore, or in their physiological tolerances to the
large temperature and salinity fluctuations of the estuary.
Crab larvae which are exported from the estuary survived
better in stressful temperature and salinity combinations
than those which are retained, and thus the
physiological stress hypothesis was rejected. However,
exported larvae were more vulnerable to predation by
fishes and invertebrates than were retained larvae. The
long spines, large body sizes and behavioral responses of
retained crab larvae were more effective at deterring
predation by two fishes and eleven invertebrates in the
laboratory, than were the short spines and small body
sizes of exported larvae.
Spines generally were more effective against planktivorous
fishes than against invertebrates. Spines operated by effectively increasing the size of the larvae and their
unpalatability to fishes. Behavioral observations revealed that fishes repeatedly attacked zoeae, but would
quickly learn to avoid the noxious prey. The armor of
crab larvae enabled them to survive attacks, and fishes
repeatedly attempting to swallow long-spined crab larvae
frequently would die. Spines did not assist in the flotation
or stabilization of crab larvae.
Electivities of the three predominant fishes sampled
from the Newport River estuary, North Carolina, also were
greater for exported than retained species of crab larvae.
Predation by larval and juvenile fishes was greater upstream
compared to downstream, and greater diurnally than
nocturnally. By hatching on nocturnal high tides, larvae
rapidly disperse downstream where the probability of predation
diminishes. Semiterrestrial crabs hatch on nocturnal spring tides to prevent stranding in tidal creeks.
Thus, the hatching rhythms and dispersal patterns of crabs
appear to have evolved in concert to transport larvae into
coastal waters where the risk of predation is reduced, or
to retain larvae within estuaries depending upon the vulnerability
of the larva to predation.