22
David THISTLE
Fig. 2.17. The proportion of asteroid feeding types at increasing
depth, showing the shift to omnivory as depth increases. Modified
from Carey (1972). Reproduced by permission of Ophelia
Publications.
living and dead material in the deep sea. For example,
Carey (1972) reports a trend for the proportion of
predaceous asteroids to decrease and the proportion of
omnivorous asteroids to increase with increasing depth
in the deep sea (Fig. 2.17).
Although prey are rare, they may be more detectable
in the deep sea than in shallow water. Flow in the
benthic boundary layer is slower and more orderly
in the deep sea than in shallow water, so chemical
gradients should be more persistent and provide better
information for prey location. Also, pressure waves
produced by prey (Ockelmann and Vahl, 1970) should
be more easily detected in the deep-sea benthic boundary layer because of its lower turbulence. Background
acoustic noise is also lower in the deep sea, making
weak acoustic signals produced by prey relatively easy
to detect. These physical attributes of the deep sea
also facilitate the transmission of information to the
prey about the approach of a predator, so that sensory
capabilities of the prey may be evolving in parallel with
those of the predators (Jumars and Gallagher, 1982).
The general decrease in food input with increasing
depth in the deep sea appears to affect predators disproportionately. For example, Rex et al. (1990) found
that abundance of predaceous gastropods decreased at a
greater rate with depth than did that of deposit-feeding
gastropods (Fig. 2.18). One possible explanation for
this pattern is that, as the distances between prey
increase with depth, the energy spent in searching
0
1
2
3
0
1000
2000
3000
4000
Depth (m)
Number per 0.09 m
2
Neogastropoda
Opisthobranchia
Mesogastropoda
Archaeogastropoda
Fig. 2.18. Logarithm of abundance of major taxonomic groups
of gastropods at different depths in the North Atlantic, showing
that predators decline more rapidly than do deposit feeders.
Neogastropoda and Opisthobranchia are predators. Modified from
Rex et al. (1990). Copyright: Elsevier Science.
increases, but the energetic return per prey item found
remains the same. Therefore, as depth increases, fewer
gastropod species (and perhaps fewer species of other
taxa) can make an energetic profit as predators.
Croppers
In the food-poor deep sea, there should be strong
selection to digest and assimilate any organic material
encountered, living or dead. Dayton and Hessler (1972)
proposed the term “cropper” for an animal that ingests
live prey, whether exclusively or in combination with
dead prey or inorganic materials. Deep-sea croppers
include species of holothurians, echinoids, ophiuroids,
asteroids, cephalopods, and some polychaetes, decapods, and demersal fishes. Most deposit feeders
in the deep sea are croppers because they feed on
living and dead material. Given the large proportion of
deposit feeders among deep-sea-floor animals, much of
the living prey may be consumed by deposit feeders
(Dayton and Hessler, 1972).
There are several corollaries of this view. Deposit
feeders vary in size from fishes to nematodes. Given
that food is in short supply, the size of the prey ingested
should be limited only by the size of the deposit feeder’s
mouth. Therefore, the smaller the prey organism, the
greater its predation risk because the number of mouths
large enough to ingest it increases as its size decreases.
Thus larvae, juveniles, and meiofauna of all life stages
should experience more intense predation than do
macrofaunal and megafaunal adults. This increase in
predation pressure with decreasing size should decrease
the probability of competitive exclusion among smaller
animals, and allow larger overlaps in their utilization of
resources. In particular, as an animal’s size decreases,
David THISTLE
Fig. 2.17. The proportion of asteroid feeding types at increasing
depth, showing the shift to omnivory as depth increases. Modified
from Carey (1972). Reproduced by permission of Ophelia
Publications.
living and dead material in the deep sea. For example,
Carey (1972) reports a trend for the proportion of
predaceous asteroids to decrease and the proportion of
omnivorous asteroids to increase with increasing depth
in the deep sea (Fig. 2.17).
Although prey are rare, they may be more detectable
in the deep sea than in shallow water. Flow in the
benthic boundary layer is slower and more orderly
in the deep sea than in shallow water, so chemical
gradients should be more persistent and provide better
information for prey location. Also, pressure waves
produced by prey (Ockelmann and Vahl, 1970) should
be more easily detected in the deep-sea benthic boundary layer because of its lower turbulence. Background
acoustic noise is also lower in the deep sea, making
weak acoustic signals produced by prey relatively easy
to detect. These physical attributes of the deep sea
also facilitate the transmission of information to the
prey about the approach of a predator, so that sensory
capabilities of the prey may be evolving in parallel with
those of the predators (Jumars and Gallagher, 1982).
The general decrease in food input with increasing
depth in the deep sea appears to affect predators disproportionately. For example, Rex et al. (1990) found
that abundance of predaceous gastropods decreased at a
greater rate with depth than did that of deposit-feeding
gastropods (Fig. 2.18). One possible explanation for
this pattern is that, as the distances between prey
increase with depth, the energy spent in searching
0
1
2
3
0
1000
2000
3000
4000
Depth (m)
Number per 0.09 m
2
Neogastropoda
Opisthobranchia
Mesogastropoda
Archaeogastropoda
Fig. 2.18. Logarithm of abundance of major taxonomic groups
of gastropods at different depths in the North Atlantic, showing
that predators decline more rapidly than do deposit feeders.
Neogastropoda and Opisthobranchia are predators. Modified from
Rex et al. (1990). Copyright: Elsevier Science.
increases, but the energetic return per prey item found
remains the same. Therefore, as depth increases, fewer
gastropod species (and perhaps fewer species of other
taxa) can make an energetic profit as predators.
Croppers
In the food-poor deep sea, there should be strong
selection to digest and assimilate any organic material
encountered, living or dead. Dayton and Hessler (1972)
proposed the term “cropper” for an animal that ingests
live prey, whether exclusively or in combination with
dead prey or inorganic materials. Deep-sea croppers
include species of holothurians, echinoids, ophiuroids,
asteroids, cephalopods, and some polychaetes, decapods, and demersal fishes. Most deposit feeders
in the deep sea are croppers because they feed on
living and dead material. Given the large proportion of
deposit feeders among deep-sea-floor animals, much of
the living prey may be consumed by deposit feeders
(Dayton and Hessler, 1972).
There are several corollaries of this view. Deposit
feeders vary in size from fishes to nematodes. Given
that food is in short supply, the size of the prey ingested
should be limited only by the size of the deposit feeder’s
mouth. Therefore, the smaller the prey organism, the
greater its predation risk because the number of mouths
large enough to ingest it increases as its size decreases.
Thus larvae, juveniles, and meiofauna of all life stages
should experience more intense predation than do
macrofaunal and megafaunal adults. This increase in
predation pressure with decreasing size should decrease
the probability of competitive exclusion among smaller
animals, and allow larger overlaps in their utilization of
resources. In particular, as an animal’s size decreases,