THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
57
predominantly to sample females. Thus, there may be
a sexual disparity in response to predatory attacks.
Such physiological characteristics, which enhance
competitive fitness within a precise range of environmental conditions found within a specific depth
range, may well limit an organism’s ability to survive
and compete elsewhere in the water column. Thus
Argyropelecus species, which are so well adapted
to the light conditions at daytime depths of 250 to
600 m, may become progressively more susceptible
to visual predation the further they are displaced
vertically, either deeper or shallower. Such high degrees
of specialization have evolutionary implications (see
p. 61).
The mesopelagic zone can usually be subdivided
faunistically at about 600 to 700 m into a shallow
and a deep sub-zone. During the day, fishes inhabiting
the upper mesopelagic zone are predominantly mirrorsided with ventral photophores, and the decapod
crustaceans (e.g., Oplophorus spinosus, Sergestes spp.
and Systellaspis debilis) are mostly half red and half
transparent. In the deeper zone, on the other hand,
fishes with mirrorlike sides are replaced with species
with non-reflective sides; the advantage of mirrorsides, it would appear, diminishes quite rapidly with
increasing depth. Presumably in the dimmer light
conditions below 600 m any fish with mirror-sides
would tend to be lit up by any nearby flashes of
bioluminescence; consequently, the selective advantage
swings towards species with uniformly dark, nonreflective coloration. However, many deep mesopelagic
fishes still have small, simple ventral photophores. The
decapod species are totally red (e.g., Acanthephyra spp.
and Sergia spp.).
At dusk, most of the diel migrants, that move
up from daytime depths in the shallow mesopelagic
zone, readily migrate up through the pycnocline into
the wind-mixed layer, whereas those that migrate up
from the deep mesopelagic zone (mostly micronekton)
tend to halt at the base of the thermocline. The
majority of macroplanktonic organisms inhabiting the
deep mesopelagic depths are non-migrants.
Ecology of the bathypelagic zone
At about 1000 m, the maximum depth to which
detectable daylight penetrates in all but the clearest
oligotrophic waters, the mesopelagic zone gives way
to the bathypelagic zone. This depth also marks the
lower limit to the range of diel migration by most
micronekton at temperate latitudes. In the temperate
Atlantic, it also approximates to the deep oxygen
minimum and the base of the permanent thermocline
(Angel, 1989a), although at equatorial latitudes these
features are displaced closer to the surface (domed). It
is also the depth at which the greatest variety of pelagic
species are caught (see p. 62), despite pelagic biomass
having fallen to about one-tenth of that found near the
surface (see p. 54).
As the availability of food diminishes both quantitatively and qualitatively with increasing depth, the
cost/benefit balance of various physiological adaptations to the pelagic mode of life shifts (see pp. 72–
75). Marshall (1971) compared the characteristics of
two congeneric fish species Gonostoma denudatum and
G. bathyphilum, which are mesopelagic and bathypelagic, respectively (Fig. 3.7). Gonostoma denudatum,
the mesopelagic species, is a dark-backed, silverysided fish with prominent ventral photophores, and
a
b
fb
ot
em
ob
a
a
b
b
Fig. 3.7. Comparison between a) the mesopelagic fish Gonostoma
denudatum and b) the bathypelagic species G. bathyphilum,
illustrating the relatively small eyes and photophores in the
bathypelagic species. Also shown are their brains drawn to the same
scale, again showing marked differences in the sizes of the olfactory
bulb (ob), the forebrain (fb), the optic tectum (ot), the eminetia
granularis (em) and the corpus cerebellum (in black) between the
two species, and also the relative sizes of the gills on the first gill
arch. Redrawn from Marshall (1971).
57
predominantly to sample females. Thus, there may be
a sexual disparity in response to predatory attacks.
Such physiological characteristics, which enhance
competitive fitness within a precise range of environmental conditions found within a specific depth
range, may well limit an organism’s ability to survive
and compete elsewhere in the water column. Thus
Argyropelecus species, which are so well adapted
to the light conditions at daytime depths of 250 to
600 m, may become progressively more susceptible
to visual predation the further they are displaced
vertically, either deeper or shallower. Such high degrees
of specialization have evolutionary implications (see
p. 61).
The mesopelagic zone can usually be subdivided
faunistically at about 600 to 700 m into a shallow
and a deep sub-zone. During the day, fishes inhabiting
the upper mesopelagic zone are predominantly mirrorsided with ventral photophores, and the decapod
crustaceans (e.g., Oplophorus spinosus, Sergestes spp.
and Systellaspis debilis) are mostly half red and half
transparent. In the deeper zone, on the other hand,
fishes with mirrorlike sides are replaced with species
with non-reflective sides; the advantage of mirrorsides, it would appear, diminishes quite rapidly with
increasing depth. Presumably in the dimmer light
conditions below 600 m any fish with mirror-sides
would tend to be lit up by any nearby flashes of
bioluminescence; consequently, the selective advantage
swings towards species with uniformly dark, nonreflective coloration. However, many deep mesopelagic
fishes still have small, simple ventral photophores. The
decapod species are totally red (e.g., Acanthephyra spp.
and Sergia spp.).
At dusk, most of the diel migrants, that move
up from daytime depths in the shallow mesopelagic
zone, readily migrate up through the pycnocline into
the wind-mixed layer, whereas those that migrate up
from the deep mesopelagic zone (mostly micronekton)
tend to halt at the base of the thermocline. The
majority of macroplanktonic organisms inhabiting the
deep mesopelagic depths are non-migrants.
Ecology of the bathypelagic zone
At about 1000 m, the maximum depth to which
detectable daylight penetrates in all but the clearest
oligotrophic waters, the mesopelagic zone gives way
to the bathypelagic zone. This depth also marks the
lower limit to the range of diel migration by most
micronekton at temperate latitudes. In the temperate
Atlantic, it also approximates to the deep oxygen
minimum and the base of the permanent thermocline
(Angel, 1989a), although at equatorial latitudes these
features are displaced closer to the surface (domed). It
is also the depth at which the greatest variety of pelagic
species are caught (see p. 62), despite pelagic biomass
having fallen to about one-tenth of that found near the
surface (see p. 54).
As the availability of food diminishes both quantitatively and qualitatively with increasing depth, the
cost/benefit balance of various physiological adaptations to the pelagic mode of life shifts (see pp. 72–
75). Marshall (1971) compared the characteristics of
two congeneric fish species Gonostoma denudatum and
G. bathyphilum, which are mesopelagic and bathypelagic, respectively (Fig. 3.7). Gonostoma denudatum,
the mesopelagic species, is a dark-backed, silverysided fish with prominent ventral photophores, and
a
b
fb
ot
em
ob
a
a
b
b
Fig. 3.7. Comparison between a) the mesopelagic fish Gonostoma
denudatum and b) the bathypelagic species G. bathyphilum,
illustrating the relatively small eyes and photophores in the
bathypelagic species. Also shown are their brains drawn to the same
scale, again showing marked differences in the sizes of the olfactory
bulb (ob), the forebrain (fb), the optic tectum (ot), the eminetia
granularis (em) and the corpus cerebellum (in black) between the
two species, and also the relative sizes of the gills on the first gill
arch. Redrawn from Marshall (1971).