The exceptionally productive areas of the epipelagic zone
are the nutrient-rich upwelling areas that occur in certain
regions along the western coasts of Africa (off Namibia and
the western Cape Province) and the Americas (off California and Peru). These upwelling areas produce vast blooms
of phytoplankton which support enormous populations of
zooplankton and plankton-feeding fishes and their predators (including man). Typical epipelagic fishes are the
tunas (Scombridae), marlin and sailfish (Istiophoridae),
dolphinfish (Coryphaena hippurus) , flyingfishes (Exocoetidae), blue shark (Prionace glauca), mako (Isurus oxyrinchus), and the whale shark (Rhincodon typus). At night the
epipelagic zone is visited by vertically migrating fishes from
deeper waters (e.g. Astronesthes, Chauliodus, Stomias and
various myctophids). These nocturnal visitors are termed
nyctoepipeJagic fishes.
The mesopeJagic region extends from the epipelagic
region to a depth of 1 000 metres. About 850 of the 1 250 or
so oceanic species of pelagic fishes live in this region. Mesopelagic fishes are generally small (few exceed 30 cm in
length) with large eyes and well-developed light organs.
Many species move up to the epipelagic region at night in
order to feed on the more abundant zooplankton of the surface waters. Typical mesopelagic fishes are the lanternfishes (Myctophidae, comprising some 250 species),
stomiiforms and barracudinas (Paralepididae).
The bathypeJagic region, from 1 000 to 4 000 metres, is
inhabited by fewer (about 150) species of fishes. This reduction in numbers of species (as well as individuals) indicates
the reduction in food with depth. The scarcity of food here
is further emphasised by the reduction in certain organ
systems of bathypelagic fishes. The skeletal and muscular
systems are greatly reduced, and the decrease in density
achieved by the reduction of these heavy tissues allows the
loss of the swimbladder. Situated below the reach of sunlight, the bathypelagic zone is lit only by the bioluminescence of the animals that live here; consequently, most
species have greatly reduced eyes. The lateral-line system
is, however, well developed in bathypelagic fishes. With
small populations scattered over a vast area, finding a mate
may be difficult. The olfactory organs in males of many
bathypelagic species are greatly enlarged, presumably as an
adaptation for tracking the scent of females in this sparsely
populated environment. In some species of ceratioid
anglerfishes the attraction of the males is carried one step
further. Once a juvenile male finds a female (which is much
larger than he is), he attaches to the female with his mouth,
becoming a sexual parasite permanently fixed to his mate.
Typical bathypelagic fishes (in addition to the ceratioids,
which represent about two-thirds of the fauna) are the
cyemid, derichthyid, serrivomerid and saccopharyngoid
eels, and the whalefishes (cetomimids).
The abyssopeJagic region, extending from 4 000 metres to
the bottom of the ocean, is the deepest and most barren of
BIOLOGY OF FISHES
the four main divisions of the oceanic zone. Because of the
difficulties of sampling this zone, the fauna of this vast abyss
is very poorly known. But we do know that the fishes here
are very few and far between. Abyssopelagic fishes include
the halosaurs, macrourids, stephanoberycids (Acanthochaenus lutkenii) and alepocephalids.
Estuaries are habitats that overlap the littoral and sublittoral zones. This environment is characterised by considerable fluctuations in temperature, salinity and turbidity.
Estuaries are relatively productive regions and serve a valuable function as "nursery areas" for the young of many sublittoral species. Some fishes spend their whole lives in estuaries (e.g. Sand goby, Psammogobius knysnaensis,
Knysna seahorse, Hippocampus capensis, and Monodactylus
falciformis); others are common in estuaries, but often
found in the ocean too (e.g. mullet (mugilids), kob, sea catfish, gobies, klipvissies (c1inids) and leervis (Lichia amia)).
Senses
VISION
Fishes show a tremendous diversity in the development
of their eyes. The major difference in the fish's eye, compared to the eye of non-aquatic vertebrates, is that the lens
is spherical, whereas most vertebrates have a "lens-shaped"
(i.e. oval in cross-section) lens. In the human eye (and also
in the eye of most other vertebrates) focusing is accomplished by the initial refraction of light rays by the cornea
and by muscles that flatten the elastic lens. The cornea of
fishes is optically non-functional, as it has the same refractive index as water, and focusing is done by moving the lens
back and forth in the eye.
Generally fishes are long-sighted, especially in their lateral field of vision, but the greatest visual acuity is in the forward field of view, because it is the posterior part of the
retina that has the greatest density of visual cells.
As in other vertebrates, the visual cells in the fish retina
are of two types: "rods" and "cones". Fishes living in depths
of 500 - 1 000 m usually have large eyes and mostly rods in
their retina, as this type of cell is the most sensitive to low
light 1evels. In the deepest parts of the sea (below about
2000 m), many of the fishes have tiny vestigial eyes. In
these stygian depths, the absence of sunlight and rarity of
bioluminescent fishes would render vision an extravagance
that could be dispensed with. Likewise, in some freshwater
fishes that live in caves and underground rivers, the eyes are
atrophied or completely absent.
Fishes of the well-lit epipelagic and near-shore waters
have a retina that is rich in cone cells, indicating not only
acuity of vision, but also the ability to distinguish various
colours. One need only see the fish fauna of a coral reef to
7
are the nutrient-rich upwelling areas that occur in certain
regions along the western coasts of Africa (off Namibia and
the western Cape Province) and the Americas (off California and Peru). These upwelling areas produce vast blooms
of phytoplankton which support enormous populations of
zooplankton and plankton-feeding fishes and their predators (including man). Typical epipelagic fishes are the
tunas (Scombridae), marlin and sailfish (Istiophoridae),
dolphinfish (Coryphaena hippurus) , flyingfishes (Exocoetidae), blue shark (Prionace glauca), mako (Isurus oxyrinchus), and the whale shark (Rhincodon typus). At night the
epipelagic zone is visited by vertically migrating fishes from
deeper waters (e.g. Astronesthes, Chauliodus, Stomias and
various myctophids). These nocturnal visitors are termed
nyctoepipeJagic fishes.
The mesopeJagic region extends from the epipelagic
region to a depth of 1 000 metres. About 850 of the 1 250 or
so oceanic species of pelagic fishes live in this region. Mesopelagic fishes are generally small (few exceed 30 cm in
length) with large eyes and well-developed light organs.
Many species move up to the epipelagic region at night in
order to feed on the more abundant zooplankton of the surface waters. Typical mesopelagic fishes are the lanternfishes (Myctophidae, comprising some 250 species),
stomiiforms and barracudinas (Paralepididae).
The bathypeJagic region, from 1 000 to 4 000 metres, is
inhabited by fewer (about 150) species of fishes. This reduction in numbers of species (as well as individuals) indicates
the reduction in food with depth. The scarcity of food here
is further emphasised by the reduction in certain organ
systems of bathypelagic fishes. The skeletal and muscular
systems are greatly reduced, and the decrease in density
achieved by the reduction of these heavy tissues allows the
loss of the swimbladder. Situated below the reach of sunlight, the bathypelagic zone is lit only by the bioluminescence of the animals that live here; consequently, most
species have greatly reduced eyes. The lateral-line system
is, however, well developed in bathypelagic fishes. With
small populations scattered over a vast area, finding a mate
may be difficult. The olfactory organs in males of many
bathypelagic species are greatly enlarged, presumably as an
adaptation for tracking the scent of females in this sparsely
populated environment. In some species of ceratioid
anglerfishes the attraction of the males is carried one step
further. Once a juvenile male finds a female (which is much
larger than he is), he attaches to the female with his mouth,
becoming a sexual parasite permanently fixed to his mate.
Typical bathypelagic fishes (in addition to the ceratioids,
which represent about two-thirds of the fauna) are the
cyemid, derichthyid, serrivomerid and saccopharyngoid
eels, and the whalefishes (cetomimids).
The abyssopeJagic region, extending from 4 000 metres to
the bottom of the ocean, is the deepest and most barren of
BIOLOGY OF FISHES
the four main divisions of the oceanic zone. Because of the
difficulties of sampling this zone, the fauna of this vast abyss
is very poorly known. But we do know that the fishes here
are very few and far between. Abyssopelagic fishes include
the halosaurs, macrourids, stephanoberycids (Acanthochaenus lutkenii) and alepocephalids.
Estuaries are habitats that overlap the littoral and sublittoral zones. This environment is characterised by considerable fluctuations in temperature, salinity and turbidity.
Estuaries are relatively productive regions and serve a valuable function as "nursery areas" for the young of many sublittoral species. Some fishes spend their whole lives in estuaries (e.g. Sand goby, Psammogobius knysnaensis,
Knysna seahorse, Hippocampus capensis, and Monodactylus
falciformis); others are common in estuaries, but often
found in the ocean too (e.g. mullet (mugilids), kob, sea catfish, gobies, klipvissies (c1inids) and leervis (Lichia amia)).
Senses
VISION
Fishes show a tremendous diversity in the development
of their eyes. The major difference in the fish's eye, compared to the eye of non-aquatic vertebrates, is that the lens
is spherical, whereas most vertebrates have a "lens-shaped"
(i.e. oval in cross-section) lens. In the human eye (and also
in the eye of most other vertebrates) focusing is accomplished by the initial refraction of light rays by the cornea
and by muscles that flatten the elastic lens. The cornea of
fishes is optically non-functional, as it has the same refractive index as water, and focusing is done by moving the lens
back and forth in the eye.
Generally fishes are long-sighted, especially in their lateral field of vision, but the greatest visual acuity is in the forward field of view, because it is the posterior part of the
retina that has the greatest density of visual cells.
As in other vertebrates, the visual cells in the fish retina
are of two types: "rods" and "cones". Fishes living in depths
of 500 - 1 000 m usually have large eyes and mostly rods in
their retina, as this type of cell is the most sensitive to low
light 1evels. In the deepest parts of the sea (below about
2000 m), many of the fishes have tiny vestigial eyes. In
these stygian depths, the absence of sunlight and rarity of
bioluminescent fishes would render vision an extravagance
that could be dispensed with. Likewise, in some freshwater
fishes that live in caves and underground rivers, the eyes are
atrophied or completely absent.
Fishes of the well-lit epipelagic and near-shore waters
have a retina that is rich in cone cells, indicating not only
acuity of vision, but also the ability to distinguish various
colours. One need only see the fish fauna of a coral reef to
7