Biology of fishes
Zones of the sea and their fishes
Consider that the oceans cover three-quarters of the
earth's surface and that fishes occupy every livable habitat
in the sea. Add in the freshwater fishes, which occur wherever lakes and rivers are found, and one could fairly say that
fishes dominate the earth.
The intertidal or littoral zone of the marine environment
is delimited by the tide marks of low and high water. On
shores with a large tidal range, the littoral zone is broad and
provides a distinct habitat characterised by wide fluctuations in temperature and salinity. In southern Africa, the
blennies, gobies and c1inids are the most common littoral
fishes. These small fishes are usually well camouflaged to
blend in with the algae-covered rocks of this region.
Juveniles of larger sublittoral fishes are often found in tide
pools, which thus serve as "nursery areas" providing food
and shelter for these vulnerable young.
Beyond the littoral region, the ocean is divided into
benthic and pelagic realms. The benthic habitat is populated
by fishes that live on or near the bottom. These fishes are
often specially modified for life on the bottom. The flattened (depressed) body of the skates and benthic rays is
adapted for resting on the substrate or swimming close over
the bottom to search for food. Likewise the compressed
bodies of the flounders and soles make it easy for these
fishes to hide on (or in) the sandy or muddy sediments they
prefer. Most benthic fishes are sedentary fishes camouflaged
to blend in with the substrate. Some even bury themselves
or burrow in the sediments. Clingfishes have their pelvic
fins modified to form a sucking disc which they use to attach
themselves to rocks - a useful anchor in a rocky surf zone.
The shallowest part of the benthic realm is the sublittoral
or continental shelf zone, extending from the littoral zone
out to a depth of 200 metres. The continental shelf zone is
characterised by high productivity and consequently dense
populations of fishes. Typical species of this sublittoral zone
are the rockcods (Serranidae), gurnards (Triglidae), kob
(Sciaenidae), stingrays (Dasyatidae), catsharks (Scyliorhinidae), flounders (Pleuronectidae and Bothidae), soles
(Soleidae), tonguefishes (Cynoglossidae), lizardfishes
(Synodontidae), klipfishes (Clinidae) and gobies.
Beyond 200 metres, the bottom usually drops more
steeply, and this region, from 200 to 2 000 metres, is known
as the continental slope or upper bathyal zone. In this dark
cold region, all of the food comes, ultimately, from above.
As might be expected, there are fewer fishes in this zone
6
than in the sublittoral zone. Typical inhabitants of the continental slope are the rat-tails (Macrouridae), chlorophthalmids, notacanthids, halosaurs, chimaeras, skates, bythitids
and morids.
The benthic habitat from 2 000 to 4 000 metres is termed
the continental rise or lower bathyal zone. Here we find
tripodfishes (Bathypterois spp.) resting on their elongated
pelvic and caudal fin rays, macrourids, bythitids, liparidids,
halosaurs and notacanthids. Again, there are fewer fishes
on the continental rise than on the slope region above. Food
is scarce in this zone and comprises mainly invertebrates.
About half of the species here lack a swim bladder. The
species that actually rest on the bottom can do without the
buoyancy of a swimbladder and thereby save energy. At a
depth of 4 000 metres, the gas in the swimbladder is under a
pressure of 400 atmospheres; gas secretion at this tremendous pressure requires considerable energy.
Below 4 000 metres, the continental rise merges with the
abyssal plain, ranging in depth from 4 000 to 6 000 metres.
Abyssal fishes are less numerous than those of the continental rise and include macrourids, ophidiids, halosaurs,
tripodfishes, liparidids and zoarcids.
The hadal zone, extending from 6000 metres to the bottom of the deepest trenches (ca. 10 300 metres), has been
difficult to sample, but it certainly contains very few fishes.
The ophidiid Holcomycteronus profundissimus and
liparidid Careproctus amblystomopsis are known from
depths greater than 7 000 metres.
The pelagic realm is divided into a neritic zone over the
continental shelf and an oceanic zone beyond the continental shelf. The neritic zone is the most productive of all the
marine habitats. The well-lit waters and plentiful nutrients
from rivers and upwelling areas result in an enormous production of algae, sea grasses and phytoplankton. This abundance of plant life supports a huge neritic community of
fishes and other animals. Consequently, much of the
world's fishery resources are harvested from this rich zone.
Typical neritic fishes are the snappers (Lutjanidae),
kingfishes (Carangidae), sardines and pilchards (Clupeidae), anchovies (Engraulidae), mackerels (Scombridae),
seabreams (Sparidae), elf (Pomatomus saltatrix), geelbek
(Atractoscion aequidens) and the galjoen (Coracinus capensis).
The epipelagic region, extending from the surface to
about 200 metres, is the uppermost of the four main divisions of the oceanic zone. Although it receives plenty of
sunshine, the epipelagic zone is (with a few localised exceptions) generally much less productive than the neritic zone.
Zones of the sea and their fishes
Consider that the oceans cover three-quarters of the
earth's surface and that fishes occupy every livable habitat
in the sea. Add in the freshwater fishes, which occur wherever lakes and rivers are found, and one could fairly say that
fishes dominate the earth.
The intertidal or littoral zone of the marine environment
is delimited by the tide marks of low and high water. On
shores with a large tidal range, the littoral zone is broad and
provides a distinct habitat characterised by wide fluctuations in temperature and salinity. In southern Africa, the
blennies, gobies and c1inids are the most common littoral
fishes. These small fishes are usually well camouflaged to
blend in with the algae-covered rocks of this region.
Juveniles of larger sublittoral fishes are often found in tide
pools, which thus serve as "nursery areas" providing food
and shelter for these vulnerable young.
Beyond the littoral region, the ocean is divided into
benthic and pelagic realms. The benthic habitat is populated
by fishes that live on or near the bottom. These fishes are
often specially modified for life on the bottom. The flattened (depressed) body of the skates and benthic rays is
adapted for resting on the substrate or swimming close over
the bottom to search for food. Likewise the compressed
bodies of the flounders and soles make it easy for these
fishes to hide on (or in) the sandy or muddy sediments they
prefer. Most benthic fishes are sedentary fishes camouflaged
to blend in with the substrate. Some even bury themselves
or burrow in the sediments. Clingfishes have their pelvic
fins modified to form a sucking disc which they use to attach
themselves to rocks - a useful anchor in a rocky surf zone.
The shallowest part of the benthic realm is the sublittoral
or continental shelf zone, extending from the littoral zone
out to a depth of 200 metres. The continental shelf zone is
characterised by high productivity and consequently dense
populations of fishes. Typical species of this sublittoral zone
are the rockcods (Serranidae), gurnards (Triglidae), kob
(Sciaenidae), stingrays (Dasyatidae), catsharks (Scyliorhinidae), flounders (Pleuronectidae and Bothidae), soles
(Soleidae), tonguefishes (Cynoglossidae), lizardfishes
(Synodontidae), klipfishes (Clinidae) and gobies.
Beyond 200 metres, the bottom usually drops more
steeply, and this region, from 200 to 2 000 metres, is known
as the continental slope or upper bathyal zone. In this dark
cold region, all of the food comes, ultimately, from above.
As might be expected, there are fewer fishes in this zone
6
than in the sublittoral zone. Typical inhabitants of the continental slope are the rat-tails (Macrouridae), chlorophthalmids, notacanthids, halosaurs, chimaeras, skates, bythitids
and morids.
The benthic habitat from 2 000 to 4 000 metres is termed
the continental rise or lower bathyal zone. Here we find
tripodfishes (Bathypterois spp.) resting on their elongated
pelvic and caudal fin rays, macrourids, bythitids, liparidids,
halosaurs and notacanthids. Again, there are fewer fishes
on the continental rise than on the slope region above. Food
is scarce in this zone and comprises mainly invertebrates.
About half of the species here lack a swim bladder. The
species that actually rest on the bottom can do without the
buoyancy of a swimbladder and thereby save energy. At a
depth of 4 000 metres, the gas in the swimbladder is under a
pressure of 400 atmospheres; gas secretion at this tremendous pressure requires considerable energy.
Below 4 000 metres, the continental rise merges with the
abyssal plain, ranging in depth from 4 000 to 6 000 metres.
Abyssal fishes are less numerous than those of the continental rise and include macrourids, ophidiids, halosaurs,
tripodfishes, liparidids and zoarcids.
The hadal zone, extending from 6000 metres to the bottom of the deepest trenches (ca. 10 300 metres), has been
difficult to sample, but it certainly contains very few fishes.
The ophidiid Holcomycteronus profundissimus and
liparidid Careproctus amblystomopsis are known from
depths greater than 7 000 metres.
The pelagic realm is divided into a neritic zone over the
continental shelf and an oceanic zone beyond the continental shelf. The neritic zone is the most productive of all the
marine habitats. The well-lit waters and plentiful nutrients
from rivers and upwelling areas result in an enormous production of algae, sea grasses and phytoplankton. This abundance of plant life supports a huge neritic community of
fishes and other animals. Consequently, much of the
world's fishery resources are harvested from this rich zone.
Typical neritic fishes are the snappers (Lutjanidae),
kingfishes (Carangidae), sardines and pilchards (Clupeidae), anchovies (Engraulidae), mackerels (Scombridae),
seabreams (Sparidae), elf (Pomatomus saltatrix), geelbek
(Atractoscion aequidens) and the galjoen (Coracinus capensis).
The epipelagic region, extending from the surface to
about 200 metres, is the uppermost of the four main divisions of the oceanic zone. Although it receives plenty of
sunshine, the epipelagic zone is (with a few localised exceptions) generally much less productive than the neritic zone.