These long-lived pelagic larvae are often carried far from
their place of origin and are thus excellent means of dispersal for shallow-water fishes.
Some oviparous species produce non-buoyant (demersal) eggs, which are larger than pelagic eggs and are generally attached to the substrate. Many species that have
demersal eggs also guard their eggs until they hatch (e.g.
damselfishes, Family Pomacentridae, and triggerfishes,
Family Balistidae). A further adaptation for protecting the
eggs is some type of "incubation". The male seahorse has a
pouch on his belly in which he keeps the eggs until they
hatch; he then releases the baby seahorses (seacolts?) from
his pouch by bending his body and apparently pushing the
juveniles out with contractions of his abdominal muscles.
Once released, the little seahorses must fend for themselves. Sea catfishes (Ariidae) and cardinal fishes
(Apogonidae) practise oral incubation, in which the male
keeps the eggs in his mouth until they hatch. In marine
fishes, it is generally the males that do the mouth brooding,
but in the freshwater cichlids that practise oral incubation, it
is the females that carry the eggs and young fry in their
mouths.
Fishes that hatch from demersal eggs are bigger than
most larvae that hatch from pelagic eggs, and their larval
stage is correspondingly briefer (sometimes even nonexistent). In chondrichthyans (chimaeras, sharks, rays, etc.)
the hatchlings are miniature copies of the adult, and the
eggs that produce these well-developed juveniles are quite
large - the egg-case of the whale shark measures 30 x 15 cm!
Although most oviparous fishes fertilise their eggs after
they have been released from the body of the female, a few
bony fishes and all chondrichthyans fertilise their eggs inside the female. Internal fertilisation is usually accomplished with special copulatory structures called "intromittent organs", but some species (e.g. the coelacanth,
Latimeria) manage internal fertilisation without intromittent organs. In bony fishes, the intromittent organ is usually
called a penis; in chondrichthyans, the male has paired intromittent organs (misnamed "claspers") that are modified
parts of the pelvic fin skeleton.
Many teleosts and most elasmobranchs are viviparous,
which means that they give birth to active, free-swimming
young. Species that provide their embryos with no extra
nourishment (other than their original yolk supply) are
called ovoviviparous (e.g. the spiny dogfishes, Squalus, the
whitespotted smooth-hound, Mustelus palumbes, and the
coelacanth). The developing embryo of most viviparous
elasmobranchs is provided with some nourishment from the
mother in addition to its original yolk supply. Carcharhinoid sharks (e.g. Carcharhinus spp., Mustelus mustelus,
and the hammerheads) have a yolk-sac - placental connection between the foetus and the oviduct (uterus) of the
mother. This placental source of nutrients develops during
the later months of the gestation period after the initial yolk
BIOLOGY OF FISHES
supply is exhausted. In some species, the shark foetus is
nourished by feeding on eggs and smaller siblings that may
be found in the oviduct. These oviphagous or cannibalistic
sharks (e.g. the ragged-tooth, Eugomphodus taurus) give
birth to only one or two large pups (one per uterus) at a
time. In this oviphagous foetal nourishment scheme, the
ovary enlarges and functions as a kind of "milk" gland,
sending little packets of yolk into each uterus to feed the developing foetus.
Hermaphroditism, the occurrence of both sexes in the
same individual, is normal and common in many bony
fishes. Functional hermaphroditism may be of either the
synchronous or the successive (sequential) types. The
gonads of a synchronous hermaphrodite produce ripe ova
and sperm at the same time. Although synchronous hermaphrodites are capable of fertilising their own eggs, this
does not usually occur, at least not in the species where
spawning has been observed. The synchronous serranids
that have been studied (Serranus and Hypoplectrus spp.)
spawn in pairs and take turns fertilising each other's eggs.
Successive hermaphrodites change sex, beginning the
reproductive period of their lives either as females (protogynous), and after a few years or spawning periods they
change to males; or they begin as males (protandrous) and
then change to females.
Although hermaphroditism is rare in freshwater fishes, it
is common in marine fishes, especially in mesopelagic and
coral-reef species. Most of the mesopelagic hermaphrodites
are of the synchronous type (e.g. Alepisauridae, Paralepididae, Notosudidae, Evermannellidae, etc.). Many coralreef fishes are successive hermaphrodites (e.g. Labridae,
Scaridae, Serranidae, Sparidae, Lethrinidae, Polynemidae,
etc.).
An interesting reproductive pattern has evolved in the
sexually dichromatic serranid fish Anthias squamipinnis.
Each small area of the reefs where this species occurs has a
little school or colony of Anthias comprising a few (sometimes only one or two) large, distinctively coloured males,
several smaller adult females and numerous juveniles coloured like the females. Each male spawns with several
females in a harem-like fashion. If a male is removed from
the group, the dominant (largest) female changes sex and
colour pattern to take the place of the missing male. This
sex and colour pattern change occurs within a few days, but
if another male is introduced into the colony before the dominant female has completed her sex change, she returns to
her female role and livery.
Growth and age determination in fishes
The growth of fishes, like that of other cold-blooded
animals, is dependent on the temperature of their environment. The availability of food is, of course, the other main
11
their place of origin and are thus excellent means of dispersal for shallow-water fishes.
Some oviparous species produce non-buoyant (demersal) eggs, which are larger than pelagic eggs and are generally attached to the substrate. Many species that have
demersal eggs also guard their eggs until they hatch (e.g.
damselfishes, Family Pomacentridae, and triggerfishes,
Family Balistidae). A further adaptation for protecting the
eggs is some type of "incubation". The male seahorse has a
pouch on his belly in which he keeps the eggs until they
hatch; he then releases the baby seahorses (seacolts?) from
his pouch by bending his body and apparently pushing the
juveniles out with contractions of his abdominal muscles.
Once released, the little seahorses must fend for themselves. Sea catfishes (Ariidae) and cardinal fishes
(Apogonidae) practise oral incubation, in which the male
keeps the eggs in his mouth until they hatch. In marine
fishes, it is generally the males that do the mouth brooding,
but in the freshwater cichlids that practise oral incubation, it
is the females that carry the eggs and young fry in their
mouths.
Fishes that hatch from demersal eggs are bigger than
most larvae that hatch from pelagic eggs, and their larval
stage is correspondingly briefer (sometimes even nonexistent). In chondrichthyans (chimaeras, sharks, rays, etc.)
the hatchlings are miniature copies of the adult, and the
eggs that produce these well-developed juveniles are quite
large - the egg-case of the whale shark measures 30 x 15 cm!
Although most oviparous fishes fertilise their eggs after
they have been released from the body of the female, a few
bony fishes and all chondrichthyans fertilise their eggs inside the female. Internal fertilisation is usually accomplished with special copulatory structures called "intromittent organs", but some species (e.g. the coelacanth,
Latimeria) manage internal fertilisation without intromittent organs. In bony fishes, the intromittent organ is usually
called a penis; in chondrichthyans, the male has paired intromittent organs (misnamed "claspers") that are modified
parts of the pelvic fin skeleton.
Many teleosts and most elasmobranchs are viviparous,
which means that they give birth to active, free-swimming
young. Species that provide their embryos with no extra
nourishment (other than their original yolk supply) are
called ovoviviparous (e.g. the spiny dogfishes, Squalus, the
whitespotted smooth-hound, Mustelus palumbes, and the
coelacanth). The developing embryo of most viviparous
elasmobranchs is provided with some nourishment from the
mother in addition to its original yolk supply. Carcharhinoid sharks (e.g. Carcharhinus spp., Mustelus mustelus,
and the hammerheads) have a yolk-sac - placental connection between the foetus and the oviduct (uterus) of the
mother. This placental source of nutrients develops during
the later months of the gestation period after the initial yolk
BIOLOGY OF FISHES
supply is exhausted. In some species, the shark foetus is
nourished by feeding on eggs and smaller siblings that may
be found in the oviduct. These oviphagous or cannibalistic
sharks (e.g. the ragged-tooth, Eugomphodus taurus) give
birth to only one or two large pups (one per uterus) at a
time. In this oviphagous foetal nourishment scheme, the
ovary enlarges and functions as a kind of "milk" gland,
sending little packets of yolk into each uterus to feed the developing foetus.
Hermaphroditism, the occurrence of both sexes in the
same individual, is normal and common in many bony
fishes. Functional hermaphroditism may be of either the
synchronous or the successive (sequential) types. The
gonads of a synchronous hermaphrodite produce ripe ova
and sperm at the same time. Although synchronous hermaphrodites are capable of fertilising their own eggs, this
does not usually occur, at least not in the species where
spawning has been observed. The synchronous serranids
that have been studied (Serranus and Hypoplectrus spp.)
spawn in pairs and take turns fertilising each other's eggs.
Successive hermaphrodites change sex, beginning the
reproductive period of their lives either as females (protogynous), and after a few years or spawning periods they
change to males; or they begin as males (protandrous) and
then change to females.
Although hermaphroditism is rare in freshwater fishes, it
is common in marine fishes, especially in mesopelagic and
coral-reef species. Most of the mesopelagic hermaphrodites
are of the synchronous type (e.g. Alepisauridae, Paralepididae, Notosudidae, Evermannellidae, etc.). Many coralreef fishes are successive hermaphrodites (e.g. Labridae,
Scaridae, Serranidae, Sparidae, Lethrinidae, Polynemidae,
etc.).
An interesting reproductive pattern has evolved in the
sexually dichromatic serranid fish Anthias squamipinnis.
Each small area of the reefs where this species occurs has a
little school or colony of Anthias comprising a few (sometimes only one or two) large, distinctively coloured males,
several smaller adult females and numerous juveniles coloured like the females. Each male spawns with several
females in a harem-like fashion. If a male is removed from
the group, the dominant (largest) female changes sex and
colour pattern to take the place of the missing male. This
sex and colour pattern change occurs within a few days, but
if another male is introduced into the colony before the dominant female has completed her sex change, she returns to
her female role and livery.
Growth and age determination in fishes
The growth of fishes, like that of other cold-blooded
animals, is dependent on the temperature of their environment. The availability of food is, of course, the other main
11