ELASMOBRANCHII (SHARKS & RAYS)
Nictitating membrane
Some sharks have a movable inner eyelid called a nictitating membrane or nictitans. This structure is variously developed in different species. In sharks with a large spiracle,
the nictitans is absent or represented by a rudimentary
groove along the lower edge of the orbit; whilst those without a spiracle (or a very small one) have a well-developed
nictitans that can be pulled up to cover the eye completely.
Sensory abilities
The sense of smell in elasmobranchs is well developed
and very important in locating their food. The common
names "smooth-hound" and "dogfish" for some sharks
were probably coined as much in allusion to their ability to
follow a scent trail through the water as to their habit of
hunting in packs. In addition to the shark's keen sense of
smell, hearing (including the related ability to detect lowfrequency vibrations) and vision are also well developed.
Most elasmobranchs have an extensive system of small
pores on the underside of the snout. These pores are the
openings into a network of small flask-shaped sense organs
called the ampullae of Lorenzini. Recent experiments have
shown that the ampullae are able to detect the minute electrical impulses from the contractions of respiratory muscles
of buried prey animals (e.g. crabs and small fishes).
Reproduction
Although fertilisation is internal in all elasmobranchs,
there are three fairly distinct modes of reproduction: (1)
Oviparous, in which the eggs are contained in a tough horny
case and the embryo develops almost completely in the eggcase after the eggs are laid; (2) Ovoviviparous, in which the
egg-cases are retained in the oviducts and the embryos subsist only on their yolk supply until they are nearly or quite
fully developed; and (3) Viviparous, in which the embryos
develop in the oviducts and are provided with nutrients
from the mother so that they are born at a relatively large
size (compared to ovoviviparous species).
Identification
Identification of elasmobranchs is often not easy, as there
are relatively few external features on which distinctions
may be based, and these sometimes vary in puzzling fashion. Colour is of limited assistance, being often drab or else
variable within one species. Most characters change significantly during growth. When the adult is large, it is not
easily captured, and few museums can provide storage for
these huge specimens. Thus an adequate series of specimens for many species is difficult to assemble.
The 700-800 species of elasmobranchs can be conve42
niently divided into two groups: the Squalomorphea
(sharks), with the pectoral fins not attached to the head,
and the Batoidea (sawfish, guitarfish, skates and rays), with
the pectoral fins expanded and fused to the sides of the head
over the gill slits, which are thus located on the underside of
the fish.
Superorder SQUALOMORPHEA
Sharks
Haaie
Cartilaginous fishes with 5-7 gill openings located above
or in front of pectoral fins, upper edge of orbit free from
eye, one or two dorsal fins (with or without spines), anal fin
present or absent.
There are about 350 species of sharks, ranging in size
from the 6 inch (15 cm) adult of the dwarf shark, Squaliolus
laticaudus, to the forty foot (12 m) whale shark - the largest
species of fish in the sea. As a group, sharks inhabit most regions of the sea, from the shore down to at least 4 000 m.
The Zambezi shark, Carcharhinus leucas, is common in
estuaries and often found well up rivers in many parts of the
world.
Shark attack
Although all sharks feed on other animals, and are
thereby considered predators, fewer than 10% of the
species are dangerous to man. Much evidence has accumulated in recent years to indicate that certain factors predispose swimmers to shark attack. All of the following practices can increase the probability of being attacked: swimming alone or far from a group of other bathers, or in deep
water far from shore; spearing fish or swimming in an area
where fish were recently speared; swimming at dusk or at
night; swimming in areas not protected by shark nets or
where sharks are frequently sighted; and molesting or provoking a shark in any way.
In addition to the factors just listed, divers should be
aware of the threat display behavioural pattern that has
been found to precede attack in some species of Carcharhinus: the shark swims with exaggerated lateral movements of the head and body; the snout is lifted, while the
back is arched and the pectoral fins are depressed. This behaviour has been observed in sharks that were situated between a stationary or approaching diver and some object
(e.g. boat or reef) restricting the shark's escape route. Also
some sharks faced with a rapidly approaching diver in open
water reacted in the same way; if the diver did not retreat,
the display was usually followed by an attack on the diver.
For treatment of shark bite see "Dangers of the Sea"
(page 17).
Nictitating membrane
Some sharks have a movable inner eyelid called a nictitating membrane or nictitans. This structure is variously developed in different species. In sharks with a large spiracle,
the nictitans is absent or represented by a rudimentary
groove along the lower edge of the orbit; whilst those without a spiracle (or a very small one) have a well-developed
nictitans that can be pulled up to cover the eye completely.
Sensory abilities
The sense of smell in elasmobranchs is well developed
and very important in locating their food. The common
names "smooth-hound" and "dogfish" for some sharks
were probably coined as much in allusion to their ability to
follow a scent trail through the water as to their habit of
hunting in packs. In addition to the shark's keen sense of
smell, hearing (including the related ability to detect lowfrequency vibrations) and vision are also well developed.
Most elasmobranchs have an extensive system of small
pores on the underside of the snout. These pores are the
openings into a network of small flask-shaped sense organs
called the ampullae of Lorenzini. Recent experiments have
shown that the ampullae are able to detect the minute electrical impulses from the contractions of respiratory muscles
of buried prey animals (e.g. crabs and small fishes).
Reproduction
Although fertilisation is internal in all elasmobranchs,
there are three fairly distinct modes of reproduction: (1)
Oviparous, in which the eggs are contained in a tough horny
case and the embryo develops almost completely in the eggcase after the eggs are laid; (2) Ovoviviparous, in which the
egg-cases are retained in the oviducts and the embryos subsist only on their yolk supply until they are nearly or quite
fully developed; and (3) Viviparous, in which the embryos
develop in the oviducts and are provided with nutrients
from the mother so that they are born at a relatively large
size (compared to ovoviviparous species).
Identification
Identification of elasmobranchs is often not easy, as there
are relatively few external features on which distinctions
may be based, and these sometimes vary in puzzling fashion. Colour is of limited assistance, being often drab or else
variable within one species. Most characters change significantly during growth. When the adult is large, it is not
easily captured, and few museums can provide storage for
these huge specimens. Thus an adequate series of specimens for many species is difficult to assemble.
The 700-800 species of elasmobranchs can be conve42
niently divided into two groups: the Squalomorphea
(sharks), with the pectoral fins not attached to the head,
and the Batoidea (sawfish, guitarfish, skates and rays), with
the pectoral fins expanded and fused to the sides of the head
over the gill slits, which are thus located on the underside of
the fish.
Superorder SQUALOMORPHEA
Sharks
Haaie
Cartilaginous fishes with 5-7 gill openings located above
or in front of pectoral fins, upper edge of orbit free from
eye, one or two dorsal fins (with or without spines), anal fin
present or absent.
There are about 350 species of sharks, ranging in size
from the 6 inch (15 cm) adult of the dwarf shark, Squaliolus
laticaudus, to the forty foot (12 m) whale shark - the largest
species of fish in the sea. As a group, sharks inhabit most regions of the sea, from the shore down to at least 4 000 m.
The Zambezi shark, Carcharhinus leucas, is common in
estuaries and often found well up rivers in many parts of the
world.
Shark attack
Although all sharks feed on other animals, and are
thereby considered predators, fewer than 10% of the
species are dangerous to man. Much evidence has accumulated in recent years to indicate that certain factors predispose swimmers to shark attack. All of the following practices can increase the probability of being attacked: swimming alone or far from a group of other bathers, or in deep
water far from shore; spearing fish or swimming in an area
where fish were recently speared; swimming at dusk or at
night; swimming in areas not protected by shark nets or
where sharks are frequently sighted; and molesting or provoking a shark in any way.
In addition to the factors just listed, divers should be
aware of the threat display behavioural pattern that has
been found to precede attack in some species of Carcharhinus: the shark swims with exaggerated lateral movements of the head and body; the snout is lifted, while the
back is arched and the pectoral fins are depressed. This behaviour has been observed in sharks that were situated between a stationary or approaching diver and some object
(e.g. boat or reef) restricting the shark's escape route. Also
some sharks faced with a rapidly approaching diver in open
water reacted in the same way; if the diver did not retreat,
the display was usually followed by an attack on the diver.
For treatment of shark bite see "Dangers of the Sea"
(page 17).