Subclass ELASMOBRANCHII
(Euselachii)
Sharks & Rays
Haaie&Roe
Denticles
Most sharks are covered with minute placoid scales or
denticles. Unlike the scales of typical bony fishes, the dermal
denticles of elasmobranchs are essentially similar to teeth,
with a hard outer layer of enamel over a core of dentine.
Also like teeth, most denticles do not increase in size after
they are completely formed; individual denticles and teeth
are continually being replaced by newly formed and slightly
larger ones during the life span of the shark. There is considerable variation in the shape and size of denticles.
The points of the denticles are usually directed toward
the tail and account for the roughness of the shark's skin if
stroked toward the head. The cured skin of some sharks has
been used as sandpaper by cabinetmakers for smoothing
hard woods.
B
. ~* .. /
~.
~
'.
- .
ri"
.
C
Shark denticles: A. Deania calcea, B. Centrophorus lusitanicus, C. Etmopterus lucifer,
D. Centrophorus scalpratus, E. Centrophorus squamosus
Teeth and feeding
The teeth of elasmobranchs are thought to be modified
dermal denticles. Sharks have a lifetime supply of teeth, as
new ones are continually being formed on the inner surface
of the jaws. The teeth are firmly anchored to a layer of
tough connective tissue, not fixed in sockets in the jaws (as
in most bony fishes). When the outer teeth are lost, the next
one in the row moves forward to take its place. In the few
species in which tooth replacement rate has been measured,
the outer teeth are replaced about once a fortnight; thus a
species that lives for 20 to 30 years produces thousands of
teeth during its lifetime.
Shark teeth are a useful taxonomic character, as their
ELASMOBRANCHII (SHARKS & RAYS)
number and shape are more or less distinctive, and a species
may sometimes be identified with certainty from a single
tooth. There are three basic types of teeth in elasmobranchs: broad, triangular, blade-like teeth (e.g. great
white and tiger sharks) which are well adapted for cutting
pieces from large prey; slender awl-like teeth (e.g. raggedtooth sharks) which are suited for catching and holding
"bite-sized" fishes and squid; and blunt, pavement-like
dentition characteristic of smooth-hound sharks and rays,
which generally feed on hard-shelled molluscs and crustaceans.
A common misconception regarding the feeding of
sharks is that they must roll on their sides or back to bite
their prey. Although the mouth of most sharks is on the underside of the head, they are not obliged to roll over when
they bite something. When feeding at the surface, a shark
often pushes its snout right out of the water in order to bite
the food item. Many sharks can protrude their jaws to a considerable degree, and this ability is important in biting large
pieces from their prey.
Spiral valve
A spiral "valve" is present in the intestine of all elasmobranchs (as well as chimaeras and some primitive bony
fishes). In most species the valve is just like a spiral staircase, in others it looks like a series of elongated cones one
inside the other, and, in the requiem and hammerhead
sharks, it resembles a scroll of rolled paper. By means of the
spiral valve, a much greater area of absorptive intestinal
surface is provided than would be the case in a straight tube.
Buoyancy
Unlike most bony fishes, elasmobranchs have no gasbladder. Buoyancy is provided in many sharks by a large
liver with considerable quantities of oil that is less dense
than seawater. The ragged-tooth shark uses its stomach as a
gasbladder by swallowing air at the surface.
Spiracle
Many elasmobranchs have an opening of varying size
situated behind the eye, known as a spiracle. Apparently
the vestige of a gill slit of ancestral forms, the spiracle is absent or greatly reduced in pelagic species (Lamnidae, Carcharhinidae and Sphyrnidae), but well developed in benthic
(bottom living) sharks and batoids. In batoids such as stingrays and skates, the spiracle is the major inlet for water
pumped over the gills, for the mouth of these sedentary
fishes is usually submerged in sand or mud, and a respiratory current of water coming in through the mouth would
soon foul the gills.
41
(Euselachii)
Sharks & Rays
Haaie&Roe
Denticles
Most sharks are covered with minute placoid scales or
denticles. Unlike the scales of typical bony fishes, the dermal
denticles of elasmobranchs are essentially similar to teeth,
with a hard outer layer of enamel over a core of dentine.
Also like teeth, most denticles do not increase in size after
they are completely formed; individual denticles and teeth
are continually being replaced by newly formed and slightly
larger ones during the life span of the shark. There is considerable variation in the shape and size of denticles.
The points of the denticles are usually directed toward
the tail and account for the roughness of the shark's skin if
stroked toward the head. The cured skin of some sharks has
been used as sandpaper by cabinetmakers for smoothing
hard woods.
B
. ~* .. /
~.
~
'.
- .
ri"
.
C
Shark denticles: A. Deania calcea, B. Centrophorus lusitanicus, C. Etmopterus lucifer,
D. Centrophorus scalpratus, E. Centrophorus squamosus
Teeth and feeding
The teeth of elasmobranchs are thought to be modified
dermal denticles. Sharks have a lifetime supply of teeth, as
new ones are continually being formed on the inner surface
of the jaws. The teeth are firmly anchored to a layer of
tough connective tissue, not fixed in sockets in the jaws (as
in most bony fishes). When the outer teeth are lost, the next
one in the row moves forward to take its place. In the few
species in which tooth replacement rate has been measured,
the outer teeth are replaced about once a fortnight; thus a
species that lives for 20 to 30 years produces thousands of
teeth during its lifetime.
Shark teeth are a useful taxonomic character, as their
ELASMOBRANCHII (SHARKS & RAYS)
number and shape are more or less distinctive, and a species
may sometimes be identified with certainty from a single
tooth. There are three basic types of teeth in elasmobranchs: broad, triangular, blade-like teeth (e.g. great
white and tiger sharks) which are well adapted for cutting
pieces from large prey; slender awl-like teeth (e.g. raggedtooth sharks) which are suited for catching and holding
"bite-sized" fishes and squid; and blunt, pavement-like
dentition characteristic of smooth-hound sharks and rays,
which generally feed on hard-shelled molluscs and crustaceans.
A common misconception regarding the feeding of
sharks is that they must roll on their sides or back to bite
their prey. Although the mouth of most sharks is on the underside of the head, they are not obliged to roll over when
they bite something. When feeding at the surface, a shark
often pushes its snout right out of the water in order to bite
the food item. Many sharks can protrude their jaws to a considerable degree, and this ability is important in biting large
pieces from their prey.
Spiral valve
A spiral "valve" is present in the intestine of all elasmobranchs (as well as chimaeras and some primitive bony
fishes). In most species the valve is just like a spiral staircase, in others it looks like a series of elongated cones one
inside the other, and, in the requiem and hammerhead
sharks, it resembles a scroll of rolled paper. By means of the
spiral valve, a much greater area of absorptive intestinal
surface is provided than would be the case in a straight tube.
Buoyancy
Unlike most bony fishes, elasmobranchs have no gasbladder. Buoyancy is provided in many sharks by a large
liver with considerable quantities of oil that is less dense
than seawater. The ragged-tooth shark uses its stomach as a
gasbladder by swallowing air at the surface.
Spiracle
Many elasmobranchs have an opening of varying size
situated behind the eye, known as a spiracle. Apparently
the vestige of a gill slit of ancestral forms, the spiracle is absent or greatly reduced in pelagic species (Lamnidae, Carcharhinidae and Sphyrnidae), but well developed in benthic
(bottom living) sharks and batoids. In batoids such as stingrays and skates, the spiracle is the major inlet for water
pumped over the gills, for the mouth of these sedentary
fishes is usually submerged in sand or mud, and a respiratory current of water coming in through the mouth would
soon foul the gills.
41