SHARKS--THE TIGERS OF THE SEA
Only a rapid systematic sketch of the class Pisces, fishes, is
possible, distinguishing the main divisions, alluding to their history,
and touching here and there the most characteristic genera and species
of the thousands that have been described by ichthyologists. The
primary division is into three subclasses:
1. _Elasmobranchii_--Sharks, skates, rays, etc., having a
cartilaginous skeleton.
2. _Teleostomi_--Ordinary fishes, having a bony skeleton.
3. _Dipnoi_--Lepidosiren, and many extinct, primitive families.
In the Elasmobranchii, or selachians, the skeleton consists of
cartilage, as in the embryos of all fishes--a sign of their primitive
and inferior rank; but parts of it in various species become hardened
by depositions of lime, especially in the vertebræ, in spines and
teeth, parts often well preserved as fossils. Sharks' teeth are among
the best known of fossils, and before science established their
true character were commonly called "birds' tongues," or "snakes'
tongues." The sharks, first to be considered, are a very ancient race,
originating in early Paleozoic times. Of the many curious extinct forms
that terrorized the Silurian, Devonian, and Carboniferous seas, a few
representatives still exist in the South Pacific, notably the cow
sharks; an eellike Japanese species with frilled gills that Dr. Garman
thought might easily fill the rôle of "sea serpent"; and the quaint
bullhead, or Port Jackson sharks of Australian waters; all these are of
small size and the last named represents the principal race in Mesozoic
seas. Their flat teeth form a sort of pavement of the mouth, enabling
them to crush the mollusks and crabs on which they chiefly feed.
Next in a rising order of classification, and of somewhat more recent
origin geologically, is the European family of dogfishes (Scyllidæ),
which includes also the "ground sharks" of warm seas--deep-water fish
eight to twelve feet long, that creep about near the bottom in search
of prey. Next come the large pelagic sharks of the family Carchariidæ,
which contains about sixty species, scattered over all the seas,
and one confined in the fresh water of Lake Nicaragua. One section,
that of the "topes" and "hounds" of temperate and warm seas, are
bottom-feeders, and have pavementlike teeth adapted to crushing and
grinding the shellfish on which they subsist; but most of the family
are swift and powerful hunters of fishes in the open sea, such as the
dreaded tiger shark of the West Indies, which is variegated in color
and sometimes twenty feet long, and the equally big blue, white, dusky,
and other ferocious bandits ranging not only the tropical seas, but
more common in northern oceans than is generally supposed. The teeth in
these and other hunting, fish-catching sharks, are shaped somewhat like
arrowheads, in some cases smooth-edged and single-pointed, in others
with sharply notched edges and side cusps. They are set in the flesh of
the jaw, unattached to the underlying bones, in concentric rows, one
close behind the other, all round the front of the mouth, both above
and below, and look and act like a set of saws, a sidewise movement of
the jaws sawing through an object seized in a single bite. As fast as
the front row of teeth are lost they are replaced by those of the row
immediately in the rear. But all gradations exist between these and the
mosaic of "pavement" teeth in the topes. The mouth of these hunting
sharks is on the underside of the head, and they must turn on their
backs to seize anything floating or swimming near the surface.
This is the group that furnishes the "man-eater" stories--tales
that have been substantiated by so many terrifying examples that
no precaution of safety against them should neglected, even on our
northern coasts, where the ravenous blue shark, or the dusky species,
may appear at any time, even in harbors. A few years ago a man was
seized by one of these sharks in a little inlet of New York Bay, at
Freeport, New Jersey.
Blue sharks are nocturnal in habit, and are sometimes seen asleep
or resting in the daytime, with the tips of the two dorsal fins,
characteristic of this family, in sight above the surface of the
water. "So gentle are they in their movements," says one authority,
"that, unlike many other monsters of the deep, they do not disturb
the luminous creatures, which at the same time will be lighting every
wavelet with their phosphorescence. Blue sharks are not very particular
as to what fish they take as food, though those which are gregarious in
their habits, like mackerel, pilchards, and herring, are most commonly
hunted by them." A curious relative of these "man-eaters" is the
hammerhead, in which the sides of the head are extended in two great
lobes, with eyes at their extremities; this kind of shark is greatly
feared in the East Indian seas. In spite of it, and the prevalence of
other huge and voracious sharks, the Arabs about the entrance to the
Red Sea, and the natives of other Oriental shores, will swim and dive
in the open sea, apparently without fear, where Europeans would be
devoured almost instantly. Another peculiar shark is the thresher, well
known in the North Atlantic as elsewhere for its strategic maneuvers.
It grows to a length of fifteen feet, of which the tail forms at least
one half. Quite inoffensive to man, the thresher feeds on the shoals
of smaller teleosts, such as pilchards, herrings, and sprats. When
feeding it swims in gradually diminishing circles around the shoal,
splashing the water with its long tail, and keeping the victims so
crowded together that they become an easy prey. Hammerheads and their
relatives, the "bonnet" sharks, frequently visit both our shores in
summer.
The porbeagles are big, fierce sharks of the family Lamniidæ, the
giant of which is Rondelet's shark, known to attain a length of more
than forty feet. The triangular, saw-edged teeth of such a one measure
nearly an inch across the base; but similar fossil teeth, and also
others dredged from the bottom of the South Pacific, are much larger,
indicating sharks beside which Rondelet's would be small, and in all
probability these monsters survived to a comparatively recent date. A
remarkable lamnoid shark of Japanese deep waters has the snout produced
into a long, flat, flexible, leaflike blade.
Closely related sharks, almost as big, are well known in the North
Atlantic. Two of them, the "bone" and the "basking" sharks, are killed
by fishermen whenever encountered for the sake of the oil in their
livers. The name of the second refers to its habit of loafing and
sleeping on the surface on fine days, when a boat may go so near it
that a harpoon may be planted in its hide before it will move. The real
"basking shark," however, is a gigantic species of Rhinodon, of the
Indian and South Pacific oceans, with a very bulky body that may exceed
forty-five feet in length. Both of these ponderous fishes are sluggish,
and are not dangerous to man, except that a blow of the tail may smash
a boat when an attempt is made to harpoon them at close quarters. They
feed on small fishes that go in shoals, and also, perhaps, on seaweeds.
The last sharks to be mentioned are American dogfish of the family
Squalidæ (another family, Scyllidæ, are known as "dogfish" and "hounds"
in Great Britain), which are numerous and greatly hated along both our
northern coasts. The common gray dogfish of the North Atlantic and
California coasts is the spiny one (_Squalus acanthias_), the larger
females of which will weigh about eight pounds. It makes its home in
deep water off the New England coast, approaching the shore when the
mackerel come in and disappearing when they depart; but dogfish are
to be found all summer in shoal places such as George's Bank, and
irregularly in shore inlets. In the late autumn they become numerous
on the Grand Banks, and stay there until the winter's cold drives them
away into deeper water. Everywhere these small sharks are a nuisance
to the fishermen, by tearing nets and by eating the cod, etc., hooked
on the trawl lines. Formerly they were regularly hunted for the oil in
their livers, which is especially valuable for certain purposes, as in
harness making, but the price of this oil is now low, and the fishery
has declined.
The economic use of sharks is not great except as producers of oil.
The flesh is good food, but not popular. In China sharks' fins are a
favorite substance for delicate soups and sauces, and a very large
trade in catching sharks for their fins is carried on near Bombay, and
in East Indian waters.
The ugly angel shark, with its squat, toadlike body, big, winglike side
fins and thick tail, occupies an intermediate place between the sharks
and the rays. It creeps along the bottom, and is remarkably voracious.
The chimæra is another queer "monster" of the deep.