-
Chondrichthyes (Sharks,Rays and Chimaeras)Michael D Gottfried,
Michigan State University Museum, East Lansing, Michigan, USA
Chondrichthyes is the class of vertebrates that includes sharks,
rays and chimaeras.
These relatives of bony fishes are characterized by an internal
skeleton of cartilage
rather than true bone.
Introduction
The chondrichthyans, or sharks, rays and chimaeras, arevery
successful animals that rst appeared over 400millionyears ago, long
before the age of dinosaurs. They arerelated to the bony shes, or
osteichthyans, which are farmore numerous and diverse unlike bony
shes, sharksand rays have an internal skeleton composed of
cartilagerather than bone. Sharks and rays, while not nearly
asdiverse as bony shes, have nevertheless maintained theirposition
as important predators in many marine and somefreshwater
environments.
Basic Design
Sharks havemaintained the same basic body plan (Figures 1and 2)
throughout their 400-million-year-plus history.Sharks typically
have a streamlinedbodywith a long snout,an underslungmouth, ve to
seven pairs of gill slits, a smallspiracular opening, paired
pectoral and pelvic ns, one ortwo dorsal ns, an anal n (absent in
some groups), and acrescent-shaped caudal (tail) n which is
heterocercal,meaning the vertebral column is upturned and extends
outto the tip of the ns upper lobe.Male sharks have claspers paired
appendages used inmating associated with theirpelvic ns. Aristotle,
in 350 bc, recognized importantdierences between sharks and bony
shes, including theobservation that sharks have separate gill slits
while thegills of bony shes are covered by an opercular ap.
Rays, closely related to sharks, are primarily bottom-dwellers
that have evolved a attened, disc-shaped body,with eyes located on
top of their heads. They use theirgreatly expanded wing-like
pectoral ns to glide gracefullythrough the water. Many rays have
modied their teethinto at crushing plates for feeding on molluscs
andcrustaceans.In addition to the sharks and rays, most modern
systematic arrangements place another group within
theChondrichthyes the Holocephali (or holocephalans),more commonly
referred to as chimaeras, elephantshes orratshes. There are around
35 living species, all marine andprimarily found in relatively deep
cold water. Holocepha-lans share with sharks and rays a
cartilaginous skeleton,and males also have claspers associated with
their pelvicns. Holocephalans, however, are rather peculiar
animalsthat dier markedly from sharks in a number of respects.Many
have oddly shaped protruding snouts, and theygenerally have large
heads and prominent eyes, ventrallypositioned mouths, jaws with
ridged knobby toothplates,smooth scaleless bodies, gills covered by
a single opercularap, and tapering whip-like tails. Holocephalans
also have
Article Contents
Introductory article
. Introduction
. Basic Design
. Diversity
. Habitats
. Reproduction
. Special Sensory Adaptations
. Fossil History
. Phylogeny
. Sharks and Humans
. Conservation
Firstdorsalfin
Seconddorsalfin
Caudalfin
Analfin
Pelvicfin
PectoralfinGill
openings
Mouth
Nostril
Snout
EyeSpiracle
Figure 1 External features of a generalized shark.Figure 2
Blacktip shark, Carcharhinus limbatus (photograph byD. Perrine
&).
1ENCYCLOPEDIA OF LIFE SCIENCES 2001, John Wiley & Sons, Ltd.
www.els.net
-
upper jaws that are rmly fused to the cranium, as opposedto the
mobile upper jaws of sharks and rays.A feature shared by all
chondrichthyans is an internal
skeleton of cartilage, rather than bone as in othervertebrates
the name Chondrichthyes means cartilagi-nous sh. Sharks and rays
uniquely possess prismaticcartilage, in which the cartilages
surface layer is composedof small crystalline prisms. Recent
research has shown thatthe jaws of some mollusc-crushing rays are
internallyreinforced at stress points by struts (or trabeculae)
ofcartilage arranged in much the same way as the bone ofother
vertebrates, and that some primitive fossil sharkspossess true bone
as well as cartilage.Cartilaginous shes do not have swim bladders,
which
bony shes use to maintain buoyancy, but instead rely onvery
large oily livers to help keep them from sinking.However, it is not
true that all sharks have to swimcontinuously to keep alive many
species, including nursesharks (Ginglymostoma cirratum) which are
commonlykept in aquaria, regularly rest on the bottom.Most sharks
are active predators and have sharp,
triangular, cutting teeth. Sharks continuously produceand shed
their teeth. It is estimated that some species mayproduce 50 000 or
more in a single lifetime. The teeth arearranged in rows on the
jaws, with each row alwaysadvancing toward the front edge of the
jaw like a slow-moving conveyor belt, and newer replacement teeth
takingthe position of the older teeth as they are shed along
themargin of the jaws. It has recently been discovered thatsome
sharks begin producing, shedding, and replacingtheir teeth even
before they are born. Rays and holoce-phalans have fundamentally
the same kind of toothreplacement but the whole process is much
slower and in
holocephalans the old teeth are kept and overgrown bynewer parts
of the tooth plates. Along with teeth, mostcartilaginous shes have
tiny placoid scales embedded intheir skin, which gives the skin its
prickly feel, and somespecies have sharp spines in front of their
dorsal ns.
Diversity
Many dierent species of sharks and rays have evolvedover the
course of their very long history, but at present thegroup is not
exceptionally diverse. There are nearly 600living species of rays
and approximately 350 shark species.In comparison, the living bony
shes (osteichthyans)number more than 23 000 species (about half of
all knownvertebrates).Despite their relatively low overall
diversity, living
sharks and rays are important and conspicuous membersof many
marine and some freshwater ecosystems. Livingsharks range in size
from the tiny dwarf lantern shark(Etmopterus perryi), with a total
length of 20 cm (8 in), tothe gigantic planktivorous whale shark
(Rhincodon typus),which grows to a length of at least 12m (40 ft).
Accordingto recent classications, living sharks and rays are
dividedbetween two major groups, the Squalea and the Galeo-morphii
(see Figure 3).
Habitats
Chondrichthyans are found in a wide range of
aquatic,predominantly marine habitats. They are most common
Carcharhiniformes requiem, hammerhead, tiger, lemon and cat
sharks
Lamniformes basking, thresher, sand tiger, mako and great white
sharks
Orectolobiformes whale, nurse, bamboo and wobbegong sharks
Heterodontiformes horn and bullhead sharks
Hexanchiformes cow and frill sharks
Echinorhiniformes bramble sharks
Squaliformes dogfish and sleeper sharks
Squatiniformes angel sharks
Pristiophoriformes sawsharks
Rajiformes skates and rays
Galeomorphii
Squalea
Figure 3 Evolutionary relationships of the major groups of
modern elasmobranchs or Neoselachii (adapted from de Carvalho,
1996).
Chondrichthyes (Sharks, Rays and Chimaeras)
2
-
and have their greatest diversity in temperate to tropicalmarine
environments, in coastal to shallow shelf areas ofthe sea. Very few
species are found below 2000 metres;among those that are the gulper
sharks (Centrophorus),which can occur down to 4000 metres. Many
shark speciesoccupy themedium-to-largemarine predator niche, and
assuch are typically found in habitats with a wide diversity ofshes
and other marine animals that are their potentialprey. These
habitats commonly include sandy-bottomedshallows o open beaches,
lagoons, estuaries, rockycoastlines, coral and rocky reefs, and
kelp forests.Particularly favoured areas include biologically rich
coast-al upwelling zones and the mouths of rivers and deltas. Afew
species, most notably the bull shark (Carcharhinusleucas), enter
freshwater environments bull sharks arewell known for swimming up
rivers and also enterfreshwater lakes. Some rays spend their entire
lives infreshwater, including the river rays (Potamotrygonidae)
oftropical South America.
Reproduction
Cartilaginous shes are unusual compared to most bonyshes in that
they produce a relatively small number ofospring, anywhere from one
to a few dozen depending onthe species (bony shes often produce
thousands or evenmillions of eggs at a time). The eggs are
fertilized internally;males use their claspers to transmit sperm
during copula-tion. Mating has only rarely been observed in sharks.
It isthought to occur seasonally for only a brief time each
year,and even less frequently in some species like the spinydogsh
(Squalus acanthias), which has a 2-year gestationperiod (the
longest known for any animal). Female sharksare often observedwith
scars around their pectoral ns andon the top of their heads,
probably the result of malesseizing them during copulation.In the
majority of sharks, the eggs develop inside the
female sharks uterus, with a long gestation period of up to2
years, and with nutrition supplied by large yolk sacs. In afew
species, for example sand-tiger sharks (Carchariastaurus), young
inside the uterus may cannibalize theirsiblings while they are
developing. Alternatively, someproduce eggs covered in a tough,
horny egg case. Theyoung develop inside the egg case but outside
theirmothers body, and also receive nourishment via a yolksac. Many
aquaria now feature exhibits in which babysharks can be watched
developing inside egg cases. In bothstrategies, once born the young
are fully functional andindependent from their parents.
Special Sensory Adaptations
Sharks and rays are often regarded as very primitive,
andtherefore simple, animals. While they are primitive withrespect
to their systematic position, they neverthelesspossess a number of
very specialized and highly sophisti-cated sensory capabilities.
Perhaps the most intriguing istheir ability to detect the weak
electrical signals producedby muscular activity in other animals,
which helps them todetect and track prey and in some cases to
locate potentialmates. Sharks detect electrical signals using
specialelectroreceptors called the ampullae of Lorenzini,
smallchambers lled with a gelatinous substance and lined
withspecial sensory cells. These ampullae, which are clusteredin
the head region particularly on the sides and undersur-face of the
snout, can detect electric signals as weak as ve-billionths of a
volt. Along with electroreception, sharksalso have a highly
sensitive lateral line system for detectingmovements andvibrations
in thewater, another very usefuladaptation for amarine predator.
The lateral line receptorsrun along the body on both sides, and
have branches on thehead. Sharks also have acute hearing, balance
and vision,and an especially keen sense of smell capable of
detectingjust a few molecules of blood in seawater.
Fossil History
Our knowledge of fossil sharks is limited in many cases totheir
teeth,which are covered in hard enamel andproducedin great numbers,
supplemented by occasional spool-likecalcied vertebral centra. Less
commonly n spines arefound. The cartilaginous skeleton of sharks
rarely fossi-lizes, although some preserved skeletons
(ironically,usually of early primitive sharks, and also of some
raysand chimaeras) have been found.Cartilaginous shes are a very
ancient groupwith a fossil
record that extends back into the Palaeozoic era, to theSilurian
period approximately 420 million years ago (Ma).The earliest fossil
chondrichthyans are known only fromisolated scales the exact nature
of these rst representa-tives is a mystery, as is the origin of the
group as a whole.Chondrichthyans may be closely related to an
extinctPalaeozoic group of jawless shes called thelodonts whichhave
somewhat similar scales, or possibly to the placo-derms, early
jawed shes with bony armour. Under-standing the early evolution of
sharks remainsproblematic due to the lack of informative specimens
ofthe earliest basal forms.The oldest well-known shark is
Cladoselache, from the
Devonian ofOhio (about 380Ma).Cladoselache resemblesmodern
sharks, with a streamlined body, ve pairs of gillslits, paired
large pectoral and smaller pelvic ns, twodorsal ns (each with a
large spine), and a cartilaginousskeleton. If Cladoselache were
alive today there would be
Chondrichthyes (Sharks, Rays and Chimaeras)
3
-
no diculty in identifying it as a shark. The teeth
ofCladoselache and a number of related primitive sharks
aredistinctive, with a high central cusp anked on either sideby
smaller lateral cusps.Following the Devonian, the Carboniferous and
Per-
mianperiods (c. 360250Ma) sawagreat early radiationofsharks,
including a number of bizarre forms characterizedby unusually
modied ns and odd dentitions. One groupcalled the iniopterygians
developed very long, wing-likepectoral ns, possibly used to y above
the water like theliving ying shes. The stethacanthids and
falcatidsinclude species with elaborately modied dorsal nelements
that project out over their heads, probably usedby males in
courtship and/or as a threat display. Theedestids have teeth
arranged in a spiral whorl with thenewest and largest teeth on the
outside of the whorl. Someedestids may have grown very large, but
only their peculiartooth whorls are well known. Among themost
unusual arethe petalodontids, poorly known until recent
discoveries,which have shown them to have deep bulbous bodies,
shortheads and strongly ridged teeth suitable for crushingshellsh
and coral. Many of the unusual sharks of this eraare known from
superb specimens found at two veryimportantCarboniferous sites
theBearGulchLimestoneof Montana, USA, and the Bearsden deposit
nearGlasgow, Scotland. Along with these, freshwater depositsfrom
this time interval commonly contain remains of thexenacanths large,
elongate sharks with tapering tails andprominent spines immediately
behind their heads. Somexenacanths are known from well-preserved
completeskeletons.The unusual sharks of the Palaeozoic largely
disap-
peared by the Mesozoic era (25065Ma). One Palaeozoicgroup that
survives into the Mesozoic, the hybodonts,supercially look like
Cladoselache but have a number ofadvanced skeletal features that
link them more closely tomodern sharks. By the middle of the
Mesozoic, the rstremains of rays are found, and by the latter part
of theMesozoic the new sharks, or neoselachians, becomeestablished
as the dominant chondrichthyans. Neosela-chians are characterized
by more mobile jaws relative totheir primitive ancestors. A number
of neoselachianspecies die out at the close of the Mesozoic era,
65Ma,but some lineages survived into the next era theCenozoic.All
Cenozoic sharks and rays, including those living today,are
neoselachians (the group also includes some fossilforms that are
now extinct).The most notorious neoselachian, and the most
specta-
cular marine predator to have ever evolved, is the
giantCarcharodonmegalodon, or Megatooth, a close relative ofliving
greatwhite sharks (C. carcharias).Megatooth sharksare found
worldwide in fossil marine deposits from about25 to 3Ma. These
superpredators reached over 16m (50 ft)in total length, but not 100
ft or more as claimed in earlierexaggerated accounts, and may have
had a mass in excessof 50 000 kg (the equivalent of several
Tyrannosaurus rex
dinosaurs!). Particularly notable are their enormousserrated
triangular teeth, which grew up to seven inchesin length and were
used to catch baleen whales and otherlarge prey. The megatooth is
probably the largest sharkever to have evolved only the living
whale shark(Cetorhinus maximus), a placid lter-feeding giant,
reachesa comparable size.Holocephalans have a fossil record
extendingback to the
Devonian, suggesting a split from sharks and raysapproximately
350400Ma.
Phylogeny
According to recent analyses, all living sharks and rays,and a
few close fossil relatives, are members of a singlemonophyletic
group, the Neoselachii (Figure 3). The oddfossil forms that
appeared in the Palaeozoic are consideredto be more primitive and
lie outside the Neoselachii.Neoselachians are united as a
monophyletic group byseveral shared characters, including features
of the gillarches, jaw musculature, pectoral girdle and teeth.
Withinneoselachians, two main lineages of living sharks
arerecognized the Galeomorphii, which includes theheterodontiforms,
orectolobiforms, lamniforms andcarcharhiniforms, and the Squalea,
comprising the hexan-chiforms, echinorhiniforms, squaliforms,
squatiniforms,pristiophoriforms and rajiforms. The monophyletic
statusof both the Galeomorphii and the Squalea is supported
byseveral shared derived features.This current framework elaborates
on and somewhat
modies what was the prevailing view for many years,which
postulated four main groups of extant sharks theGaleomorphii,
Squalomorphii, Squatinomorphii and Ba-toidea. This earlier
hypothesis left unresolved the questionof the interrelationships of
the batoids (Rajiformes),which are now considered to be members of
the Squalea,closely related to the pristiophoriforms
(sawsharks).Sharks and rays are grouped together within Chon-
drichthyes as the Elasmobranchii (or elasmobranchs),
withholocephalans as their next most closely related (or
sister)group.
Sharks and Humans
Sharks are a subject of great fascination to humans, nodoubt
because of the perception that they pose a threat tous. In fact,
given themanymillions of peoplewho engage invarious forms of
aquatic recreation, the number ofconrmed deadly shark attacks is
actually relatively low(although the attacks that do occur
receivewide attention).Attacks have most commonly happened along
the coast-lines of Australia, South Africa and the USA, all of
whichhave large populations of coastal shark species. In an
Chondrichthyes (Sharks, Rays and Chimaeras)
4
-
average year, about 20 human deaths worldwide can beattributed
to sharks (while at the same time we catch andkill millions of
sharks, often as incidental, wastefulbycatch).Shark attacks are,
therefore, uncommon, and in many
cases may even be accidental on the part of the shark.
Theattacks can be divided into two types provoked, in whichthe
shark is reacting to aperceived threat, andunprovoked,which are
more mysterious. Certain species, such as thegrey reef shark
(Carcharhinus amblyrhynchos), are moreaggressive than others, and
may attack if approached tooclosely. Other aggressive species
include the tiger (Galeo-cerdo cuvier), bull (Carcharhinus leucas),
blue (Prionaceglauca), hammerhead (Sphyrna species) and great
white(Carcharodon carcharias) sharks. Often aggressive
speciessignal their intent by body language, including archingtheir
back into a hump. One hypothesis is that sharksmistake people in
the water for favourite prey, includingthe observation that surfers
resemble seals when viewedfrombelow.This idea remains controversial
andmay be anoversimplication.Some simple precautions will reduce
the unlikely
possibility of being attacked by a shark. Swimmers,
divers,surfers, etc., should avoid shallow, murky water near
rivermouths at dawn or dusk, or at night, conditions underwhich a
number of attacks have occurred. Recently,researchers in South
Africa have experimented withelectromagnetic elds as a means of
repelling sharks; whilesome promising results have been obtained,
the eective-ness of these devices is still inconclusive.Sharks are
also benecial to humans in many ways. A
number of species are caught for food, and shark meat hasgained
in popularity in recent years (to the detriment ofsome
overharvested species). Shark skin is an eectiveabrasive, and is
also used to make wallets, shoes and otherproducts. Shark liver oil
is thought bymany to have healthbenets, and the corneas from shark
eyes have been usedfor corneal transplants in humans. Recent
research has,however, cast serious doubt on the eectiveness of
sharkcartilage derivatives as a treatment for cancer.Sharks occupy
an important position in the art and
traditional religious beliefs of many sea-based cultures.Such
beliefs are particularly strong among the nativeislanders of the
Pacic, and in the Aboriginal culture ofAustralia, where sharks are
greatly revered and respected.
Conservation
In recent years it has become increasingly clear that
thepopulations ofmany shark species are in sharpdecline. The
primary cause of this is commercial overshing; sharks arecaught
deliberately in very large numbers, and are alsokilled as
accidental bycatch in other sheries. Much of thementality that
contributed to this current state of aairsarose in the 1970s, when
sharks were both popularized anddemonized in the popular media,
most notoriously in thebook and lm versions of Jaws.We nowhave a
better andmore sophisticated appreciation of the crucial role
thatsharks and rays play as predators, and in some casesscavengers,
in helping to maintaining healthy balancedecosystems.Reduced
population levels are of particular concern in
sharks because of their long gestation periods and
lowreproductive rates. To address this issue, shark biologistsand
marine conservation groups have strongly urged thatshark shing be
reduced to sustainable levels, that thewasteful practice of nning
(killing sharks only for theirns) be halted, and that critical
habitats, such as shallow-water nursery areas, be preserved and
protected. There aresome promising recent signs that this problem
is beingtaken seriously. For example, South Africa, Australia
andthe USA have taken steps to protect the spectacular
butthreatened great white shark Carcharodon carcharias, anda number
of countries are working towards restricting thecommercial catch of
declining shark species so that theirpopulations have a chance to
recover. Organizationsactively involved in shark research and
conservationinclude the American Elasmobranch Society, the
Eur-opean Elasmobranch Association and the Pelagic SharkResearch
Foundation.
Further Reading
Compagno LJV (1984) FAO Species Catalogue. Sharks of the
World.
FAO Fisheries Synopsis 125, volume 4(12): 1655.
Compagno LJV, Simpfendorfer C, McCosker J et al. (1998)
Readers
Digest Explores Sharks. Readers Digest: Pleasantville, NY.
de Carvalho MR (1996) Higher level elasmobranch phylogeny,
basal
squaleans, and paraphyly. In: Staissny M, Parenti L and Johnson
G
(eds) Interrelationships of Fishes, pp. 3562. San Diego:
Academic
Press.
GottfriedMD,CompagnoLJVandBowmanSC (1996) Size and skeletal
anatomy of the giant megatooth shark Carcharodon megalodon.
In:
Klimley AP and Ainley DG (eds) Great White Sharks: the Biology
of
Carcharodon carcharias, pp. 5566. San Diego: Academic Press.
Klimley AP andAinleyDG (eds) (1996)GreatWhite Sharks: the
Biology
of Carcharodon carcharias. San Diego: Academic Press.
Paxton JR and Eschmeyer WN (1994) Encyclopedia of Fishes.
San
Diego: Academic Press.
Tricas TC, Deacon K, Last P et al. (1997) The Nature Company
Guide:
Sharks and Rays. Hong Kong: Time-Life Books.
Chondrichthyes (Sharks, Rays and Chimaeras)
5