Gnathostomes fishes (1) Chondrichthyes

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Chapter 5



Two major innovations make this group successful:

jaws developed from the first gill arch
paired appendages (fins)

These two characteristics allowed them to become predaceous jaws allowed them to catch and consume prey

paired appendages gave fishes more maneuverability - fins add stability and control
there is no good indication for the origin of appendages - one simply the fin-fold theory

Gnathostomes developed into a number of clades:

Ancestral gnathostomes =>
Placoderms - extinct
Acanthodians - extinct
Flourished in the late Silurian and Devonian
Placodermi - armoured fishes An extinct group well-represented in the fossil record
Name means “plate skin” because skin had dermal bone plates
much like Ostracoderms; however, placoderms had jaws and paired appendages
no evidence of vertebral column
had a notochord but no vertebral column
varied in size- some as large as 6m in length
well-represented in the Devonian Period - some were benthic while others appeared to be more predaceous


skeleton composed of calcified cartilage
bone is thought to be more primitive to cartilage
500-700 species; almost all marine
have jaws and paired appendages
have 5-7 external gill openings or gill slits and one reduced gill called a spiracle or false gill
no swim bladder
internal fertilization
may be viviparous, oviparous, or ovoviviparous

Two Parental Clades for Chondrichthyes

Subclass Elasmobranchii
the sharks, rays, and skates
~700 species
several gill openings
placoid type of scales

Elasmobranchii subdivided into two groups:

pleurotremata (“side hole”) - refers to location of gills on the side of the head: sharks
hypsotremata (“below hole”) - refers to location of gills on the ventral surface: rays and skates
gills of both groups ventilated by a double pump and by the spiracles
in some sharks ventilation requires a continuous movement for water to pass over the gills
sharks lack a swim bladder; have an oil (squalene) produced by the liver which acts as a density control device by adjusting overall body density


Oviparous - egg layers
Ovoviviparous - live birth
during development the young are nourished by the yolk
when the egg is fertilized, it has a large yolk and the embryo develops in the uterus while obtaining nutrient from the stored yolk
Viviparous - live birth
young receive nourishment from some type of placental attachment
nourishment comes from the mother’s blood, secretions, or other tissues

Generalized Classification of Elasmobranchii:

Order Squalimorphes (Squaloids) - spring and green dogfish sharks, basking and megamouth sharks
~80 species
live in cold deep waters
Squalus acanthius - dogfish shark

Order Galeomorphs (Galeoids) - contains most of the sharks

~250 species
all with five gill openings
dominant carnivores of shallow waters
hammerhead, great white, whale shark

Order Batoidea (Rajiformes)- skates and rays

~425 species
skates have pointed nose and lay eggs, rays give live birth

Family: Echinorhinidae - Bramble sharks Order: Squalimorphes (bramble, sleeper and dogfish sharks) Subclass: Elasmobranchii - sharks and rays

Large (3-4 m), wide-ranging, deepwater sharks in cold-temperate to tropical seas.
Circumglobal distribution on continental and insular shelves and slopes from 11 to 900 m, on or near the bottom.
Short-nosed, cylindrical sharks with no anal fin
Skin covered with coarse denticles or enlarged thorns
Feed on a variety of benthic and neritic fishes, including other sharks as well as crabs, octopuses and squids
Thought to suck in their prey by suddenly expanding their mouths and pharynxes when in range
Ovoviviparous and lack a yolk sac placenta

Family: Squalidae - Dogfish sharks Order: Squalimorphes (bramble, sleeper and dogfish sharks) Subclass: Elasmobranchii (sharks and rays)

Atlantic, Indian and Pacific Oceans
No lateral teeth or barbels. Dorsal fin with or without spines
5 gill openings anterior to pectoral fin
Maximum length at 6.3 m, reported for Somniosus microcephalus
This family includes the smallest sharks in the world.

Family: Ginglymostomatidae - Nurse sharks SubOrder: Orectolobiformes (carpet sharks) Order Squalimorphes Subclass: Elasmobranchii (sharks and rays)

Marine, all oceans. Small to large sharks with nasoral grooves, short to long barbels, small spiracles behind eyes.
Two spineless fins and an anal fin

A short precaudal tail

Common, small to large, nocturnal, inshore bottom sharks with a circumglobal distribution in subtropical and tropical waters, in depths from the intertidal down to at least 70 m.
Cruise and clamber on the bottom with their mouths and barbels close to the substrate while searching for food.
They use their short, small mouths and large mouth cavities as a bellows to suck in their prey: bony fish, crabs, shrimps, lobsters, squids, octopuses, corals, sea urchins, sea squirts.

Family: Cetorhinidae - Basking sharks Order: Squalimorphes Subclass: Elasmobranchii (sharks and rays)

Marine, all oceans, highly migratory.
Gill openings exceptionally large; gill rakers elongate, plankton feeders, teeth reduced.
The tail is nearly symmetrical with keel on caudal peduncle.
Fifth gill opening in front of pectoral fin.
The family contains the world's second largest fish, reportedly reaching 40 feet (12 m) in TL.

Family: Sphyrnidae - Hammerhead, Bonnethead, and Scoophead sharks SubOrder: Carchariniformes (ground sharks) Order Galeomorphes Subclass: Elasmobranchii (sharks and rays)

Marine, coastal; occasionally in brackish water.
Global (chiefly warm waters).
Head laterally expanded, with eyes and nasal openings much widely set than in other sharks.
No spiracle.
A maximum length of 4.5 m was reported for Sphyrna tudes.
Feed on a wide variety of bony fish, elasmobranchs, cephalopods, crustaceans, and other prey.

Family: Lamnidae - Mackerel sharks, White sharks SubOrder: Lamniformes (mackerel sharks) Order Galeomorphes Subclass: Elasmobranchii (sharks and rays)

Distribution: global. Large sharks with pointed snouts and spindle-shaped bodies.
Large gill openings.
First dorsal fin large, high, erect and angular or somewhat rounded; second dorsal and anal fins minute.
Caudal peduncle with a distinct keel; large teeth; fifth gill opening in front of pectoral fin; spiracle sometimes absent.
Maximum length up to 6.4 m or more.
Fast swimming predators - some are man-eaters.
Ovoviviparous without a yolk-sac placenta but with uterine cannibalism.

Family: Pristidae - Sawfishes SubOrder: Pristiformes (sawfishes) Order Galeomorphes Subclass: Elasmobranchii (sharks and rays)

Atlantic, Indian, and Pacific Oceans.
Snout elongated into a long flat blade with teeth of uniform size deeply embedded on each side.
No barbels. Head depressed with the body more or less resembling a shark.
Two distinct dorsal fins.
Giant fishes of coastal waters, entering estuaries and ascending large rivers for great distances. Used for food and regarded as exciting game fishes because of their size. Although generally harmless to humans, their large size and sawlike beak make them dangerous when hooked or speared.
Of the many nominal species only four appear to be valid.

Family: Rajidae - Skates Order: Rajiformes (skates and rays) Subclass: Elasmobranchii (sharks and rays)

Benthic rays occurring in all oceans, from Arctic to Antarctic waters and from shallow coastal shelves to abyssal regions; some species enter brackish waters.
Disc quadrangular to rhomboidal.
Mouth transversed to arched, with numerous teeth.
Five pairs of ventral gill slits.
Tail very slender, with lateral folds, usually 2 reduced dorsal fins and a reduced caudal fin.
Electric organs weak, developed from caudal muscles.
Skin prickly in most species, the prickles often in a row along midline of dorsal.
Oviparous; eggs in a horny capsule with four long tips.
Skates feed on other benthic organisms. Skate wings are considered good eating.

Family: Anacanthobatidae - Smooth skates Order: Rajiformes (skates and rays) Subclass: Elasmobranchii (sharks and rays)

Known from South Africa, Natal, and the tropical western Atlantic; also from Taiwan.
A terminal filament of varying length arising from a small, bluntly rounded protuberance at the tip of the snout.
Dorsal fins absent, but membranous caudal fin present. Five pairs of small, ventral gill slits.
Dorsal and ventral surfaces of disc smooth, without dermal denticles.
Tail slender, a bit shorter than disc. Small skates of slope regions in tropical/subtropical waters.

Family: Platyrhinidae - Thornbacks Order: Rajiformes (skates and rays) Subclass: Elasmobranchii (sharks and rays)

Atlantic, Indian and Pacific Oceans.
Body form intermediate between that of a shark and a skate.
Tail stout, not well demarcated from body.
Dorsal fins 2; distinct.
Caudal fin present.
Denticles arranged in a row on dorsal midline.
No spine in tail.

Family: Rhinobatidae - Guitarfishes Order: Rajiformes (skates and rays) Subclass: Elasmobranchii (sharks and rays)

Atlantic, Indian and Pacific Oceans mostly in tropical coastal waters.
Body form intermediate between that of a shark and a skate.
Also called shovelnose sharks.
Numerous small, blunt teeth in jaws.
Two large dorsal fins; caudal fin well developed.
Denticles arranged in a row on dorsal midline.
No spine in tail.
They reach moderate to large size and are important commercial species in many coastal nations.
Feed on bottom organisms, including mollusks and crustaceans, but will also take small fishes.

Family: Dasyatidae - Sting rays SubOrder: Myliobatiformes (eagle rays, stingrays and mantas) Order: Rajiformes Subclass: Elasmobranchii (sharks and rays)

Chiefly marine; also in brackish and freshwater - Atlantic, Indian and Pacific Oceans.
Side of head continuous with anterior margin of pectoral fin.
Dorsal fin totally absent or indistinct, when present.
Disc about 1.2 times as broad as long.
No caudal fin.
Tail long and whip-like.
Most species with at least 1 long venomous spine on tail, which can cause excruciating pain to humans.
Largest species to about 4m length or width.
Live-bearing (ovoviviparous).

Family: Mobulidae - Manta rays and devil rays SubOrder: Myliobatiformes (eagle rays, stingrays and mantas) Order: Rajiformes Subclass: Elasmobranchii (sharks and rays)

Atlantic, Indian and Pacific Oceans.
Head differentiated from disc.
Eyes and spiracles on sides of head.
Mouth ventral in Mobula and terminal in Manta.
Size of gill openings almost the length of eye. Length of tail much greater than disc. Some with spines.
Dorsal fin small. Caudal fin lacking. Three functional paired limbs. The cephalic pair of fins aid in feeding and are said to be essentially the anterior subdivision of the pectorals.
Some reach about 6.1 m and weigh over 1360 kg.

Subclass Holocephali
chimaeras or ratfish
~25 species
gills covered by an operculum
no placoid scales

Chimaeridae - Chimaeras or ratfishes Order: Chimaeriformes          Subclass: Holocephali

Atlantic, Pacific, and Indian oceans in temperate to tropical waters, mostly below 200 m
Short and rounded snout
First dorsal fin erectile, with a strong spine; second dorsal fin long and low
Diphycercal tail - vertebral column runs straight to the tip, dividing the caudal fin symmetrically
Anal fin confluent with caudal fin in Hydrolagus ; separate in Chimaera
Feed on small fishes and bottom invertebrates
Males with head clasper
Oviparous; large tadpole-shaped egg capsules are deposited on substrate
Dorsal spine with associated poison gland, the venom of which hurts humans

Family: Rhinochimaeridae - Longnose chimaeras Remark: Found at scattered localities worldwide in temperate and tropical seas, in 200 to more than 2,000 m depth. Long and pointed snout, lacking a hooklike process. First dorsal fin erectile, with a strong, mildly toxic spine; second dorsal fin long, low and not falcate. Diphycercal tail, i.e., the vertebral column is extending to the tip and divides the caudal fin symmetrically; caudal fin may be confluent with or separate from anal fin. Oviparous: egg-cases spindle-shaped, with broad filamentous horizontal flanges

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