Chordates: Class Chondrichthyes (Sharks, Skates, Rays)

• The text presents the fourth lecture, focusing on chordates.

Chondrichthyes

• Chondrichthyes includes both predators and harmless mollusk eaters.
• Sharks, skates, and rays belong to the Chondrichthyes.
• Members have skeletons made of cartilage.

General Characteristics of Class Chondrichthyes

• The skin is typically covered with placoid scales, which is an exception in chimaeras.
• The skeleton is entirely cartilaginous.
• Teeth are modified placoid scales, typically cone-shaped with expanded triangular bases.
• Gills open separately on the body, without an opercular flap, except in chimaeras.
• Pelvic fins in males are modified to form claspers.
• Jaws are simple and tooth-bearing.
• Lungs or swim bladders are absent.

Study of Scoliodon (Indian Dog Fish)

• Systematic Position
• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Class: Chondrichthyes
• Subclass: Elasmobranchii
• Habitat
• The Scoliodon is a vertebrate found in oceans.
• Widely distributed in the Indo-Pacific oceanic region.
• Predaceous, active swimmers, and voracious feeders.
• Description
• Has a laterally compressed, long body (about 60 cm), spindle-shaped that includes a head, trunk, and tail.
• Its ventral surface is white or pale, while dorsal and lateral sides are pigmented dark grey.
• The head is dorso-ventrally compressed to form the snout, containing oblique nostrils, a slit-like mouth, and lateral protuberant eyes.
• Five pairs of lateral gill clefts are located behind the eyes.
• The trunk portion has lateral paired fins and median unpaired fins:
  • Lateral paired fins: two posterior pelvic and two anterior pectoral fins
  • Median unpaired fins: small second dorsal and large first dorsal fins.
• The heterocercal tail, with its vertebral column and musculature is turned upwards.
• Two pigmented lateral lines extend from head to tail.
• Scoliodon shows sexual dimorphism, as males have a pair of claspers.
• The cloaca is present between the two pelvic fins.
• Fins
• Fins are thin, flat outgrowths of skin with muscles supported by cartilaginous rods or rays.
• Scoliodon has unpaired median fins i.e., 1st dorsal fin, second dorsal fin, caudal fin, anal fin, and paired fins (anterior pair pectoral fins and posterior lateral pelvic fins).
• Pectoral fins are large and situated posterior to the gill clefts.
• Pelvic fins are simple; in males, the inner margin bears a pair of rod-shaped copulatory organs called claspers.
• Claspers are intermittent organs that introduce sperm into the female genital tract.
• Fins propel the body forward during swimming

Scoliodon Body Openings

• Mouth
• Placed a little behind the apex on the ventral side of the anterior region.
• Bounded by upper and lower jaws, each with 1 or 2 rows of sharply pointed and backwardly directed teeth
  • Teeth are for holding and tearing prey.
• Nares
• These are two crescent apertures, the nares or nostrils, located ventro-laterally and anterior to the mouth.
• Exclusively olfactory; having no role in respiration, connected to the mouth cavity by internal nostrils.
• External Gill Slits
• Anterior to each pectoral fin, present on either side of the body, are vertically elongated external gill slits or branchial clefts in a series of 1 to 5
  • These are main respiratory organs.
• Cloacal Aperture
• Found on the tail region between two pelvic fins is an elongated median groove or cloacal aperture.
  • Which leads to a small chamber, the cloaca, a common exit for digestive and the urinogenital system.

Lateral Line and Pores

• The lateral line runs along each lateral side of the body.
• It marks the position of a sensory lateral line canal system that opens via minute pores.

Digestive System

• Consists of the alimentary canal and associated glands.
• Alimentary Canal
• Complete, divided into the buccal cavity, pharynx, oesophagus, stomach, intestine, and rectum.
• The stomodeal portion is lined with ectoderm, the mesodaeum with endoderm, and the proctodaeum with ectoderm.
• Buccal Cavity
• Wide cavity containing sharp, pointed, and backwardly directed homodont and polyphyodent teeth.
• Mainly to prevent the escape of prey.
• The mucous membrane prevents prey from escaping.
• A glandular and muscular tongue is formed.
• Pharynx
• The pharynx is a large wide chamber, which opens through five vertical slits called internal gill slits into the gills.
  • The mucous membrane contains denticles to prevent the escape of prey.
• Oesophagus
• The pharynx opens into a short, narrow, thick, muscular oesophagus containing rugae.
• It remains closed, except when swallowing food, to prevent water from entering the stomach.
• It opens into the stomach through an oesophageal valve.
• Stomach
  • A 'J' shaped tube divided into two parts: -- Cardiac Stomach: a long, broad muscular part containing longitudinal folds, ending with a cardiac valve. -- Blind Sac: A small outgrowth of unknown function that starts at the end of the cardiac stomach.
• Intestine
  • The intestine is a straight, wide tube divided into the duodenum and ileum. -- Duodenum: the narrow, anterior part receiving the common bile duct dorsally and the pancreatic duct ventrally. -- Ileum: the remainder, with the mucous membrane produced into a scroll valve, increasing absorptive area.
• --The passage of food slows down for proper absorption
• The intestine opens into the rectum with a rectal valve between them. -- Rectal gland similar to medulla of the kidney, excreting excess salts.
• The rectum leads to the cloaca.

Scoliodon Reproductive System

• The cloaca is a short terminal chamber, which receives ducts of the urinogenital system, and it has a pair of abdominal pores.

Osteichthyes (Bony Fishes)

• About 30,000 species of bony fishes are known to exist in this class
• These are spindle-shaped, oval in section, or flattened.

Characteristics of Osteichthyes

• Their endoskeleton is made of bones.
• Paired and median fins are present, supported by long rays of cartilage or bone
• Fleshy, lobed fins are present in sarcopterygians
• These types of fins are supported by bones having joints and are adapted to live at the bottom of the sea.
• The tail is mostly homocercal.
• Contain a swim bladder or air bladder for buoyancy and gaseous exchange.
• Mouth is terminal.
• Contain four pairs of gills, protected by a bony flap called the operculum.
• Sarcopterygians, lungfishes, and lobe-finned fishes have lungs.
• The skin is covered by bony dermal scales, classified as ganoid, cycloid, or ctenoid scales.
• Cold-blooded animals, lacking the capacity to regulate internal body temperature. -- but larger marine fishes like tuna and swordfish have some endothermy.
• The heart is two-chambered (one auricle and one ventricle) except for lungfishes -- Lungfishes have a three-chambered heart (two auricles and one ventricle).
• The nervous system is poorly developed compared to other vertebrates.
• The brain is divided into the forebrain (smell), midbrain (vision/learning/motor), and hindbrain (medulla oblongata and cerebellum for coordination).
• The spinal cord and a matrix of nerves serve the rest of the body.
• The lateral line organ contains hydrodynamic receptors (neuromasts)
• It helps to sense vibration, water pressure, and assistance in navigation and locating their prey.

Osteichthyes Digestive System

• The esophagus is short and expandable
• Most species have a stomach with gastric glands for digestion.
• Many bony fishes have blind sacs called pyloric caeca at the end of the stomach (increase gut area and digest food).
• There is an absence of cloaca, different genital and anal openings are present.
• Possess mesonephric kidneys
• Sexes separate, but some hermaphrodites exist.
• Mostly oviparous, laying many eggs Fertilization is mostly external with direct development.
• The male pelvic fins are not modified into claspers.

Scales in Fishes

• Placoid Scales
• Found in cartilaginous fishes (sharks, rays, and chimaeras).
• Structurally homologous to vertebrate teeth, with a central pulp cavity, conical dentine layer, and rectangular basal plate on the dermis.
• The outermost layer is enamel-like.
• Cycloid Scales
• Smooth texture, uniform with a smooth outer edge or margin, common on fish with soft fin rays (salmon and carp).
• Ctenoid Scales
• Are like cycloid scales, with small teeth along their outer edges, usually found on fishes with spiny fin rays (perch-like fishes).
• Cosmoid Scales
• Found in several ancient lobe-finned fishes.
• Composed of a dense, lamellar bone layer, a spongy bone layer with blood vessels, and a complex dentine layer called cosmine.
• Ganoid Scales
• Found in sturgeons, paddlefishes, gars, bowfin, and bichirs.
• Derived from cosmoid scales, with a layer of dentine (instead of cosmine).
• Usually diamond-shaped, connected by peg-and-socket joints are usually thick and non-overlapping.