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Cartilaginous Fish

Chondrichthyes

Cartilaginous fish, or chondrichthyans, are jawed aquatic vertebrates distinguished by their cartilaginous skeletons. Unlike the bony fish of the class Osteichthyes, cartilaginous fish belong to the class Chondrichthyes. They typically have paired fins and nares, placoid scales, and a pouch-like conus arteriosus in their hearts and generally lack opercula (gill coverings) and swim bladders. 

Currently, more than 1,000 species of cartilaginous fish are classified into two broad subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras or ghost sharks).

Description

Size

The whale shark, the biggest among cartilaginous fish, can grow up to approximately 39 ft (12 m) in length. In contrast, the finless sleeper ray, one of the smallest species, measures just about 3.9 in (10 cm) long.

Body Plan

Skeleton

As their name suggests, these fish have an endoskeleton made of cartilage instead of bone. In Elasmobranchii members, like sharks and rays, the notochord, a flexible, rod-like structure, is replaced by a vertebral column as they develop. In Holocephali, the notochord remains unchanged throughout their lives.

Scales and Teeth

Most cartilaginous fish have tough, warty skin covered by tooth-like placoid scales or dermal denticles, which streamline their bodies. However, electric rays have soft, smooth skin without these scales.

Early cartilaginous fishes had pectoral and pelvic girdles without dermal elements. Over time, the pelvic and pectoral fins evolved to connect in the middle, forming scapulocoracoid and puboischiadic bones. In rays, the pectoral fins are flexible and attached to the head.

It is believed that the oral teeth in these fish evolved from dermal scales, which gradually migrated toward the mouth over time.

Organ System

Respiratory

Gaseous exchange in cartilaginous fish occurs through five to seven pairs of gills. Small circular or slit-like openings called spiracles, located behind each eye, also assist in their respiration. These are tiny and circular in nurse sharks and slit-like in wobbegongs. However, thresher and mackerel sharks have completely lost these spiracles.

Pelagic species continuously swim to keep water flowing over their gills, while bottom-dwelling species use spiracles to draw in water, which is then expelled through the gills.

Nervous

Their nervous system includes a small, relatively simple brain, 8 to 10 pairs of cranial nerves, and a spinal cord with numerous spinal nerves. They have large, well-developed eyes and powerful nostrils for detecting smells.

Their auditory system can detect a limited range of sounds, but the inner ear contains three large semicircular canals that help maintain balance and body orientation.

In addition to these basic features, cartilaginous fish possess specialized sensory organs. 

Immune

Cartilaginous fish have a spleen and an epigonal organ, which consists of tissue clusters around the gonads. Since they lack bone marrow, these organs produce red blood cells.

Some sharks and rays also have a specialized immune structure called Leydig’s organ, located along the esophagus, which generates blood cells.

Taxonomy

Presently, the subclass Elasmobranchii contains 13 extant orders, which include modern sharks, rays, skates, and sawfish. Holocephali, on the other hand, consists of a single extant order (Chimaeriformes) containing chimaeras.

  *Position uncertain

Cartilaginous Fish (Chondrichthyes)

Evolution and Fossil Records

Cartilaginous fish are believed to have evolved from acanthodians (class Acanthodii). Fossils like Entelognathus suggest that cartilaginous fish are paraphyletic to this primitive class. Some traits thought to be unique to acanthodians have also been found in early cartilaginous fish. A few characteristics initially considered exclusive to acanthodians have also been discovered in basal cartilaginous fish. Moreover, recent phylogenetic studies show that modern-day cartilaginous fish are closely related to acanthodian fish like Doliodus and Tamiobatis.

The earliest acanthodian ancestors of cartilaginous fish are Qianodus and Fanjingshania, which date back to the early Silurian Period (around 439 million years ago). Shenacanthus vermiformis, another fossil from the Silurian, possessed thoracic armor plates similar to that of ancient cartilaginous placoderms.

By the Early Devonian Period (419 million years ago), jawed fish had diversified into three groups: placoderms, bony fishes, and early cartilaginous fish. The first abundant sharks were Cladoselache of the Devonian Period, while bony fish appeared in the late Silurian or early Devonian (416 million years ago).

Distribution and Habitat

Most cartilaginous fish are marine, and only a few species, like the giant freshwater stingray (Himantura chaophraya), inhabit freshwater habitats. They usually live in shallow waters of continental margins or offshore islands but are sometimes found in midwaters or at the bottom of the oceans.

Diet

Sharks are apex predators that feed on other fish, turtles, and marine mammals, such as whales and seals. Rays and skates primarily feed on invertebrates like crustaceans, mollusks, and cephalopods.

These aquatic vertebrates also scavenge on carrion (decaying flesh) of dead whales and garbage thrown from cargo ships.

Reproduction and Life Cycle

During mating season, mature sharks typically separate by sex. Males grip females with their teeth and insert their clasper into the female’s cloaca for internal fertilization. Pregnant females then separate from other similarly sized females. Although most sharks are ovoviviparous (eggs hatch within the parent’s body), some, like bull and hammerhead sharks, are viviparous and give birth to live young. When inside the eggs, the young are generally nourished by the yolk sac or even the egg capsule.

In skates, the males seize and bite the pectoral fins of the females, forcing their claspers into the cloaca. They hold the females using the clawlike spines on the dorsal side of their own pectoral fins. After fertilization, the females lay eggs in series (oviparous), usually two eggs at a time.

The young continue to grow, attain sexual maturity, and may live for up to 50 to 100 years.

Adaptations

References Article last updated on 11th July 2024
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