Elasmobranchs are an ancient group of cartilaginous fish, including sharks, rays, and skates. They have inhabited the world’s oceans for approximately 400 million years, since the Devonian period. Found across diverse marine environments, from shallow coastal waters to deep oceanic trenches, elasmobranchs are globally distributed in tropical and temperate regions. Some species even inhabit freshwater or hypersaline conditions.
Defining Features
Elasmobranchs have distinct characteristics that differentiate them from bony fish. Their skeletons are composed entirely of cartilage, rather than bone, providing flexibility and allowing for continuous growth throughout their lives. This cartilaginous structure is often reinforced with calcium salts, offering rigidity without the weight of bone.
They have five to seven pairs of gill slits that open individually to the exterior, unlike bony fish with a single gill cover. Their skin is covered by tiny, tooth-like placoid scales (dermal denticles), which are made of enamel and dentine, resembling miniature teeth embedded in the skin. These denticles minimize drag and enhance swimming efficiency while also providing a tough, protective armor. Elasmobranchs also lack a swim bladder, instead relying on large, oil-filled livers to maintain buoyancy.
Diversity of Forms
The subclass Elasmobranchii includes diverse forms: sharks, rays, and skates. Sharks, with around 400 species, are recognized by their streamlined bodies and are often apex predators. Examples include the Great White Shark and the filter-feeding Whale Shark, the largest fish in the ocean.
Rays and skates, comprising about 500 species, share a flattened body shape with pectoral fins expanded into broad, wing-like structures. Rays possess a whip-like tail, often with a venomous barb, and are adapted for bottom-dwelling, feeding on mollusks and crustaceans. Examples include the Manta Ray, a filter feeder, and various stingrays. Skates, similar to rays, lack the venomous barb and are smaller, laying egg cases on the seabed.
Remarkable Adaptations
Elasmobranchs have evolved adaptations for success in diverse marine habitats. Their sensory systems are highly developed, including an acute sense of smell for detecting odors over long distances. They also have a lateral line system, which detects weak water movements and vibrations.
Electroreception is a unique adaptation, facilitated by specialized organs called the Ampullae of Lorenzini. These ampullae are gel-filled canals that detect minute electrical fields generated by muscle contractions of prey, even when hidden in sediment. Elasmobranchs display various feeding strategies, from suction-feeding in benthic species to ram-feeding (swimming towards prey) and biting in pelagic predators. Their jaws are highly mobile, with multiple rows of continually replaced teeth that are adapted for different prey types.
Reproduction involves internal fertilization, unlike many bony fish. They exhibit diverse reproductive methods, including oviparity, where females lay eggs encased in protective coverings, such as the “mermaid’s purses” of skates and some sharks. Ovoviviparity involves eggs hatching inside the mother, with young nourished by yolk before live birth, while viviparity means the young develop inside the mother, receiving nourishment directly from her.
Ecological Importance and Conservation
Elasmobranchs play a role in maintaining marine ecosystem balance and health. As apex or mesopredators, they influence food web dynamics, regulating prey populations and contributing to nutrient cycling. Their presence can prevent overgrazing by herbivores and maintain species diversity within their habitats.
Despite their ecological significance, elasmobranch populations face global threats. Overfishing is a primary concern, driven by demand for their fins, meat, and liver oil. Their slow growth rates and low reproductive output make them vulnerable to depletion. They are also frequently caught as unintentional bycatch in various fishing gears, including bottom trawls, longlines, and gillnets. Habitat degradation, pollution, and climate change further jeopardize their populations by altering their distribution, affecting prey availability, and disrupting migration patterns. Currently, over one-third of all shark and ray species are considered threatened with extinction.