Skates are an ancient and diverse group of fish that glide across the world’s ocean floors. They belong to the class Chondrichthyes, which includes all fish with skeletons made entirely of cartilage rather than bone, a feature shared with sharks and chimaeras. As members of the superorder Batoidea, they are known as elasmobranchs, distinguished by their flattened bodies and broad, wing-like pectoral fins. Skates occupy a wide variety of marine habitats across the globe.
Defining the Skate: Classification and Characteristics
Skates are scientifically classified within the order Rajiformes, a group of cartilaginous fish defined by their depressed body shape. Their most prominent physical feature is the rhomboidal or disc-shaped body, formed by the fusion of their large pectoral fins to the sides of the head and trunk. The endoskeleton is composed of flexible, lightweight cartilage, contributing to their low-density structure.
The skate’s skin is covered in small, tooth-like structures called dermal denticles, giving the surface a texture similar to sandpaper. Many species also possess small, thorn-like prickles concentrated along the midline of the back and on the tail. A stiff snout, or rostrum, projects forward from the disc. Their five pairs of gill slits, along with the mouth and nostrils, are located on the ventral, or underside, of their flattened bodies.
Skates vs. Rays: Key Distinctions
The differences between true skates and rays are based on distinct anatomical and reproductive features. The most reliable distinction lies in their reproductive strategy: skates are oviparous, meaning they lay eggs, while rays give birth to live young. Skates also lack the venomous, barbed spine characteristic of many stingrays. Instead, they possess a thicker, fleshier tail that often features two small dorsal fins near the tip.
Locomotion also differs. Rays typically propel themselves through the water by flapping their pectoral fins like wings in an oscillating motion. Skates, by contrast, move using an undulating motion along the edges of their pectoral fins, creating a wave-like ripple. Some skates also engage in “punting,” where they push off the seabed using their pelvic fins. Skates often exhibit a more triangular body outline, while rays generally have a rounder or more kite-like disc shape.
Unique Reproductive Strategy
Skates lay their fertilized eggs externally in protective cases. These casings are commonly referred to as “mermaid’s purses” when found washed ashore, easily recognized by their tough, leathery, rectangular shape. The casing is composed of keratin, the same protein found in human hair and fingernails, providing a durable shell for the developing embryo.
Each corner of the rectangular case features a horn or tendril that anchors the egg to the seabed, often attaching to structures like seaweed or rocks. The embryo develops inside the case for an extended period, which can range from a few months up to more than a year depending on the species and water temperature. Specialized slits in the horns allow water to be siphoned in, ensuring a constant supply of oxygen for the growing skate. Once development is complete, the miniature skate emerges from an opening in the case, resembling a small adult.
Life on the Seabed: Habitat and Diet
Skates are exclusively benthic organisms, adapted to life on the ocean floor, and rarely venture into the water column. They inhabit a wide range of depths, from shallow coastal waters to deep-sea abyssal plains over 3,000 meters down. These fish are most commonly found over soft substrates, such as sandy or muddy bottoms, where their flattened bodies allow for effective camouflage.
The skate’s diet consists primarily of bottom-dwelling invertebrates. They actively seek out and consume crustaceans, such as crabs and shrimp, as well as mollusks like clams and oysters. Their mouths are equipped with flat, pavement-like teeth designed for crushing the hard shells of their prey. Skates also opportunistically feed on small demersal fish that they ambush from their concealed position.