Skate (fish)

Early Eocene fossil stingray Heliobatis radians

Skates belong to the ancient lineage of cartilaginous fishes. Fossil denticles (tooth-like scales in the skin) resembling that of today's chondrichthyans date at least as far back as the Ordovician, with the oldest unambiguous fossils of cartilaginous fish dating from the middle Devonian. A clade within this diverse family, the Neoselachii, emerged by the Triassic, with the best-understood neoselachian fossils dating from the Jurassic. This clade is represented today by sharks, sawfish, rays and skates.[5]

The skate belongs to the class Chondrichthyes. This class consists of all the cartilaginous fishes, including sharks and stingrays. Chondrichthyes is divided into two subclasses; of which Elasmobranchii includes skates, rays, and sharks. Skates are the most diverse elasmobranch group, comprising over 20% of the known species. The number of species is likely to increase as taxonomic issues are resolved and new species are identified.[6] There are 17 genera recognised in the family Rajidae:[2]

• Amblyraja
• Beringraja
• Breviraja
• Dactylobatus
• Dentiraja
• Dipturus
• Hongeo
• Leucoraja
• Malacoraja
• Neoraja
• Okamejei
• Orbiraja
• Raja
• Rajella
• Rostroraja
• Spiniraja
• Zearaja

Skates have more valid species (574) than any other group of cartilaginous fishes. Since 1950, 126 new species of skates have been discovered. Five scientists take credit for the rapid increase of findings. The Rajidae are considered monophyletic because of their similarity in appearance. There are 13 genera and about 245 valid species. However, there is little information about the diets of about 24% of these species. There are at least 45 dubious species of skates worldwide.[7]

Skates are cartilaginous fishes like other Chondrichthyes, however, skates, like rays and other Rajiformes, have a flat body shape with flat pectoral fins that extend the length of their body.[8] A large portion of the skate's dorsal body is covered by rough skin made of placoid scales. Placoid scales have a pointed tip that is oriented caudally and are homologous to teeth. Their mouths are located on the underside of the body, with a jaw suspension common to Batoids known as euhyostyly.[9] Skate's gill slits are located ventrally as well, but dorsal spiracles allow the skate to be partially buried in floor sediment and still complete respiratory exchange.[10] Also located on the dorsal side of the skate are their two eyes which allow for predator awareness.[8] In addition to their pectoral fins, skates have a first and second dorsal fin, caudal fin and paired pelvic fins. Distinct from their rhomboidal shape is a long fleshy slender tail. While skate anatomy is similar to other Batoidea, features such as their electric organ and mermaid's purse create clear distinctions.

Full view of a skate's mermaid's purse. Roughly 125 mm in length.

Skates produce their young in an egg case called a mermaid's purse. These egg cases have distinct characteristics that are individualized to each species.[11][12] This makes a great tool for identifying different species of skates. One of these identifiable structures is the keel. The keel is a flexible ridge that runs along the outside of the structure. Another characteristic is the number of embryos in the egg case. Some species contain only one embryo while others can have up to seven. The size of the fibrous shell around the case is another characteristic. Some species have thick layers on the exterior, however other species do not have the presence of this layer.

The fibers of the skate electric organ are embedded in the muscles located lateral to the notochord in the tail.

The electric organ (EO) is a characteristic exclusive to aquatic species. Among the Chondrichthyes, the only species to possess electric organs are the electric ray and the skate. Unlike many other electrogenic fishes, the skate is unique in that it has two paired electric organs, which run longitudinal through the tail in the lateral musculature of the notochord.[13] The impulses put out by the electric organs of the skate are considered to be weak, asynchronous, long-lasting signals.[14] Although the anatomy of the skate EO is now commonly studied and well understood, there is not enough strong evidence to suggest what the actual function of the EO is. Some research suggests the electric impulses are too weak to be a mechanism used for defense or hunting. It is also too irregular to be useful for electrolocation purposes.[14] The most reasonable explanation in the literature suggests that the electric organ discharges may be used as a form of communication used for reproduction purposes.[14]

Four developing embryos located in an opened big skate egg case (mermaid's purse)

Skates mate at the same nursery ground each year. In order to fertilize the egg, males use claspers, a structure attached to the pelvic fins. The claspers allow them to direct the flow of semen into the female's cloaca. Skates are oviparous, meaning they lay eggs with very little development in the mother. This is one major difference from rays, which are viviparous, meaning they give birth to live young. When a female skate is fertilized, a protected case forms around the embryo called an egg case, or more commonly mermaid's purse. This egg case is then deposited out of the mother's body onto the ocean floor where the skates develop for up to 15 months before they enter the external environment.

The majority of skates feed on bottom dwelling animals, such as shrimp, crab, oyster, clams, and other invertebrates. To feed on these animals they have grinding plates in their mouths. Filter feeding rays that eat plankton have gill rakers. Skates are an influential part of the food webs of demersal marine communities. They utilize similar resources to those of other upper trophic-level marine predators, such as seabirds, marine mammals, and sharks. The flattened body shape, ventral eyes and well developed spiracles of the skate allows them to live benthically, buried in the sediment or using a longitudinal undulation of the pectoral fins known as Rajiform locomotion to glide along the water floor.[17] Current research suggests that some species of skates, in addition to their Rajiform locomotion, use their pelvic fins to perform ambulatory locomotion.[18] This form of locomotion performed by the skate is being explored as a possible origin for our own development of walking by looking for similar neural pathways used for movement between skates and animals walking on land.[19]