Thornback Ray

Lore & Background

The thornback ray is a demersal species usually found on sedimentary seabeds such as mud, sand or gravel at depths between 10 and 60 m, though it has been found from inshore to 300 metres (980 ft) and more infrequently as deep as 1,020 metres (3,350 ft). Juvenile fish feed on small crustaceans, particularly amphipods and bottom-living shrimps; adults feed on crabs, shrimps and small fish. The back is covered in numerous thorny spines, as is the underside in older females. In sexually mature fish, some of the spines are thickened with button-like bases (known as bucklers), particularly well developed on the tails and backs of sexually mature females. Named by Carl Linnaeus in the 10th edition of Systema Naturae in 1758, Raja clavata is one of about 13 species of skate (family Rajidae) known from the North Sea and adjacent Atlantic waters. Thornback rays are an oviparous and polyandrous species; paired eggs are laid and deposited on shallow sand, mud, pebble or gravel bottoms. Up to 170 egg cases can be laid by a single female in a year, with average fecundity around 48–74 eggs. Egg cases are oblong capsules with stiff pointed horns at the corners, each containing one embryo, and hatch after about 4–5 months. Mating season is from February to September, peaking in June, and adults have been observed to form same-sex aggregations during the mating season with females moving to shallower inshore waters approximately a month before the males. Mating does not occur in the Baltic Sea. Thornback rays reach maturity at between 7-8 years old, and have a maximum known age of 15 years.

In Their Own Story

In the Solway Firth, part of the border between Cumbria, England and Dumfries and Galloway, Scotland, a study of the growth and maturation of Raja clavata showed that males and females appear to mature at 42 and 45 cm in disc width respectively. The Solway population is heavily exploited by an unrestricted commercial fishery and a considerable proportion (48.6%) of the retained catch is immature. It is suggested that fishing pressure has brought about a reduction in the size at which female fish mature. This ray is also one of the fish used by the marine leech Pontobdella muricata as a host. Common names across Europe include: Danish — Sømrokke; Dutch — Stekelrog; English — Roker / Thornback ray / Dorn; French — Raie bouclée; German — Nagelrochen; Norwegian — Piggskate; Spanish — Raya de clavos; and many others.

Reader's Guide

The thornback ray is a demersal species best observed on sedimentary seabeds such as mud, sand or gravel at depths between 10 and 60 m, though it can be found from inshore to 300 metres (980 ft) and occasionally as deep as 1,020 metres (3,350 ft). Divers often encounter it in Atlantic coastal waters of Europe and western Africa, as well as the Mediterranean and Black Seas. It is probably one of the most common rays encountered by divers. The ray's body is kite-shaped with a long, thorny tail; the back is covered in numerous thorny spines, and the underside is also prickly in older females. Colours vary from light brown to grey with darker blotches and numerous small darker spots and yellow patches; the underside is creamy-white with a greyish margin. When threatened they can appear black. Juvenile fish feed on small crustaceans, particularly amphipods and bottom-living shrimps, while adults feed on crabs, shrimps and small fish. Egg cases are oblong capsules with stiff pointed horns at the corners, 5.0–9.0 cm long without the horns and 3.4–6.8 cm wide, anchored with an adhesive film. They hatch after about 4–5 months, and pups are about 11–13 cm.

Did You Know?

• The thornback ray was named by Carl Linnaeus in the 10th edition of Systema Naturae in 1758.
• Up to 170 egg cases can be laid by a single female in a year, with average fecundity around 48–74 eggs.
• Thornback rays reach maturity at between 7-8 years old and have a maximum known age of 15 years.
• In the Solway Firth, 48.6% of the retained catch is immature, suggesting fishing pressure has reduced the size at which female fish mature.
• The thornback ray is one of the fish used by the marine leech Pontobdella muricata as a host.