Thornbacks are rays, as opposed to skates. The main difference is in their appearance – rays have much shorter ‘noses’ than skates. Though both skates and rays look very like flatfish such as plaice, sole and halibut, there is no relationship between their two families. The resemblance is due to the similarity of their bottom-dwelling lifestyle. In fact, rays are very closely related to sharks. The ‘backs’ of both flatfish and rays are coloured to suit the type of ground they inhabit, as a means of camouflage.
The dorsal surface of the thornback is mottled and blotchy and varies from brown to grey with many dark spots and yellowish patches. In juveniles these lighter patches tend to be more distinct, with a dark outline. The underside is off-white with darker edges.
Follow the feeding
Thornbacks feed mainly on the sea bed and are not particularly fussy about what they eat. Immature flatfish and other bottom dwelling fish such as gobies and sand eels will be eaten but the preferred diet consists of crabs, shrimps and shellfish. They tend not to pursue prey over long distances; instead they lie in wait, camouflaged against the bottom.
The mouth is on the underside, well suited to bottom feeding. The flattened triangular teeth are useful for grinding up molluscs and crustaceans. Despite this, large thornbacks have been known to chase herrings and sprats in the shallows.
Life at the bottom
Thornbacks are shallow water seafish, rarely, if ever, found in the depths. Juveniles and mating adults can be found in very shallow water of only a couple of metres but most of the year is spent at 9-60m (30-200ft). They prefer to live and feed over mud, sand, shingle and gravel, though they will also forage over mixed and rough ground from time to time.
While not migratory, there is some movement towards shore during the late winter and early spring, probably to breed. The spawning period offers the best chance of catching a thornback from the beach. Mature females are the first to move inshore and do so when almost ready to spawn. A few weeks later, when the eggs have ripened, the adult males also make the journey. This segregation may account for the large catches ofone sex that tend to occur from time to time.
Fertilisation takes place internally by means of the claspers. The egg capsules, known as mermaid’s purses, are laid in shallow coastal waters between March and August, with each female laying about 140 in all. These ‘purses’ are dark rectangles about 7cm by 9cm (2%in by 3Hin) with a conical horn at each corner. They can often be seen washed up during the summer months.
After 16 to 20 weeks, the young hatch out, folly formed though still sporting a yolk sac, and soon begin to feed on small crustaceans. They gradually move into deeper water as they mature. Sexual maturity is reached at seven years for males and nine years for females.