Skates flattened themselves after genomic furniture moved
Skates of family Rajidae are flattened cartilaginous cousins of sharks and rays, with more than 150 described species across seventeen genera—or about 250 valid names if counting wider. Softnose and pygmy forms now sit in separate families, while egg cases called mermaid's purses still fingerprint each kind.
Skates descend from a very old line. Tooth-like skin denticles resembling those of modern cartilaginous fish go back at least to the Ordovician, clear cartilaginous fish fossils appear by the middle Devonian, and the Neoselachii, the group that includes today's sharks, rays and skates, was present by the Triassic. When skates split from sharks roughly 286 to 221 million years ago, rearrangements in their genome changed how genes are switched on in three dimensions, producing the flattened body.
That body is a broad disc. The pectoral fins run the length of the trunk and fuse to the head, and rippling them in rajiform locomotion lets a skate glide over the bottom. The mouth and five pairs of gill slits sit underneath, while the eyes and a pair of spiracles on top let the fish breathe and keep watch while half buried in sediment. Rough placoid scales, homologous to teeth, cover much of the back, and grinding plates in the mouth crush shrimp, crabs, clams and oysters.
Unlike stingrays, skates have no whip-like tail or venomous spine, and they lay eggs instead of bearing live young. Males use claspers on the pelvic fins to fertilise females, which return to the same nursery grounds each year and leave egg cases, mermaid's purses, on the sea floor. The keel, the shell thickness and the number of embryos, from one to seven, differ by species, so a washed-up case can identify its maker. Skates also carry paired electric organs along the tail that give off weak, long-lasting pulses whose purpose is still unclear.
They range from the tide line to below 3,000 metres and, unusually for cartilaginous fish, grow more diverse toward the poles. Researchers have named 126 new species since 1950.
Source: Skate (fish)
