Sei and Bryde's are both baleen whales; they have a series of fringed, overlapping plates that hang from the upper jaw where teeth would be. The plates are composed of a fingernail-like material called keratin that frays out into fine hairs on the ends and inside the mouth next to the tongue. Sei whales have 320 to 380 baleen plates on each side, which are about 48 cm (19 inches) long. The plates are black outside, with a white fringe that is very fine and silky. Bryde's whales have 250 to 350 baleen plates on each side, which are about 42 cm (16 1/2 inches) long. Some of the baleen plates in the front of the mouth are white, while the rest of the plates are black. The fringe is stiff and coarse. The fine baleen of the sei whale adapts it for feeding on its favorite food, copepods (small crustaceans). There is some variation in their diet: they frequently eat fish in the North Pacific and krill (small, shrimp-like crustaceans) on occasion. The sei feeds on plankton (marine animal and plant organism that drift or float with currents and waves), which means it spends prolonged periods on the surface. Like right whales, they often open their mouths and skim the surface for their food. When doing this they blow or spout once every 1 to 2 minutes. Bryde's whales are more usually fish eaters, often feeding on schools of anchovies, sardines, herring, or mackerel. While feeding, the Bryde's whale displays a more regular up-and-down pattern, frequently arching its back quite high and diving for 5 to 15minutes.