mackerels. This tribe consists of 21 species in all—18 of those are classified into the genus Scomberomorus, two into Grammatorcynus, and a single species into the monotypic genus Acanthocybium. Other mackerel In addition, a number of species with mackerel-like characteristics in the families Carangidae, Hexagrammidae and Gempylidae are commonly referred to as mackerel. Some confusion had occurred between the Pacific jack mackerel (Trachurus symmetricus) and the heavily harvested Chilean jack mackerel (T. murphyi). These have been thought at times to be the same species, but are now recognised as separate species. The term "mackerel" is also used as a modifier in the common names of other fish, sometimes indicating the fish has vertical stripes similar to a scombroid mackerel: Mackerel icefish—Champsocephalus gunnari Mackerel pike—Cololabis saira Mackerel scad—Decapterus macarellus Mackerel shark—several species Shortfin mako shark—Isurus oxyrinchus Mackerel tuna—Euthynnus affinis Mackerel tail goldfish—Carassius auratus By extension, the term is applied also to other species such as the mackerel tabby cat, and to inanimate objects such as the altocumulus mackerel sky cloud formation. Characteristics Most mackerel belong to the family Scombridae, which also includes tuna and bonito. Generally, mackerel are much smaller and slimmer than tuna, though in other respects, they share many common characteristics. Their scales, if present at all, are extremely small. Like tuna and bonito, mackerel are voracious feeders, and are swift and manoeuvrable swimmers, able to streamline themselves by retracting their fins into grooves on their bodies. Like other scombroids, their bodies are cylindrical with numerous finlets on the dorsal and ventral sides behind the dorsal and anal fins, but unlike the deep-bodied tuna, they are slim. The type species for scombroid mackerels is the Atlantic mackerel, Scomber scombrus. These fish are iridescent blue-green above with a silvery underbelly and near-vertical wavy black stripes running along their upper bodies. The prominent stripes on the back of mackerels seemingly are there to provide camouflage against broken backgrounds. That is not the case, though, because mackerel live in midwater pelagic environments which have no background. However, fish have an optokinetic reflex in their visual systems that can be sensitive to moving stripes. For fish to school efficiently, they need feedback mechanisms that help them align themselves with adjacent fish, and match their speed. The stripes on neighbouring fish provide "schooling marks", which signal changes in relative position. A layer of thin, reflecting platelets is seen on some of the mackerel stripes.