BANDS AGAINST STRIPES ON THE BACKS OF MACKEREL, SCOMBER-SCOMBRUS L

A thin band of reflecting platelets overlies the central parts of the light and dark stripes found on each side of the dorsal surfaces of th e body of the mackerel (Scomber scombrus L.). When this fish has its a nteroposterior axis horizontal and its mid-dorsal and mid-ventral line s in the same vertical plane, V, the surfaces of the reflecting platel ets in these bands are within a few degrees of being vertical. These s urfaces are, however, tipped about 17 degrees from plane V towards the tail. In the angular distributions of radiance commonly found in the sea, the reflections from these bands can mask parts of the pattern of light and dark stripes seen by neighbours in ways that depend on the orientation of the fish in the external light field and the position o f the fish relative to its neighbours. With this arrangement, when the fish changes its orientation and/or its velocity with respect to neig hbouring fish, this is signalled to its neighbours as changes in the p atterns of brightness of its dorsal surfaces. Relatively small changes in roll, pitch and yaw can produce large changes in appearance and, a s vision is a most important sense in the mackerel, it seems likely th at these changes are important for signalling.