Mantas move through the water by the wing-like movements of their pectoral fins, which drive water backwards. Their large mouths are rectangular, and face forward as opposed to other ray and skate species with downward-facing mouths. The spiracles typical of rays are vestigial, and mantas must swim continuously to keep oxygenated water passing over their gills. :2–3 The cephalic fins are usually spiralled, but flatten during foraging. The fish's gill arches have pallets of pinkish-brown spongy tissue that collect food particles. Mantas track down prey using visual and olfactory senses. They have one of the highest brain-to-body mass ratios and the largest brain size of all fish. Their brains have retia mirabilia which may serve to keep them warm. M. alfredi has been shown to dive to depths over 400 metres (1,300 ft), while their relative Mobula tarapacana, which has a similar structure, dives to nearly 2,000 metres (6,600 ft); the retia mirabilia probably serve to prevent their brains from being chilled during such dives into colder subsurface waters.