The megamouth shark (Megachasma pelagios) is an extremely rare and unusual species of shark discovered in 1976. Its body is stout, tapering posteriorly. The head is bulbous, wide and long; snout very short and broadly rounded. Gill slits moderate long but not reaching dorsal surface of head. Mouth broad, terminal, corner extending behind eyes. Caudal peduncle without keels or ridges.
First dorsal fin origin closer to pectoral fin bases than to pelvic fin bases; dorsal fins relatively low; second dorsal fin less than half size of first dorsal fin. Pectoral fins shorter than head length in adults. Caudal fin asymmetrical, with pronounced ventral lobe.
Although only 38 confirmed sightings of megamouth shark are reported, this species is now known from Indian, Pacific and Atlantic Oceans. As with the two other filter-feeding sharks, the basking and whale sharks, this species is wide-ranging. However, the megamouth is considered to be less active and a poorer swimmer than the basking or whale sharks. Poor mobility likely is a reflection of its flabby body, soft fins, asymmetrical tail, lack of keels and weak calcification.
As its species name (pelagios, the Greek word for "of the sea') suggests, the megamouth lives epipelagicly (in the upper part of the water column) in open ocean. Although only few sightings of megamouth have been reported, the capture of the 6th megamouth was very important in augmenting our understanding of the ecology of this species. This specimen was tagged and followed for two days, allowing insight into its habitat preference and behavior. It remained at a depth of 15m during the night, then dove to 150m at dawn and returned to shallow waters at dusk. So the megamouth is presumed to be a vertical migrator on a diel cycle, spending the daytime in deep waters and ascending to midwater depths at night.
Only two observations of megamouth provide information about this species behavior. The 6th specimen from Dana Point, California (21 October 1990) offered the most important insights into the behavior of this species. The male specimen, with 494cm in total length, was tagged and tracked for two days. One of the conclusions of these observations is that megamouth is probably a vertical migrator on a diel cycle spending the daytime in deep waters and ascending to midwater depths at night. This vertical migration may be a response to the movements of the small animals on which it feeds. The krill that make up part of megamouth's diet are known to migrate from deep waters to the surface.
Megamouth, in contrast to many other deep-water sharks, shows a decrease in specific gravity in the form of a soft, and poorly calcified cartilaginous skeleton; very soft, loose skin; and loose connective tissue and muscles. Others epibenthic (live in the water just above the bottom) and epipelagic sharks often have an enlargement of their abdominal cavity and increased liver volume. The huge liver allows for greater production of liver oil in order to reduce specific gravity and increase hydrostatic support.
The 13th sighted also offered important megamouth behavioral observations. This sighting documented sperm whales attacking megamouth shark. Observers reported that the megamouth was swimming slowly and apparently confused at the surface. The shark showed signs of the whales' attack, on the base of its dorsal fin and gills.
Precise details of feeding behavior are unknown due to the lack of observations on a live, feeding specimen. However, some inferences can be made from morphological observations on the captured specimens. Scientists believe that this shark swims slowly through aggregations of euphasiid shrimp ("krill") and other small prey with its mouth open.
The ovary of the megamouth is similar to other mackerel sharks and this suggests that megamouth embryos are oophagous (the first well-developed embryo eats the other eggs in the uterus).