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  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.
The megamouth has approximately fifty rows of very small and relatively numerous teeth on each jaw, but only three rows are functional. Females seem to present fewer teeth rows than males. Upper and lower jaws have a symphyseal (where the two halves of the jaw meet) toothless space, but it is larger in upper jaw. A difference between the upper and lower teeth was recognized on a female specimen. The first five upper teeth are smaller than the first five lower teeth; the more distal upper teeth are smaller than the lower teeth; the cusps of the lower teeth are more acute and longer than those of the upper teeth.
  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.The only confirmed register of a megamouth predator is an isolated event of sperm whales (Physeter macrocephalus) attacking this shark. This occurred in Manado, North Sulawesi, Indonesia (30th August 1998) near midday, while some researchers were observing the whales. The base of the dorsal fin and the gills of the shark showed signs of the whales' attack.
Sperm whales are usually considered squid feeders but there are a few notes about small deep-sea sharks in their diet. This behavioral observation significantly alters our views on the relationship between whales and sharks




Sea Horses