The megamouth shark (Megachasma pelagios) is one of the most mysterious and least understood of all the sharks. It was first recorded in 1976 and is so different from other shark lineages that it has been placed in its own family: Megachasmidae (3). Megamouth sharks can reach over 5 metres in length (4); the head is large with a short snout and, as the name would suggest, an extremely large mouth (5). The mouth contains over 50 rows of very small, hooked teeth. The body is tapered with a fleshy appearance; it is a blackish-brown colour above and white below (3). There are two unequal sized dorsal fins and the tail has a longer upper lobe (3).
Since its discovery in 1976, only 17 sightings have been recorded of this elusive fish (3) and the majority of these have been of dead specimens; either accidentally caught or stranded individuals (5). Consequently, very little is known of the megamouth shark's natural ecology and behavioural observations have only been possible from the tagging of one individual for a brief two-day period in 1990 (3). This shark was observed to undergo vertical migrations, spending the day in deep water and ascending to midwater at night; it is likely that this migration is undertaken in response to the movements of prey species such as krill (3). Megamouth sharks are thought to feed by swimming through groups of small prey items with their mouths open; however, no direct observations have yet been achieved (3).
The lack of data concerning either the distribution or behaviour of megamouth sharks makes it particularly difficult to assess the severity and types of threats faced by this species. A number of specimens have been caught accidentally as by-catch of deepwater fishing methods and it is likely that this practice will increasingly affect population numbers (4).
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