Few fish live up to their name as well as this week's aquatic oddity: the Mega-mouth shark, Megachasma pelagios.
Mega-mouths really don't look like any other shark. With their huge rounded head and gaping, down-turned mouth they always have a rather depressed look about them.
Their massive body, which can grow in excess of 5.5m long, tapers away after the head making them look rather like an gigantic tadpole until you notice their huge asymmetrical tail fin, the upper lobe of which is very long, similar to those of thresher sharks.
The whole shark has a soft and flabby appearance, not the sleek, streamlined profile most of us think of when picturing sharks.
They are harmless plankton feeders sifting their prey from the water with specially adapted gill rakers. Their feeding method is thought to differ from the other giant plankton eating sharks, (whale and basking) in that they are believed to suck prey into their mouth, close it and expel the water through their gills where the rakers filter our their prey, rather than swimming forward mouth agape, constantly filtering like their planktivorous cousins.
It was believed they had evolved from the same ancestor as basking sharks, but it is now thought they evolved their feeding adaptations independently.
Their diet seems to consist largely of euphausiid shrimp (krill), but copepods and jellyfish have also been found in their stomachs.
Very little is known of their natural behaviour, especially as the majority of specimens found have been dead, or near death, but a male specimen captured and re-released successfully with a radio tag attached in 1990 gave a brief insight into their behaviour. The tag showed the fish swam at a depth of between 120-160m during daylight hours but rose to between 12-25m at night. This behaviour is typical of many species that track to day/night migration of plankton. The tagged fish lived up to its sluggish looks, swimming at no more that 2.1kph during the recorded period.
The first Mega-mouth was discovered tangled in the sea anchor of a US naval ship off Hawaii in 1976 and was finally described scientifically in 1983. Since then only 50 specimens have been caught or sighted, making it one of the rarest and enigmatic shark species.
At adult size they have few predators, but one was seen being attacked by sperm whale. However all adult specimens discovered so far have shown scars or wounds left by the parasitic Cookie cutter shark, Isistius brasiliensis.
They are ovoviviparous, giving birth to live young which develop in eggs inside the mother before birth. As far as can be ascertained from the fish recorded so far they are distributed throughout the world's temperate and tropical oceans.
Why not check out some of our other Weird fish of the week features?