Megamouth Shark

Order - Lamniformes
Family - Megachasmidae
Genus - Megachasma
Species - pelagios

When the first megamouth was captured a new shark family, genus and species had to be erected.
There are conflicting hypotheses regarding the evolutionary relationships between the Megachasmidae and other shark families. One theory suggests that the Megachasmidae is evolutionary derived and form a family with the basking shark. Others disagree with this idea and suggest that the Megamouth is relatively derived and forms a sister group to the mako, white and porbeagle sharks. Recent studies suggest that heis the most primitive living species within the order Lamniformes, which contains all the aforementioned families, and has independently evolved the filter feeding mode, shared with the basking shark.

Geographical Distribution

Although only 14 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.

Habitat

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.

Size

The sizes of all reported megamouth sharks were maximum at least 550cm (17ft). Males mature by 400cm (13ft) and female by 500cm (16ft).

Behavior

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.

Feeding Behavior

The megamouth shark, which reaches more than 5 m in length, is one of the three giant filter-feeding sharks in the sea. The other two are the basking shark (Cetorhinus maximus) and the whale shark (Rhincodon typus).

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. The megamouth shark, as its vernacular name indicates, has a huge mouth that is terminal and extends behind the eyes. Scientists believe that this shark swims slowly through aggregations of euphasiid shrimp ("krill") and other small prey with its mouth open. It feeds by protruding its jaws and expanding its buccal cavity in order to sucks the prey inside. Then the mouth is closed and the jaws retracted. This action decreases the pharyngeal volume and makes possible the expelling of water through the gill openings.

Reproduction

The megamouths II and VI were both mature males, and both showed evidence of recent mating. The claspers of megamouth II were oozing spermatophores and those of the sixth specimen were abraded and bleeding, a common occurrence in sharks that have just mated. The 6th megamouth also had a fresh wound on the lower jaw, a feature found in other sharks that grasp one another's mouth during mating. Megamouth II was taken in late November and megamouth VI in late October, these factors make scientists believe that Southern California might be a mating area for megamouth sharks in the fall.
The 7th megamouth observed, a female with 471cm in total length was heavily studied and one of the conclusions is that this specimen is immature. This judgement was based on uteri that were enlarged only posteriorly, a poorly developed ovary and ostium and the small size of the oocytes. 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). The 12th megamouth captured is the only known mature female. The total length of this specimen was 5.44m and the expanded uteri measured 260mm. The right ovary possessed a large number of whitish yellow eggs.

Predators

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.

Carol Martins