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

Geographical Distribution
Habitat
Biology

Geographical Distribution

Although only 15 sightings (see Table) 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.

Biology

· Distinctive Features

Body stout, tapering posteriorly. Head 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.

· Size

The sizes of all reported megamouth sharks are listed in a table (see Table). Maximum size is at least 550cm (17ft). Males mature by 400cm (13ft) and female by 500cm (16ft).

· Coloration

Dorsal surface of body, pectoral and pelvic fins, dorsal fins, center of anal fin and caudal fin are blackish brown. Ventral surface of body, below level of pectoral end pelvic fins, tips and posterior margins of pectoral and pelvic fins, abruptly white, as the posterior margins of dorsal and anal fins and postventral caudal margins.

Dorsal side of head is blackish brown, except areas between nostril and eye and between eye and spiracle, which are paler than the rest of dorsal area of head. Lateral side of head behind mouth corner is darker. Lower jaw dark with silver tint and many small dark blotches. Ventral side of head behind lower jaw dirty white.
Mouth roof on dorsal and lateral parts, and oral membranes, silvery. Tongue purplish brown with slight silvery tint on both sides, dorsally and ventrally. Both sexes seem to have a white band on the anterior surface of the snout. This white band could be considered a feeding behavioral characteristic, because it is so contrasted by the dark coloration of the snout and upper jaw, and becomes very prominent when the upper jaw is protruded. Probably under low light this white band may be more visible. This band might also be related to recognition of other individuals of the same species.

· Dentition

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.
Megamouth tooth in A, lingual; B lingual; C lateral; and D, basal views, from Compagno

· Mucous and Dermal Denticles

Megamouth sharks have very small mucous (on the tongue) and dermal (on the skin) denticles that differ in shape and size at each region of the body. There are many pigment cells on the dorsal side, but none on the ventral side.

· 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 over than 500cm 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 (A). It feeds by protruding its jaws and expanding its buccal cavity in order to sucks the prey inside (B). 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 (C).

· Stomach Contents

All analyzed specimens presented euphausiid shrimp in the stomach, indicating a filter-feeding habit. The stomach of the first megamouth captured contained only one type of shrimp, Thysanopoda pectinata. The second megamouth's stomach contents included fragments of euphasiids, copepods (animals that make up plankton) and the jellyfish, Atolla vanhoeffeni. In general, euphasiids are the main alimentary item of megamouth diet.

· Origin of Filter-Feeding

There are two conflicting hypotheses about the origin of filter feeding in megamouth exist. The first theory suggests a single origin for this habit between megamouth and basking sharks. In contrast, the other suggests that adaptations for filter feeding evolved independently in the two lineages from different ancestral origins. Results of recent studies argue strongly in favor of the latter hypothesis. This makes sense when considering morphological and behavioral differences between the two sharks. The second hypothesis also suggests that megamouth may have evolved its distinctive feeding apparatus from an ancestral sand tiger-like shark (family Odontaspidae) through jaw size exaggeration, acquisition of papillose (bud-like) gill rakers, and modification of jaw protrusion for suction feeding.

· 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 claspers of the first captured megamouth were fully described. Its claspers were relatively slender, with tip elongated, forming a very narrow, slender process.

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.

· Parasites

Almost all megamouth reported presented scars in different parts of body. These scars are considered bite marks of the cookiecutter shark (Isistius brasiliensis). Cookiecutter sharks are small dogfish sharks that attach to prey with the help of suctorial (aiding in suction) lips, and a modified pharynx. The cookiecutter removes a conical plug of flesh from the sides of the prey, leaving a crater-like wound. This parasitic shark is presumed to be a vertical migrator on a diel cycle, spending the daytime in deep waters and ascending to midwater depths and to the surface at night. The similar suggested pattern of megamouth combined with its slow swimming speed would make it easy prey for the active cookiecutter shark. Some specimens, males and females, of a parasitic copepod (Dinemoleus indeprensus), also have been found on the megamouth body surface.

A few species of parasites are found internally in megamouth sharks, including specimens of a cestode worm species (Corrugatocephalum ouei) and specimens of a poorly known trypanorhynch (Mixodigma leptaleum) recovered from the intestine contents of the 7th megamouth. A microscopic parasite (Chloromyxum) was also found in this megamouth from its gall bladder.

· 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 (see notes on the webpage).

Fishery Importance

Megamouth had probably been encountered more times than we know, but often such large sharks are returned to the sea by their captors because of the difficulty in handling them. However, those encounters were never been confirmed. Actually, the megamouth is too rare to be harvested.