Despite being a massive shark (measuring up to 16 feet/5 meters in length), there is a lot shark scientists don’t know about megamouth sharks (Megachasma pelagios). With only 117 specimens observed and documented so far, this filter-feeding shark is one of the rarest big sharks known in the world!
The megamouth shark was first discovered in 1976 by a U.S. navy research vessel that was operating in Hawaiian waters. The shark had became tangled in some underwater cables and due to their mouths they were dubbed the ‘megamouth’ shark. Such a peculiar-looking animal, the deepwater shark is the sole member of its genus, Megachasma. Believed to be diurnal, it is thought to swim between the shallows and deep waters to follow swarms of zooplankton. “Unlike the other filter-feeding sharks, M. pelagios seems to possess a unique feeding method likely derived from the ram-filter mode used by the basking shark,” say the authors of a new study. “By creating a negative pressure when it expands maximally the oral cavity, it fills it and expulses the water through its gill slits when it closes it.”
As one can imagine, due to their rarity, very little is known about the shark’s behaviour or how their unique physical traits help them out. For example, it was believed their lips could possibly be bioluminescent, attracting prey into their mouth. Another hypothesis is that they use the unusual white band on their upper jaw to produce bioluminescence as a lure trap. This white band, hidden in a groove between their snout and the jaw, is only visible when their upper jaw is protruded. Could this somehow be linked to feeding?
Researchers led by scientists Laurent Duchatelet and Victoria C. Moris of Université Catholique de Louvain, Belgium wanted to find out what this band does for these mysterious sharks. After collecting tissue patches from specimens that were accidentally caught by local fishermen along the Japanese coast, the team used different techniques to find out more information about the samples collected (ratio of measures, shape, density, arrangement, surface features, calcification structure, and reflective properties).
Their analysis found that the band doesn’t emit its own bioluminescence but seems to reflect the light produced by bioluminescent planktonic preys, thanks to the denticles of the white band.