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, Mpelagios 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. This has been talked about before, with a study in 2001 proposing that megamouth sharks are not luminous but instead use the reflectance of their white band to attract their prey of choice. However, no study had ever been performed to confirm these assumptions so far… until now! “The reflection of light, previously suggested [in 2001], was confirmed by [our] reflectance and transmittance measurements,” the authors explained. “Older observations of ‘light emission’ in Mpelagios would be due to the white band denticles highly reflecting the spotlights of observers and divers. Since white band denticles reflect wavelength in the entire visible spectrum, megamouth sharks could use the white band to reflect either downwelling light or luminescence produced by the plankton to attract and feed on it.”

Is there a chance that megamouth sharks could produce bioluminescence, though? “Based on bacterial detection as well as histological and reflectance analyses, our results strongly support the statement that the megamouth shark, Mpelagios, is not a luminous shark species,” the researchers reported.

Just goes to show we are always learning from our oceans… and how many mysteries there still exist for us to uncover.