Abstract
Despite its five meters length, the megamouth shark (Megachasma pelagios Taylor, Compagno & Struhsaker, 1983) is one of the rarest big sharks known in the world (117 specimens observed and documented so far). This filter-feeding shark has been assumed to be a luminous species, using its species-specific white band to produce bioluminescence as a lure trap. Another hypothesis was the use of the white band reflectivity to attract prey or for social recognition purposes. However, no histological study has ever been performed to confirm these assumptions so far. Two hypotheses about the megamouth shark's luminescence arose: firstly, the light emission may be intrinsically or extrinsically produced by specific light organs (photophores) located either on the upper jaw white band or inside the mouth; secondly, the luminous appearance might be a consequence of the reflection of prey luminescence on the white band during feeding events. Aims of the study were to test these hypotheses by highlighting the potential presence of specific photophores responsible for bioluminescence and to reveal and analyze the presence of specialized light-reflective structures in and around the mouth of the shark. By using different histological approaches (histological sections, fluorescent in situ hybridization, scanning electron microscopy) and spectrophotometry, this study allows to unravel these hypotheses and strongly supports that the megamouth shark does not emit bioluminescence, but might rather reflect the light produced by bioluminescent planktonic preys, thanks to the denticles of the white band.
Highlights
One rare and mysterious shark species, the megamouth shark, Megachasma pelagios, described in 1983 (Taylor, Compagno & Struhsaker) is assumed to display light emission capabilities [1,2,3]
No bacterial labeling is observed in the megamouth shark white band, membrane, or back of the throat skin patch sections treated with EUB probes (Fig 3A–3C)
Masson’s Trichome staining of E. spinax sections reveals the presence of typical lanternshark photophores in the ventral skin (Fig 4). These organs situated within the integument are recognizable in histological sections (Fig 4B and 4C): embedded in a dense connective tissue, colored in green, with denticules, colored in orange and green, they harbor, at their base, a hemispherical cup-shaped layer of black-pigmented cells, covering the central photocytes, colored in brown with red nuclear, covered by iris-like structure cells containing black pigments and externally topped by a single or multiple lens cells colored in orange (Fig 4B and 4C)
Summary
One rare and mysterious shark species, the megamouth shark, Megachasma pelagios, described in 1983 (Taylor, Compagno & Struhsaker) is assumed to display light emission capabilities [1,2,3].
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