The swell shark gets its name from its ability to inflate itself to roughly double its normal size by swallowing water as a form of protection from predators. But that’s not its only amazing property – the hero of the second coin in the Luminous Marine Life series can also fluoresce green all over.
To do so, the swell shark converts incoming ultraviolet light, or blue light, into green light. A special pigment in the sharks’ eyes enables it to see the neon green of its conspecifics. Human eyes do not have that pigment, which is why we need an ultraviolet torch to see the swell shark’s green glow. The molecules that make swell sharks fluoresce are unique in the animal world. The light areas of their skin glow more intensely than the dark ones. Why the shark does that is not yet fully understood, but it is assumed that the process helps swell sharks to recognise each other and to distinguish between the males and females of the species. It could also be a form of camouflage and because the molecules offer protection against bacteria and algae and just happen to glow.
Although the swell shark is not targeted for fishing, humans are its main predator as the sharks still end up as bycatch in lobster and crab traps as well as in bottom trawls. This is a problem because swell sharks reproduce slowly and have few young. Global warming and overfishing are further challenges, as the swell shark prefers temperate water temperatures and industrial fishing reduces the shark’s feeding opportunities. The swell shark is just one of 12 fascinating examples in the Luminous Marine Life series of why we absolutely have to protect the marine habitat – not least from ourselves.
All 12 of the magical sea creatures in the Luminous Marine Life series are shown in silhouette on the coin’s obverse. Air bubbles up between them and a tail fin disappears beneath the waves to the right. A wide band, reminiscent of an old-fashioned porthole frames the aquatic scene on the coin’s reverse. A swell shark appears to swim through the porthole, its body brown in colour when illuminated with ultraviolet light glows green. As on all coins in the series, to the left and below a shell and a starfish can be seen.