Smooth Lanternshark (Etmopterus pusillus)
Photo: Brandi Noble (Wikimedia Commons)
Bioluminescence, the production of light by organisms, is a marvelous ability possessed by a broad range of species including troglodytic glow worms, forest fireflies, thumb-sized click beetles, Caribbean zooplankton, parasitic fungi and even lowly bacteria. There are hundreds of bioluminescent species on land and in water, but arguably, the phenomenon finds its finest (and trippiest) expression in the murky ocean depths.
Deep sea organisms live in a dark world, with little or no sunlight penetration. But its not a world entirely without light, thanks to bioluminescence. Most deep sea organisms utilize bioluminescence for communication, warning, or hunting, by combining a light-emitting luciferin pigment (the most common of which is coelentarazine) with an oxidative luciferase enzyme; the result, as luciferin reacts with oxygen, is the production of light.
Perhaps the most famous of these deep sea illuminators are the anglerfishes, diverse array of whimsically named species from a suite of different families, including goosefishes, frogfishes, handfishes, sea toads, footballfishes, dreamers, whipnoses and seadevils. Typical anglerfish, let's take the Humpback Anglerfish (Melanocetus johnsonii), which is perhaps better known as the Black Seadevil, thanks to its meteoric rise to fame on YouTube in 2014, features a light-producing organ called a esca, which is mounted on a specialized ray called an illicium. The esca hangs in front of the anglerfish's generously-toothed and highly destendable mouth, and illuminates with the help of bioluminescent symbiotic bacteria. The light serves to attract potential prey and mates.
Beyond anglerfish, there is a ensemble of other bioluminescent deep sea denizens, including squid, snails, worms, jellies, an astounding diversity of zooplankton, and many more fish. Among the more unique are the loosejaws (Malacosteus spp.), which produce a red bioluminescence. Red light cannot be seen by most of the loosejaw's prey and so this flashlight-wielding predator has a distinct hunting advantage - it can see its prey, but its prey can't see it. Most bioluminescent deep sea organisms haven't received much scientific study, but one group that has are the lanternsharks (Etmopterus spp.), and they've proven even more interesting than your average bathypelagic swimmer.
Lanternsharks (family Etmopteridae), are a diverse lineage found all over the world's oceans, most of them living between 200 and 5000 m deep. As their name suggests, lanternsharks are bioluminescent, but they aren't the only sharks that sparkle and shine: cookiecutter sharks (Isistius spp.), Viper Dogfish (Trigonognathus kabeyi), Pygmy Shark (Euprotomicrus bisinatus), and Taillight Shark (Euprotomicroides zantedeschia), for example, do so too. But it is among the 40 species (others probably await discovery) of lanternsharks that bioluminescence is most pronounced. In fact, their bioluminescence may have played an important role in promoting the great diversity of Etmopterus we see in the world's oceans today.
Most of the lanternsharks have bioluminescent organs (photophores) on their bellies and their sides, although some species don't seem to have any at all. Sadly, the only lanternshark that occurs in Canadian waters, Great Lanternshark (Etmopterus princeps), doesn't glow, though, strangely, it lives deeper than many of the other etmopterids, down to 4500 m, where having some bioluminescent abilities ought to be highly advantageous. But for those species that do light up the abyssal depths, they do so in a three of different ways. Their bellies are covered in photophores that glow a dull blue and are used as a form of camouflage called counter-illumination. Most lanternsharks are less than a metre long, thus potential prey for many larger fish. Even in the near-complete darkness of the deep sea, the large sensitive eyes of many predatory fish can detect the silhouettes of their prey swimming above them, but the subtly glowing bellies of lanternsharks makes their silhouettes invisible against the faint background of blue light emanating from above. Lanternsharks, in effect, hide in plain sight. Many surface dwelling fish have a similar but less sophisticated adaptation called counter-shading, wherein their bellies are a light shade to match the sky above, thus making them more difficult for predators to spot from below.
Other sets of photophores lines the flanks of lanternsharks and some species have them on their backs as well. These photophores, rather than acting as camouflage, make their bearers more obvious, either as aposmatic (warning) signals to potential predators or as advertisements to potential mates. We're all familiar with aposmatic colours, bees and wasps for example, are patterned black and yellow to announce to predators that they shouldn't be messed with, lest they deliver a painful sting. In a similar fashion, lantersharks have generously large spines at the base of both dorsal fins, and at least one species, the Velvet Belly (Etmopterus spinax), illuminates its spines to advertise this formidable defense mechanism. The researchers who made this discovery facetiously called the illuminated spines lightsabers.
Green Lanternshark (Etmopterus virens). The dark patch on the belly is a concentration of bioluminescent photophores used in counter-illumination, while the linear dark patches near the base of the tail are photophores used in species recognition. Each dorsal fin sports a spine at its lead edge.
Photo: Brandi Noble (Wikimedia Commons)
Lanternsharks also employ bioluminescence when advertising to potential mates. The band of photophores along the flanks and tail base are unique for each species. This novel adaptation may have allowed Etmopterus to rapidly diversify into more species than almost any other shark genus. Species-specific photophore patterns act as a means of genetic isolation, keeping species from hybridizing. Just as bird species tell each other apart by the differences they display in plumage, lanternsharks distinguish between members of their species (potential mates) and other species (non-suitable mates) by the unique light patterns each produces. A lanternshark's light display isn't terribly bright, it doesn't shine like a spotlight through the pelagic depths, instead it's more subdued, often visible from only a few metres away. But it's enough to do the trick, especially when aided by special ocular adaptations that make lanternshark eyes extremely sensitive. What to a human observer may seem like a meagre glow emanating from the flank of a Splendid Lanternshark (Etmopterus splendidus), may actually appear to be a magnificent lightshow to the sharks themselves.
