Did you know that there are 50 bioluminescent shark species? One of them is the velvet belly lanternshark, a tiny deep sea shark. In this dark world, they would be seen from below as a dark shadow against a slightly brighter, green glowing background, caused by residual light from the surface of the ocean. Thus their sides and bellies glow themselves due to thousands of tiny photophores (light-emitting organs) emitting a long lasting blue-green luminescence – a process called counterilluminating. This “cold light” is produced by a 98% efficient chemical reaction between a protein (luciferin) and an enzyme (luciferase) in the presence of oxygen. The shark’s bioluminescence develops before birth (the complete pattern is laid down by the time the embryo is 95 mm -3.7 in- long) to protect newborn sharks. Each species has its own pattern of photophores, which perhaps aids in intraspecific communication, too.

But the velvet belly doesn’t stop with the belly. It has got defensive spines in front of its dorsal fins (like the spiny dogfish) and uses spine-associated photophores (SAPs) to discourage predators from several meters away.

velvet belly lanternshark
velvet belly lanternshark – Source: Julien M. Claes, Mason N. Dean, Dan-Eric Nilsson, Nathan S. Hart & Jérôme M allefet: “A deepwater fish with ‘lightsabers’ – dorsal spine-associated luminescence in a counterilluminating lanternshark”

As another peculiarity of the velvet belly laternshark, the chemical reaction within the cells of its photophores is stimulated by hormones (melatonin or prolactin).

To deal with the higher concentrations of heavy metals in the deep sea, the velvet belly’s immune system can identify and detoxify cadmium, copper, mercury, zinc, and other toxic contaminants. Additionally, enzyme important in metabolism and movement have been modified to withstand the extreme cold of the deep sea.

To avoid that adults of this species competing against their own young for food, velvet bellies undergo a dietary and locational shift. Whereas little sharks feed on a specific krill and small fish, adult velvet bellies move to depths greater than 500 metres -1,600 feet- and feed primarily on a specific shrimp and various deep-sea squids living there.

In the northeastern Atlantic velvet belly lanternsharks are considered as near threatened, since bycatch in bottom trawls and deepwater longlines decline their numbers, reducing them to fishmeal.

Sources: here, here and here