Glowing in the Deep
Updated: Apr 21
Believe it or not, there are over 530 different species of sharks known to science and new species are being found every year! The discovery of a new species is always exciting, but in some cases, the fabulousness of the new species makes for particularly intriguing news... A newly discovered species of kitefin shark is particularly spectacular - the American pocket shark (Mollisquama mississippiensis) - because it can glow in the dark!
What is a Kitefin Shark?
There are only about a dozen species of kitefin sharks, known to scientists as the family Dalatiidae. These small sharks only reach a maximum of about 2 metres in length and are very rarely encountered by humans, because they generally live in very deep regions of the oceans (Compagno, 1987).
Kitefin sharks can be distinguished from their close relatives because they have spineless dorsal fins, lack an anal fin, have a robust lower jaw and "heterodont dentition" (meaning they have more than one shape of tooth) (Grace et al, 2019).
Several species in this family, such as the pocket shark (Mollisquama parini), have "photophores". These cells allow patches of their skin to glow in the dark! This is known as "bioluminescence" (Grace et al, 2019).
A New Species of Glowing Pocket Shark
A newly described species dubbed the American pocket shark (Mollisquama mississippiensis) differs from its cousins because on top of glowing photophores, it also has a "pit organ". This specialised organ produces luminous fluid which can be ejected into the water (Grace et al, 2019; Claes et al, 2020).
This new species has only been described based a very small number of individuals which have been found in the southeast Pacific Ocean and some specimens that had been found back in 1979. Therefore, we don't know very much about them at all (Grace et al, 2015).
What we do know about the morphology of this species, we have learned from a 40.0 cm female specimen and a neonate male measuring 14.2 cm total length. The American pocket shark has a cylindrical body and wide head, with bulbous snout. The mouth is located on the underside and when the jaw is closed, the lower jaw teeth cover those of upper jaw. These sharks have two pectoral fins, five small gill slits and agglomerations of photophores on their underside (Grace et al, 2015; Grace et al, 2019).
Why Do They Glow!?
As we know the depth at which these sharks were caught in trawls, we know that American pocket shark is a deep-water species. The specimens were found at around 580 metres down in the "mesopelagic zone" (aka twilight zone) of the open ocean. At this depth, light can penetrate, but ambient light levels are low compared to shallower depths in the "epipelagic zone" (Grace et al, 2015).
Scientists think that the American pocket shark has evolved the ability to squirt luminous clouds of fluid so that they can stay concealed from their prey when they are hunting or to escape from their predators. A few other marine animals do something similar - like squids squirting ink - to defend themselves from threats (Grace et al, 2019; Claes et al, 2020).
We still have a lot to learn about these little sharks, but no matter why they glow, they are certainly fascinating and delightful!
Claes JM, Delroisse J, Grace MA, Doosey MH, Duchatelet L & Mallefet J (2020). Histological evidence for secretory bioluminescence from pectoral pockets of the American Pocket Shark (Mollisquama mississippiensis). Scientific Reports, 10:18762. Access online.
Compagno LJV (1987). Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Access online.
Grace MA, Doosey MH, Bart HL & Naylor GJP (2015). First record of Mollisquama sp. (Chondrichthyes: Squaliformes: Dalatiidae) from the Gulf of Mexico, with a morphological comparison to the holotype description of Mollisquama parini Dolganov. Zootaxa, 3948:3, 587–600. Access online.
Grace MA, Doosey MH, Denton JSS, Naylor GJP, Bart HL Jr. & Maisley JG (2019). A new western north Atlantic Ocean kitefin shark (Squaliformes: Dalatiidae) from the Gulf of Mexico. Zootaxa, 4619:1, 109–120. Access online.