POCKET SHARKS ARE SO SINGULAR THAT AS FAR AS THE WORLD KNOWS there are only two known species – and only one specimen of each. The first resides in a museum in Russia. The second, found in the Gulf of Mexico and just identified, has been given a scientific name but it’s probably going to be known as the American pocket shark.
If having one specimen each of two species makes them the world’s rarest sharks, they stand out for a different reason: Deep sea denizens, these cartilaginous fishes sport “pockets” adjacent to each pectoral fin that may play a role in bioluminescence.
MEET MOLLISQUAMA MISSISSIPPIENSIS
The first one, the 16 in/40 cm female Mollisquama parini, was discovered off the coast of Chile in 1979. Information about it is surprisingly scarce but it was presumably discovered by Russian scientists. It’s named in honor of a Russian ichthyologist and resides in the Zoological Museum of St. Petersburg.
American researchers netted a second one in the Gulf of Mexico in 2010 while studying the deep sea feeding habits of sperm whales. They got around to studying it several years later and this year identified it as a separate species. They named it Molllisquama mississippiensis in honor of its discovery point off the Louisiana coast. In street talk, it’s likely to be the American pocket shark.
NINE-YEAR-OLD CATCH, NEW SPECIES
The new guy, a male, may be a juvenile. Only 5.5 in/14 cm long, with a blunt head and a tapered body, it somewhat resembles a tiny sperm whale.
In 2015, a scientist at the National Oceanic and Atmospheric Administration began an evaluation of specimens collected during the whale-feeding survey and realized he had a unique animal. The mission to determine how unique brought together scientists from the NOAA, Tulane University, the University of Florida and the National Museum of Natural History. Their results were published the journal Zootaxa.
SO, WHAT IS A POCKET SHARK?
It’s a small shark, usually found in deep waters, with “pockets,” or internal sacs located on each side of its body near the pectoral fins. Their purpose is not clear, but it’s surmised that they release some kind of glowing substance that may play a role in attracting prey and/or deterring predators. It’s also suggested that they play a role in releasing pheromones to attract mates. In both the Russian specimen and the American pocket shark, the pockets measure about four percent of the body length.
Additionally, Molllisquama mississippiensis, at least, features luminous photophores irregularly distributed among many areas of its body.
On the other hand, as Washington Post science writer Clare Fiessler noted, the bioluminescent factor attracted headlines in news about the fish but “To scientists, the shark’s glow was the least exciting part.”
After all, bioluminescence among deep sea creatures – sharks and others – is not unusual. These animals prowl, after all, in the mesophotic and photic zones where light from the surface is minimal if present at all. Some 75 percent of all deep sea denizens and at least 10 percent of shark species are believed to produce their own light, Ms. Fiesler notes.
THE BYCATCH FACTOR
A difficulty is that both pocket sharks were brought up in the nets of researchers studying something else – in the case of M. mississippiensis the feeding habits of sperm whales. M. Palini was collected at a depth of some 1,100 ft/330 m, M. mississippiensis at a similar depth.
In other words, scientists have been able to analyze their anatomical features but no one has had a glimpse of them as live animals to see what they actually do in their habitats.
Normally, scientists would carefully dissect a representative specimen to understand its characteristics and architecture. With only one American pocket shark to study, they passed on doing this. Instead, they did a “digital dissection” using computed tomography.
They found the American pocket shark differs enough from its Russian cousin to warrant classification as a new species. M. mississippiensis differs from the Russian version in its sharp, tiny teeth, 10 fewer vertebrae and possession of a temparture-sensitive “pit organ” just behind its lower jaw, presumably helping it detect prey.
The fact that two different species of similar sharks were captured in two different oceans, from depths difficult to explore, underscores how little is known about deep sea life.
PRINCIPAL SOURCES: A new Western North Atlantic Ocean kitefin shark (Squaliformes: Dalatiidae) from the Gulf of Mexico, Zootaxa; Researchers identify new species of pocket shark, Tulane University News Office; The Pocket Shark: A Digital Dissection, NOAA; Scientist Just Identified a Tiny New Species of Shark That Glows in The Dark, Science Alert; The more we exploit the oceans, the more glow-in-the-dark sharks we discover, The Washington Post; Pocket Shark, Britannica.com; Pocket Shark, Wikipedia,com.