Sunday 17 August 2014

Pelagic Stingray

Image: Makoto Nakashima
Pteroplatytrygon violacea
Stingrays are most famous for their sting, but they're also well-known as flattened fish that spend all their time gliding an inch above the ocean floor. Not so the Pelagic Stingray. They're just as flat, but they're not one to be tied down. Their home is the wide, open sea!

The pelagic zone is that part of the sea that lies away from the sand, rocks and reefs of the seabed and the coast. Life here is dominated by a featureless expanse of various shades of blue in all directions. It's a life constantly on the fin; there can be no hiding in the nooks and crannies of a coral reef or resting on the soft sand of the ocean floor.

We've seen some very strange and unexpected pelagic creatures in our time, like the Blanket Octopus, Tomopteris worms and peculiar snails like the Sea Butterfly. We've also seen ocean-faring stingrays like the enormous, filter-feeding Manta Ray and the Cownose Ray which descends to the seabed to snatch up crustaceans and molluscs. Those are both members of a family of unusual, frequently massive stingrays known as the Eagle Rays.

Image: Lyle & Han-Yu's Wedding.
The Pelagic Stingray is different. It doesn't belong to the Eagle Ray family, but is instead the only pelagic member of Dasyatidae, the Whiptail Stingrays. All 70 of its closest relatives are bottom-dwelling fish, so this is quite the rebel!

Pelagic Stingrays are not huge, usually reaching about 60 cm (2 feet) across. Yet they can still get over 1.2 metres (4 ft) long! As in other Whiptails, the tail is a lot longer than the rest of the body, and comes armed with a serrated, venomous stinger.

Video: Aaron Rana

Most stingrays swim using mesmerising undulations of their gigantic pectoral fins, but the Pelagic uses more of a flapping motion akin to the Eagle Rays. On the one hand (pectoral fin) this reduces their manoeuvrability, on the other hand (pectoral fin) it increases their lift and power.

Pelagic Stingrays also use their pectoral fins to manhandle (fishfinnle?) prey and direct it to their mouth. They primarily feed on crustaceans like krill and amphipods, but they'll also tackle squid, fish and jellyfish. So that's most things, really. Pelagic Stingrays have pointy teeth for gripping onto slippery prey, in contrast to most rays that crush through tough shells using their blunt teeth.

Image: Scott Perry
Pelagic Stingrays probably spot their food using their small eyes, which would also be why they usually spend all their time in the top 100 metres (330 ft) of the water column, where it's bright and sunny. Other stingrays use sensory organs called the ampullae of Lorenzini to detect the electromagnetic fields produced by animals who may be completely buried beneath the sea floor. Pelagic Stingrays have ampullae of Lorenzini, too, but far fewer than other stingrays. They will of course need at least some to figure out how close their food is to their mouth.

It's so odd to have your mouth and your eyes so far apart that you can't see what you're eating while you're eating it.

Range. Confirmed sightings in dark blue
With their ability to snatch prey right out of the water, Pelagic Stingrays have no need to visit the bottom and can roam far and wide across the open ocean. They've been reported here and there in tropical and subtropical waters all over the world and presumably range everywhere in between.

Pelagic Stingrays migrate to keep up with warmer waters. There are populations in the Pacific Ocean who spend the winter months near the equator and swim north to California or Japan in the spring. Some Atlantic populations seem to divide their time between the Gulf Stream and more northerly waters.

Image: Scott Perry
Most Pelagic Stingrays give birth to their young while they're closer to the equator, just before migrating north. Like other stingrays, they produce eggs but never deposit them, so they give birth to live young. Pelagic Stingrays produce around 4 to 12 pups, each one measuring 15 to 25 cm (6 to 10 in) across. They do so after a gestation period of just 2 to 4 months. That's less time than any other shark or ray!

There's that jet-setting lifestyle. No use getting weighed down by the kids for months on end!


TexWisGirl said...

they've got to fly! :)

Porakiya said...

Eagle rays and whiptailed rays. there must be a ray for everything! Also, loving those long tails and wide roaming life style

Crunchy said...

The proper aquatic adaptation of "manhandle" is "fin-nagle."

Joseph JG said...

@TexWisGirl: Yup! Always on the move.

@Porakiya Draekojin: Ha! Yeah, there are a lot of rays. And I agree, those tails are fantastic!

@Crunchy: Ah, I shall remember this!