domingo, 7 de abril de 2013

Blue Zoo: Skates and Rays

Sarah Bedolfe
February 21 2013

Featuring one amazing marine animal per week. 

Skates and rays are fish that are closely related to sharks. Together these animals make up the elasmobranchs, a subclass of cartilaginous fish. Instead of having skeletons made of bone, sharks, rays, and skates have skeletons made of soft, pliable cartilage.

Like rays, skates are usually flat, with a long tail. Most species live on the seafloor, where their shape makes it easy for them to hide by burrowing into the sand.

Southern stingrays. Photo by Barry Peters via Wikimedia Commons, Creative Commons.

A broad skate. Photo by NOAA/MBARI via Wikimedia Commons, Public Domain. 

There are some differences between skates and rays, however. Unlike some rays, no skates have venomous stinging barbs on their tails, but they can have rows of spikes instead for protection.

Skates also have a shorter tail and their pelvic fin can have two lobes instead of just one; the first lobe sometimes resembles a small modified foot, called a crus, and can be used to help movement.

Skates and rays have eyes on top of their heads. Behind the eyes are spiracles, which pump water to the gills. Photo by John Gullo via Flickr, Creative Commons License. 
Skates and rays have special adaptations for living on the seafloor. For example, their eyes are on the top of their bodies, facing up. This lets them see potential predators above them. Next to their eyes, they also have something called spiracles. Since spiracles are next to the eyes and often look like they’re blinking, they can be mistaken for the eyes but they are actually part of the respiratory system.

Sharks normally pass water over their gills by taking water in through their mouth while swimming. Skates and rays are unable to do this, however, because their mouths are on the bottom, facing the sand. This lets them easily burrow around in the seafloor sediment for food such as shellfish. Meanwhile, to breathe, they use spiracles to pump water to the gills without swallowing sand through their mouths. 

Photo by Thomas Kohler via Flickr, Creative Commons License. 
A male skate, top photo, can be identified because its pelvic fin (at the base of the tail) has two lobes, one of which is a modified foot. The additional appendages are claspers, which females lack.

Photo by Shmulik Blum.
A male stingray, lower photo, only has a single-lobed pelvic fin in addition to its claspers. You can also see their gill slits, mouths, and nares. 

When looking at a skate or ray from the bottom, you can see the mouth, gill slits, and nares (they're similar to nostrils; they are not eyes). The pelvic fin, which is at the base of the tail, on rays is a single lobe, but in skates is a double lobe with a modified foot. Below the pelvic fin, males have two extra appendages called claspers, which females lack. Both individuals pictured above are males; a female is pictured below. 

Though they have many common features, perhaps the biggest difference between them is that skates lay eggs, while rays give birth to live young (sharks can do either, depending on species). A skate egg case is called a mermaid's purseand is made of chitin, which is a material similar to your fingernails. 

Photo by silvereye via Flickr, Creative Commons License. 
Photo by Bill Abbott via Flickr, Creative Commons License. 
In an aquarium it is possible to safely replace a panel in the egg sac with a plastic window. This allows you to watch the embryo develop until it is ready to hatch.
Viewing a baby skate from underneath, you can see the crus (modified feet) and mouth. Some internal organs are visible through its thin skin. It is lacking a pair of claspers beneath the pelvic fins, indicating it is a female.

While most skates and rays are confined to the seafloor and look more or less rounded, these groups are still quite diverse, and include species like the oddly-shaped guitarfish (a skate), to the majestic manta ray. 

Out of all pelagic (open ocean) species of sharks, skates, and rays, 32% of them are threatened with extinction, mainly due to overfishing. While sharks are pursued most for their fins, manta rays are hunted for their gill rakers, which they use to filter plankton from the water, and skates are sometimes used to make mock scallops.

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