Chondrichthyes

By Anonymous - April 15, 2016

Sharks and rays belong to a group of cartilaginous fishes (Class Chondrichthyes). A majority of chondrichthyans live in marine environments, however some are able to travel into estuaries and up rivers. They are often located in shallower waters of the ocean, but some can roam into deeper oceanic waters. Cartilaginous fishes have unique structures that differentiate them from bony fishes (Class Osteichthyes). For starters, their bones are composed of cartilage, although they are often calcified, they are not considered true bones. They have 5-7 gill slits and a modified first gill slit called a spiracle, located behind the eye, that helps many of the bottom dwelling rays take in water for respiration as they wait for prey. Chondrichthyans have placoid scales, which are unique to other types of scales. Placoid scales closely resemble teeth. They contain blood vessels and are covered in enamel that gives the chondrichthyan protection from other species. They also help reduce drag. Teeth in chondrichthyans are not firmly attached to the jaw, but many rows of teeth below wait below to replace a lost or damaged teeth, but don’t worry, teeth are made continuously throughout the organism’s life.

basic tooth anatomy http://palaeos.com/vertebrates/bones/teeth/teeth.html

During our trip to Cozumel, we saw four different cartilaginous fishes, including the nurse shark (Ginglymostoma cirratum), the giant spotted eagle ray (Aetobatus narinari), the yellow stingray (Urobatis jamaicensis) and the southern stingray (Dasyatis americana).

Nurse Shark (Ginglymostoma cirratum)

Nurse sharks are nocturnal, bottom dwelling sharks. During the day they rest in caves, crevices, and under rocks to sleep and at night they prey upon stingrays, crustaceans, and mollusks. They are often found in coral reefs, where many caves and rock out-croppings are formed. This is why we readily saw them on our trip. They reach up to 8-9 feet in length and can weigh up to 200 pounds. Nurse sharks have been known to attack humans, but are typically docile unless bothered. Female nurse sharks are considered ovoviviparous, meaning that fetal development takes place inside of egg cases inside of the mother.

http://www.stlzoo.org/visit/thingstoseeanddo/stingraysatcaribbeancove/nurse-shark/

Eggs inside of their cases of a nurse shark.

http://www.flmnh.ufl.edu/fish/gallery/photoout.asp?id=14021

Giant Spotted Eagle Ray (Aetobatus narinari)

Giant Spotted Eagle Rays are called giant for a reason! They can grow up to 4 meters wide and 9 meters long. They are brown or black with small white spots. They have a long, rounded snout and a thick head. Their teeth make up plates in the form of bands. This allows them to crush and grind their prey, such as bivalves, shrimps, and crabs. Eagle Rays have a long tail, with a venomous barb at the base. The barb may seem scary, but they are rather docile creatures and very rarely cause harm to humans. Like the nurse shark, they are ovoviviparous and copulate abdomen to abdomen.

Giant Spotted Eagle Ray

Yellow Stingray (Urobatis jamaicensis) and Southern Stingray (Dasyatis americana)

The Yellow Stingray and Southern Stingray have similar features. They are about the same size and commonly found in shallow, sandy areas. Being stingrays, they both have tails with the poisonous barbs on the end, but are not a threat to humans unless extremely bothered. With their fins, the rays like to wave the water to create holes in the sand to find their prey, which are mostly made up of crustaceans. And like the other chondrichthyes, both are ovoviviparous.

However, the physical description of the Yellow Stingray and Southern Stingray are different. The Yellow Stingray is, well, yellow. It also has pale and darker blotches. It is very round in shape and only get up to about 26 inches long. The Southern Stingray however, is more diamond shape and has an olive brownish coloration. Also, the Southern Stingray is more closely associated with the cleaner wrasses because they have more parasites than the Yellow Stingray. The cleaner wrasse can take upwards of half an hour to clean the Southern Stingray to its approval.