Cownose ray

The cownose ray (Rhinoptera bonasus) is a species of Batoidea found throughout a large part of the western Atlantic and Caribbean, from New England to southern Brazil (the East Atlantic populations are now generally considered a separate species, the Lusitanian cownose ray (R. marginata)).[2] These rays also belong to the order Myliobatiformes, a group that is shared by bat rays, manta rays, and eagle rays.[3]

This article is about the ray common in the western Atlantic. For other species of cownose rays, see Rhinoptera.

Cownose ray
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Superorder: Batoidea
Order: Myliobatiformes
Family: Rhinopteridae
Genus: Rhinoptera
Species:
R. bonasus
Binomial name
Rhinoptera bonasus
(Mitchill, 1815)

Cownose rays prefer to live in shallower, coastal waters or estuaries.[2] Size, lifespan, and maturity differ between male and female rays. Rays have a distinct shape, and it has two lobes at the front of its head, resembling a cow nose. Cownose rays can live between 16 and 21 years, depending on sex.[4] Rays feed upon organisms with harder shells, such as clams, crustaceans, or mollusks.[2] They are migratory creatures, where they migrate South in the winter and North in the summer.[4] The rays are known to occupy the Chesapeake Bay in the summer months.

In 2019, the species was listed as vulnerable on the IUCN Red List.[1] The species has been subjected to overfishing due to the perceived threat of overpopulation in the Chesapeake Bay.[5] There are not many conservation strategies or efforts for cownose rays.

Taxonomy

The genus name Rhinoptera is named for the Ancient Greek words for nose (rhinos) and wing (pteron). The species name bonasus comes from the Ancient Greek for bison (bonasos).

Description

A cownose ray is typically brown-backed with a slightly white or yellow belly. Although its coloration is not particularly distinctive, its shape is easily recognizable. It has a broad head with wide-set eyes, and a pair of distinctive lobes on its subrostral fin. It also has a set of dental plates designed for crushing clams and oyster shells. Male rays often reach about 2 and 1/2 feet in width, while females typically reach about 3 feet in width.[4] The cownose ray is often mistaken for being a shark by beach-goers due to the tips of the rays fins sticking out of the water, often resembling the dorsal fin of a shark.[6]

When threatened the cownose ray can use the barb at the base of its tail to defend itself from the threat.[6] A cownose ray has a spine with a toxin, close to the ray's body. This spine has teeth lining its lateral edges, and is coated with a weak venom that causes symptoms similar to that of a bee sting.[7]

Habitat and Distribution

Cownose rays are migratory and social creatures and reside on the east coast of the United States, Brazil, as well as in the Gulf of Mexico.[2] They prefer to live in near coastal waters and in estuarian ecosystems.[2] Cownose rays are able to tolerate a wide range of salinities because of the areas they occupy.[8] This allows for the rays to have the potential to live in a wider range of habitats if one area gets too crowded and competition for resources is high.[8] Cownose rays are known to be abundant in the Chesapeake Bay and migrate to the area for mating and nursery purposes, typically in the late spring and summer time.[2] Rays are typically spotted near the surface of waters as well.[2]

Behavior

Diet and feeding
Cownose ray teeth and mouthparts. Stingray teeth consist of interlocking bars (dental plates) that crush food.

The cownose ray exhibits a durophagous diet, meaning it feeds upon hard-shelled organisms, such as mollusks, crustaceans, but they prefer scallops or clams, which have softer shells and are categorized as bivalves.[2] The cownose ray tends to feed either in the early morning hours or in the late afternoon hours, when the waves are calm and visibility is higher than during the day.[9] Their feeding occurs in the benthic zone, or at the bottom of the ocean.[2]

The rays are able to capture their prey through suction and the opening and closing of their jaw.[9] Because of the type of prey cownose rays consume, their jaw needs to be able to handle the hard-shell organisms. Their jaws are extremely robust and have teeth with a hardness comparable to that of cement.[9] Their cephalic lobes also assist with capturing and handling their prey by pushing them towards their mouth.[9]

Predation

The cownose ray sits fairly high up on the food chain, and as a result only has a few natural predators. These predators include; cobia, hammerhead sharks, and humans who like to fish for them.[10][11]

Cownose rays swimming in shallows in the Gulf of Mexico

Reproduction and lifespan

Cownose rays breed from April through October.[2] Rays will not reach a mature age until they are roughly 70% of the way to their maximum size.[2] Females reach maturity between ages 7–8, while males reach maturity between ages 6–7.[2] The lifespan of the cownose ray varies by sex; the oldest female ray that has been recorded was 21, and the oldest male ray was 18, which were both observed in the Chesapeake Bay.[4]

Cownose rays are ovoviviparous, meaning that the embryo grows within its mother until it is ready to hatch.[4] Rays have a longer gestation period due to their K-selected species attributes. The length of gestation is believed to last between 11 and 12 months, and at full term, the offspring are born live, exiting tail first.[4]

Migration

Rays often travel and migrate in large schools based on size and sex.[12] Their migration pattern consists of rays moving north in late Spring and moving south in late Fall.[2] Much of what we know about their migration has been from studies done in the Chesapeake Bay. Male and female rays will come to the Bay in the late spring and leave in the fall.[2] While occupying the Chesapeake Bay, the female rays and her pups will live in the estuarine waters.[2] Males have been observed leaving the Bay earlier than the females to arrive at a second feeding ground, and the reason for taking a longer migration route is not fully known.[2] One hypothesis is that males exit the Bay to reduce competition of certain resources, such as food and shelter.[2]

Threats and Conservation

The cownose ray is currently listed as vulnerable by the IUCN Red List[1] due to extensive overfishing and commercial fishing.[13] The overfishing is due to the perception that rays destroy oyster beds meant for the shellfish industry.[5]

The trophic cascade in the northwest Atlantic Ocean has been cited and used to link cownose ray overpopulation to the decrease in large coastal sharks, which therefore cause bivalves populations valuable for commercial reasons to be depleted; however, there is little evidence that supports this hypothesis.[5] Campaigns such as "Save the Bay, Eat a Ray" in the Chesapeake Bay used these claims to promote the fishery of these rays in hopes of preserving the Bay, which can be detrimental to this species.[5] Cownose rays reach a mature age later in their lifecycle and long gestation periods, meaning that they are a K-selected species.[5] This suggests that they are vulnerable and sensitive to overfishing, and their populations cannot easily bounce back after these events. Even though rays have been used as a scapegoat to explain the decline in bivalves, some studies have found that cownose rays do not consume a great deal of oysters or clams.[5] Other studies have found that much of the shellfish prey that the cownose ray consumes is influenced by the size of the shell, so it has been suggested that oyster growers protect their shellfish until their shell reaches a certain size.[14]

There are not many conservation strategies or efforts for cownose rays, besides the fact that cownose ray killing contests have been banned in the state of Maryland.[13]

Relationship to humans

Risk to humans

Stingrays, including the cownose ray, can pose a low to moderate risk to humans. Rays will lash their tails when threatened, posing a risk of being whipped. If threatened, the cownose ray can also use their barb as a weapon to sting the aggressor. A sting from a cownose ray can cause a very painful wound that requires medical attention once stung. While the sting is not usually fatal, it can be fatal if stung in the abdomen.[15] There is also a risk associated with eating meat from the sea animal that has not been prepared correctly. Shigella may be acquired from eating meat from a cownose ray that has been contaminated with the bacteria. This bacteria causes shigellosis, and can result in dysentery. Symptoms can include diarrhea, pain, fever, and possible dehydration.[16]

Aquariums

Cownose rays can be seen in many public aquaria worldwide and are often featured in special 'touch tanks' where visitors can reach into a wide but shallow pool containing the fish, which have often had their barbs pinched or taken off (they eventually regrow, similar to human nails), making them safe enough to touch.

The following aquariums and zoos are known to have touch tanks featuring cownose rays (alone or with other fish):

USA

Canada

References
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