The cownose ray (Rhinoptera bonasus) is a captivating species of Batoidea inhabiting a significant portion of the western Atlantic and Caribbean, stretching from New England to southern Brazil. These rays, easily recognized by the distinctive lobes on the front of their head resembling a cow's nose, play a crucial role in their ecosystems. This article delves into the diet and feeding habits of the cownose ray, exploring their preferred foods, hunting strategies, and ecological impact.
Cownose Ray: An Overview
Cownose rays exhibit a preference for shallower, coastal waters and estuaries. These migratory creatures travel south in the winter and north in the summer, with the Chesapeake Bay being a popular location during the warmer months. Male rays typically reach a width of about 2 and 1/2 feet, while females can grow to approximately 3 feet. They can live between 16 and 21 years.
Anatomical Adaptations for Feeding
The cownose ray's physical characteristics are well-suited to its diet. A typical cownose ray is brown-backed with a slightly white or yellow belly. They possess a broad head with wide-set eyes and distinctive lobes on their subrostral fin. Most importantly, they have specialized dental plates designed for crushing the shells of their prey. Their jaws are extremely robust, and their teeth have a hardness comparable to cement, allowing them to handle hard-shelled organisms.
Dietary Preferences: A Durophagous Lifestyle
The cownose ray exhibits a durophagous diet, primarily feeding on hard-shelled organisms. Their diet consists of organisms with harder shells, such as clams, crustaceans, or mollusks. While they consume a variety of shellfish, they show a preference for scallops or clams, which have softer shells and are categorized as bivalves. Studies of cownose ray populations in the Chesapeake Bay, Virginia, have revealed that their diet consists primarily of bivalve mollusks. Examination of their stomachs and spiral valves has shown large quantities of crushed valves of thin-shelled bivalves, small to moderate amounts of bivalves with intermediate thickness, and only a few small valve fragments of the thick-shelled.
Hunting and Feeding Strategies
Cownose rays typically feed either in the early morning or late afternoon hours, when the waves are calm and visibility is higher. They are adept at capturing prey using suction and the opening and closing of their jaw. One way the cownose ray will feed is by using its pectoral fins to stir up the bottom sediments to reveal its prey. Cownose rays are able to tolerate a wide range of salinities because of the areas they occupy. 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.
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It has been proposed that cownose rays use a very specific mechanism to obtain deep-burrowing prey. They locate food on the bottom substrate (benthos) through mechano- or electroreceptive detection. Once they suspect prey is there, they employ a combination of stirring motions of the pectorals while sucking/venting both water and sediment out through the gills and away from the area to create a central steep-sided cavity depression. The continued movement of the pectoral fins aids in dispersing the sediments released from the gills and increases the depth of the depression.
Feeding Ecology and Behavior
Cownose rays are migratory and social creatures, residing on the east coast of the United States, Brazil, as well as in the Gulf of Mexico. They prefer to live in near coastal waters and in estuarian ecosystems. Rays often travel and migrate in large schools based on size and sex. Their migration pattern consists of rays moving north in late Spring and moving south in late Fall. 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. While occupying the Chesapeake Bay, the female rays and her pups will live in the estuarine waters. 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.
Conservation Status and Threats
The cownose ray is currently listed as vulnerable by the IUCN Red List due to extensive overfishing and commercial fishing. Cownose rays reach a mature age later in their lifecycle and long gestation periods, meaning that they are a K-selected species. 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.
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. 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.
Interesting Facts about Cownose Rays
The cownose ray was first named Raja bonasus (Mitchill, 1815). This name was changed to the currently valid name Rhinoptera bonasus that same year. The genus name is derived from the Greek “rhinos” meaning nose and “pteron” meaning wing. The species name bonasus is from the Greek “bonasos” meaning bison.
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The cownose ray has smooth skin. This ray is set apart from all of its relatives by the indented anterior contour of its cartilaginous skull (chondrocranium), with the conspicuously bilobed subrostral fin. The exception to this is the ticon cownose ray (Rhinoptera brasiliensis), which so closely resembles R. bonasus in both appearance and bodily proportions, that they can only be differentiated by their number of teeth. Normally, R. The disc is approximately 1.7 times as broad as it is long. The eyes and the spiracles of the cownose ray are located on the sides of their broad head. The main portions of the pectorals arise from the sides of the head, close behind and below the eyes. The outer corners of the pectorals are pointed and become concave toward their posterior margins. The tail, round to oval in cross-section, is moderately stout near the anterior spine, and narrows rearward, tapering to a lash-like tip. The length of the tail, measured from the center of the cloaca, is about twice as long as the body, measured from the cloaca to the front of the head, but can be three times as long on small specimens. There are one or two tail spines. The first spine (posterior spine) is located directly behind the base of the dorsal fin. The anterior spine varies in length from very short (tip hardly emerges from skin) to as long as the posterior spine. The spines contain marginal teeth with broad bases and sharp tips that curve rearward at a 45º angle. The number of teeth ranges from 22 to 45.
The dorsal surface of the cownose ray is light to dark brown, and can sometimes have a yellowish tint. The ventral surface is white or yellowish white with the outer corners of the pectorals being more or less brownish. Some specimens are marked above and below with many narrow obscure dark lines or bands that radiated outward from the center of the disc. There are usually 7 series of teeth, on the dental plate, in each jaw that contain up to 11 to 13 rows exposed and functioning simultaneously. The disc width at birth is about 14 inches (36 cm). There seems to be a considerable variation in the size when maturity is reached, which is independent from geographical influence. A male from the North Carolina waters measured only 26-28 inches (66-71 cm) wide with testes enlarged, whereas a Brazilian male measured 31 inches (79 cm) having the claspers only reach about halfway along the inner margins of the pelvic fins. A female containing embryos measured 24 inches (61 cm).
Reproduction and Development
Cownose rays breed from April through October. Rays will not reach a mature age until they are roughly 70% of the way to their maximum size. Females reach maturity between ages 7-8, while males reach maturity between ages 6-7. Cownose rays are ovoviviparous, meaning that the embryo grows within its mother until it is ready to hatch. Rays have a longer gestation period due to their K-selected species attributes. The exact gestation period is not currently known for the cownose ray. The gestation period is believed to be 11-12 months, although some evidence suggests that cownose rays might have two gestation periods lasting 5-6 months. Development is ovoviviparous, producing eggs that develop within the maternal body and hatch within. The embryos lose the shell capsule in early August. The initial nutrition provided to the embryos is from the yolk, which gradually diminishes between August and October. After October, histotroph, a viscid yellowish secretion from the uterus, provides the remaining nutrition. Cownose rays make long migrations. It is suspected that only one cownose ray embryo is carried to full term but six embryos have been found to make it to full term in one female. Female cownose rays may lift their pectoral fins out of the water to avoid mating. According to observations in July of 2000 by biologists from the NMFS Apex Predators Program, a large school of cownose rays of varying ages and sexes was spotted in the shallows of Delaware Bay. A female cownose ray was seen swimming with the edges of her pectoral fins sticking out of the water. Occasionally, there would be major splashing as the males tried jumping out of the water to grasp the pectoral fins with their mouths.
Health Considerations
Common bacteria found in cownose ray include Shigella sp. (in intestine) and Serratia liquefaciens (on teeth). Shigella sp. causes inflammatory dysentary (Shigellosis). Cestodes (tapeworms) have been found in cownose rays. Most of these cestodes infect the spiral valve of the cownose ray. Benedenella posterocolpa, a monogenetic trematode, has been found on the ventral surface of cownose rays. Shigella may be acquired from eating cownose ray meat that has been contaminated with this bacteria.
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