It’s cownose ray mating season again!
It’s cownose ray mating season. It’s also the time of year when I’m frequently asked “what kind of fish is that swimming at the surface with two fins out of the water?” That fish is a female cownose ray and if one looks carefully they will see at least one male, often several, following her at a slightly deeper depth. When a female cownose ray displays her pectoral fin tips above water she is ready to mate.
In Charlotte Harbor, cownose ray mating behavior can occur between October and June but mating usually takes place from April to June. Like all sharks and rays, fertilization is internal. Female cownose rays have two ovaries, but only the left one is functional. Males have modified pelvic fins called claspers that they use to deposit sperm when mating. During mating, the male bites the female to hold onto her. This often leaves visible wounds along the pectoral fins. Sharks and other rays also exhibit this biting behavior. Don’t worry, the wounds heal quickly.
Unlike skates which lay eggs, commonly called mermaid purses, rays give live birth. The gestation period for cownose rays is 11-12 months after which time one pup is born. Very rarely a female may produce two offspring. To maximize space prior to birth offspring wrap their pectoral fins around their body. At birth, a cownose ray is about 9 inches from wing tip to wing tip.
The cownose ray, Rhinoptera bonasus, is found in coastal and estuarine waters from southern New England to Brazil. Despite their large range, they are actually quite under-studied. Fortunately for us, a number of publications have been derived from the Charlotte Harbor area, and that’s good because there are some very distinct geographical differences.
Up until the mid-2000s, much of what we knew about cownose rays came from studies in the Chesapeake Bay and northern Gulf of Mexico. These included age and growth, reproduction, foraging, and migration studies. The oldest animal from these studies was a 21 year old female, so we know cownose rays get at least that old.
In the Chesapeake, cownose rays become sexually mature at 6-7 years for males and 7-8 years for females. In the northern Gulf, sexual maturity occurs earlier for both sexes (4-5 years). In Charlotte Harbor, sexual maturity occurs at smaller sizes than those from the Chesapeake and northern Gulf, but it is not yet known if that correlates to a younger animal.
In northern climates, cownose rays migrate south beginning around October each year, however in Charlotte Harbor, cownose rays can be observed year round. This is probably due to our warmer climate. Cownose rays do not do well when the water temperature dips below 60 degrees. In addition to our year round population, we also likely see a population increase as migrating cownose rays from the northern Gulf make their way south for the winter. This may explain the very large aggregations we sometimes see.
Cownose rays are pelagic swimmers and can cover considerable ground, however many tagged individuals from a Charlotte Harbor study exhibited small home ranges and could be observed in these areas for months at a time. This is a good indication that they were finding ample prey items to feed on within their limited home range. What causes cownose rays to move when they do is unclear but it is probably related to prey availability and predator avoidance.
Speaking of prey, cownose rays have a diverse diet of invertebrates including bi-valves, marine worms, urchins, sand dollars, and a host of other things. Cownose rays eat during the day and at night.
Recently, cownose rays have become a controversial species. In the Chesapeake, some advocate the harvest of cownose rays because they believe these rays are negatively affecting oyster restoration and bi-valve aquaculture efforts. Scientists however are concerned that this species is vulnerable to collapse because it is slow to reach maturity, only produces one pup a year, is understudied, and is unregulated.
I’ll leave you with some cool new research findings. Scientists recently determined that there is a second species of cownose ray in the Gulf of Mexico. Rhinoptera brasiliensis, appears to be more prevalent in the western Gulf of Mexico, but they have been found in the eastern Gulf, including one from Charlotte Harbor. The species look very similar and are best distinguished by examining their DNA.
Colins, A.B., M.R. Huepel, and C.A. Simpfendorfer. 2008. Spatial distribution and long-term movement patterns of cownose rays Rhinoptera bonasus within an estuarine river, Estuaries and Coasts, 31: 1174-1183.
Colins, A.B., M.R. Huepel, and P.J. Motta. 2007. Residence and movement patterns of cownose rays Rhinoptera bonasus within a south-west Florida estuary, Journal of Fish Biology, 71: 1159-1178.
Collins, A.B., M.R. Huepel, R.E. Hueter, and P.J. Motta. 2007. Hard prey specialists or opportunistic generalists? An examination of the diet of the cownose ray, Rhinoptera bonasus, Marine and Freshwater Research, 58, 135-144.
Fisher, R.A., G.C. Call, and R.D. Grubbs. Age, growth, and reproductive biology of cownose rays in Chesapeake Bay, Marine and Coastal Fisheries, 5:1, 224-235.
Neer, J.A. and B.A. Thompson. 2005. Life history of the cownose ray, Rhinoptera bonasus, in the northern Gulf of Mexico, with comments on geographic variability8in life history traits, Environmental Biology of Fishes, 73: 321-331.
Poulakis, G.R. 2013. Reproductive biology of the cownose ray in the Charlotte Harbor estuarine system, Florida, Marina and Coastal Fisheries, 5:1 159-173.
Smith, J.W., and J.V. Merriner. 1987. Age and growth, movements and distribution of the cownose ray, Rhinoptera bonasus, in Chesapeake Bay, Estuaries, 10:2 153-164.