Spotted Eagle Ray
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Myliobatidae
Spotted Eagle Ray (Aetobatus narinari)
Characteristics, Distribution and Range
The Myliobatidae family is made up of three subfamilies containing seven genera and about 42 species. Myliobatidae includes the eagle rays, manta or devil rays, and cownose rays. Myliobatidae are known for their large, disc-like, pectoral fins or “wings” that are pointed at the tips (Tricas et al. 2002, Tinker and De Luca 1973). These pectoral fins, unlike those of stingrays, do not extend forward and envelope the head but end at the side of the head, giving these rays heads distinct from the rest of the body. Pairs of eyes and spiracles are located on the side of the head. The skin is mostly smooth besides the few tubercles around the eyes and along midline on the backs of males. The long, slender, tapering tails have a venomous spine on the dorsal surface of most species, located closer to the body than in stingray spines. The flat, bony, plate-like teeth, set in one or more series, make up the beak designed to crush hard-shelled sand dwellers.
Myliobatids are mainly shoreline rays that live in shallow coastal waters of tropical or warm temperate seas (ADW 2007). Rarely, members of this marine family, such as eagle and cownose, may venture into estuaries and mangrove areas. Myliobatidae can be found near reefs, in coastal lagoons, and, especially in the case of manta rays, far out to sea. Many members make summertime migrations into temperate waters.
The Spotted Eagle Ray (Aetobatus narinari), also given the Hawaiian name Spotted Duck-billed Ray, are easy recognizable by the white or bluish white spots scattered on the greenish to pinkish slate colored backs (Tricas et al. 2002, Tinker and De Luca, 1973). The slender tails can be as long as four times the length of its body, with a poisonous spine at the base of the upper surface and a small dorsal fin just in front of the spine. Spotted Eagle Rays are the most common and most widely distributed of the eagle rays, occupying shallow tropical waters of the Atlantic, Pacific and Indian oceans. These and other myliobatids will swim near the surface and occasionally leap into the air sometimes somersaulting (Nelson 1994).
Physiology and Environmental Tolerances
Myliobatids perceive and interact with their environment using the sensory channels sight, hearing, smell, taste and touch (Tricas et al. 2002, ADW 2007). These rays belong the group elasmobranchs, giving them exceptional electrical sensitivity. Elasmobranchs have ampullae of Lorenzini which are electroreceptor organs containing receptor cells and canals leading to pores in the animals skin. Myliobatids can detect electrical patterns created by nerve conduction, muscular contraction, and even the ionic difference between a body, such as prey and water. Experimental observation reveal the tendency
of certain rays to change their feeding location according to artificially induced changes in the electrical field around them. Cartilaginous fishes have also been observed to use electrosensory information not only to locate prey, but also for orientation and navigation based on the electrical fields created by the interaction between water currents and the earths magnetic field.
Rarely, myliobatids are found in estuary habitats. Most elasmobranches avoid low-salinity water because their body fluids have salt concentration similar to seawater so in a less concentrated solution, the osmotic uptake of water through gills and skin would be intolerable (Taylor, 2001). It is hypothesized that some ray species have adapted to low-salinity water by boosting urine output to expel excess water and reduce body fluid concentrations, therefore minimizing inward osmotic flow.
Tropic Web
Members of the Myliobatids mainly prey on molluscs, crustaceans, planktonic organisms, and small fishes as their primary food sourse (Marine Bio 2007). The Myliobatids search for concentrations of prey but mechanisms for retrieval vary. Eagle rays search for mollusks and crustaceans by digging on bottom substrate using their snouts like a spade, whereas Cownose rays feed in schools, beating their powerful pectoral fins to expose buried shellfish (ADW 2007).
Humans and other animals are threatened by the poisenous tail spines of species such as the spotted eagle ray due to their severe wounding capabilities. Tail spines are only inflicted defensively and are less dangerous then spines of other rays due to the near body location making it harder to powerfully inject the spine with their whip-like tail (Tinker and DeLuca, 1973). Humans are the biggest mollusc competition for rays and many species populations of myliobatidae are threatened by human activities.
Even large stingrays have large fish predators, especially sharks such as the silvertip shark (Carcharhinus albimarginatus) and the great hammerhead (Sphyrna mokarran) (Bester 2007). Hammerheads rely on stingrays as a food source and are specialized for hunting them. The shark uses its head to knock the rays to the bottom and pin them so it can pivot around to bite the rays