Species Description: The Atlantic Stingray, Dasyatis sabina, is a small ray distributed throughout shallow water habitats in the southeastern United States. Body color on the dorsal surface ranges from a medium brown to dark brown, and is generally uniform. The ventral surface is white. Rays are dorsoventrally flattened, with pectoral fins expanded toward, and fusing with the head. Gill openings are ventral in all rays. The caudal and dorsal fins are reduced or absent in some species.
There are 9 genera in the Dasyatidae family, most of which posses a hardened spine or barb near the base of the tail. Of the spine-bearing species, including Dasyatis sabina, many have venom-secreting cells which may be either glandular or scattered throughout the integumentary sheath surrounding the spine (Bond 1996; Amesbury and Snelson 1997).
Potentially Misidentified Species: Can be confused with the sympatric species Dasyatis sayi and Dasyatis americana.
Regional Occurrence: The Atlantic Stingray, Dasyatis sabina, is distributed along the Atlantic coast from Chesapeake Bay to South Florida and the Gulf of Mexico. It is most abundant throughout coastal waters in Florida, but also has stable populations in Georgia, Louisiana and Texas. Although D. sabina has been reported to occur in some areas of South America, it is believed that these reports resulted from misidentification of specimens (Lee et al. 1980; Robins and Ray 1986).
IRL Distribution: D. sabina occurs throughout the Indian River Lagoon, and is most common in shallow water habitats. It is one of 3 species of ray occurring in the IRL. The others are D. americana and D. sayi (Snelson 1988).
Age, Size, Lifespan: Dasyatis sabina has a life span of approximately 9 years, and achieves a size of 60 cm DW (disc width, or the width of the body including the pectoral fins, but excluding the tail), and 109 cm TL (total length). Snelson et al. (1988) reported a maximum male size for Florida Dasyatis of 32.6 cm DW. The largest females were 37 cm DW. Maximum body weight for males was approximately 1.6 kg, while for females, it was 2.2 kg.
Abundance: D. sabina is the most abundant and widely distributed ray in shallow water estuaries (Snelson et al. 1988: Maruska and Tricas 1996).
Reproduction: Sex ratios in Florida populations of D. sabina are approximately 1:1. Males mature at approximately 2 years of age, after they have achieved a size of 20 cm DW. Females mature at a slightly larger body size of between 22 - 24 cm DW (Snelson et al. 1988; Johnson and Snelson 1996). Dasyatis sabina has the longest preovulatory mating period reported for any elasmobranch (Kajiura et al. 2000). Mating takes place between October and April in Florida, with a peak in March (Snelson et al. 1988; Maruska et al. 1996; Kajiura et al. 2000). Testes are active from September through March, with peak sperm production from August through January. Egg development in females occurs over 5 -6 months. Ovulation occurs almost synchronously among females in a population in late March and early April, with young being born in late July or early August following a 3.5 - 4 month gestation period. Brood size is between 1 - 4, with an average of approximately 2.6. Neonates are 10 - 13 cm DW.
Before the onset of the breeding season, changes in male tooth morphology occur. In the non-mating season, male tooth structure is similar to that of females, being principally rounded and molariform. However, as breeding season approaches, male teeth become sharp and narrow for improved grasping of females (Kajiura et al. 2000; Kajiura and Tricas 1996) and subsequent internal fertilization. Mating behavior in D. sabina follows the general pattern for sharks and rays. During early courtship, males follow closely behind females, biting at their bodies and pectoral fins (Kajiura et al. 2000; Tricas 1980). When breeding is initiated, males grasp the pectoral fins of females with their teeth to assist in providing leverage for clasper insertion and for body alignment. It is thought that this fin gripping behavior is a precopulatory releasing mechanism that facilitates female cooperation in breeding (Kajiura and Tricas 1996).
Embryology: The developmental mode in D. sabina is aplacental viviparity, meaning that once the supply of yolk in an egg has been depleted, nourishment to the embryo is provided from maternal secretions, though not via a placenta (Bond 1996). Yolk sacs in this species weigh up to 10 times that of an early embryo and are fully absorbed by day 60, approximately 75% of the total gestation period of 3.5 - 4 months (Johnson and Snelson 1996). Following absorption of the yolk sac, embryos are fully dependent upon secretions of uterine milk, or histotroph, from glandular cells in the female.
Embryos first become recognizable at approximately 4-6 weeks post ovulation and measure 8.6 mm TL (total length), or approximately 1.2 mm DW (disc width). Early embryos do not have the characteristic rounded or oval appearance of most stingrays, but rather, are elongate and resemble sharks. At this stage, the eyes and olfactory bulb in the head region are undeveloped, and the external gill filaments first begin to emerge. At 1.8 mm DW, pectoral fins begin to develop and expand laterally toward the posterior region of the body. The head is ventrally flexed at this stage, and the external gills become more fully enlarged. By 3.6 mm DW, the pelvic fins begin to develop, and the pectoral fins begin to expand toward the head. At 5.0 mm DW, the eyes develop, and become pigmented when the embryo has reached approximately 7.0 mm DW and becomes dorsoventrally flattened. Growth after this stage is rapid, with body weight increasing as much as 100 fold. By the time embryos attain a size of 40 mm DW, they can be sexed, because the presence of claspers becomes obvious in males. At 60 mm DW, the tail spine first begins to emerge. At 70 mm DW, embryos are morphologically similar to adults. Neonates are approximately 100mm DW (Johnson and Snelson 1996).
Temperature: D. sabina is sensitive to colder temperatures but tolerates higher temperatures up to 35°C well. The critical thermal minimum for this species is estimated to be 15 - 17 °C (Snelson et al. 1988). Snelson et al. (1988) reported that it became harder to locate D. sabina in winter months when water temperatures drop below 15 °C because many individuals were believed to migrate into the deeper water of channels and holes.
Salinity: D. sabina is a common inhabitant of shallow estuaries, but also strays into freshwater areas. It is broadly euryhaline. Salinity in the IRL can range from approximately 7 - 45 ppt. D. sabina tolerates this range well, showing no obvious signs of discomfort in response to changing salinity, even when subjected to rapid changes (Snelson et al. 1988). The only known permanent freshwater population of D. sabina occurs in the southern (upper) reaches of the St. John's River, Florida, more than 350 km from the coast. The St. John's population completes its entire life cycle in freshwater, in spite of there being no physical barrier to fish migration. Many have speculated as to how this population may have come into existence. The possibility exists that the St. John's population is perhaps a remnant of the estuarine population that inhabited the area when sea level was higher, and the St. John's River basin was a brackish coastal lagoon (Amesbury and Snelson 1997). If this is indeed the case, then the St. John's population may have begun a genetic separation from the estuarine population as early as the late Pleistocene (Cook 1939; Amesbury and Snelson 1997). Johnson and Snelson (1996) have suggested that one major reason why D. sabina is able to maintain a permanent population in the St. John's River is that the surrounding river basin has a high content of Pleistocene mineral salt deposits that increase chloride levels in otherwise freshwater systems.
In comparing freshwater and marine adapted D. sabina, Piermarini and Evans (1998) noted that the salt-secreting rectal glands in the freshwater rays from the St. John's River had a gland weight to body weight ratio that was 80% lower than those found in marine D. sabina. Since there is less of a need to excrete excess salts in a fresh water environment, freshwater elasmobranchs have adapted decreased size and function in their rectal glands. Further, many freshwater elasmobranchs have serum/plasma urea levels that are 30 - 50% lower than in marine species (Piermarini and Evans 1998).
Trophic Mode: The diet of D. sabina is composed primarily of benthic invertebrates such as anemones, Polychaete worms, amphipods, mysids, isopods, bivalves, and the calcified discs of brittlestars (Turner et al. 1982; Cook 1994; Kajiura and Tricas 1996).
D. sabina is a highly electroreceptive fish having bilateral rows of neuromasts (sensory cells) that detect water motion over the dorsal surface, and a pored ventral canal system composed of ampullae of Lorenzini (see Bond 1996) that function in detecting and localizing prey (Maruska and Tricas 1998). Living prey of any species give off characteristic electrical potentials which can be detected by the ampullae of Lorenzini. D. sabina uses this electrical input to aim strikes at potential prey buried in the substratum (Blonder and Alevizon 1988).
Olfaction is also used in combination with electroreception to detect prey (Maruska and Tricas 1998). Feeding behavior involves the ray slowly swimming approximately 0.1 - 0.2 m above the surface of sand bottoms, followed by abrupt stops to evaluate an area for the presence of prey items. To inspect an area, the ray lies motionless on top of the sand. If prey is detected, the ray begins an undulatory movement of its pelvic fins to mechanically excavate prey from the benthos. Excavation creates a feeding depression which helps retain prey. Deep-dwelling prey such as polychaete worms are further exposed by increased mouth suction. Prey consumption involves rapid biting motions of the jaws, and movement in the spiracles and gills (Maruska and Tricas 1998).
Competitors: Competes for benthic prey with other ray species and with fishes that feed on similar prey items.
Habitats: D. sabina is typically considered abundant to common in shallow estuarine waters less than 1 m in depth (Snelson et al. 1988). Preferred habitat areas have bottoms of sand or silt, and include shoreline, spoil islands, and seagrass beds. D. sabina exhibits pronounced seasonal movement patterns. From spring through fall, it is typically found in inshore shallows. During late fall and winter, it migrates into the deeper water of channels and holes. It resumes its normal activities when water temperatures return to 16 - 18 °C (Snelson et al. 1988). In spite of its ability to migrate, D. sabina shows high site fidelity and does not often migrate from the IRL (Johnson and Snelson 1996).
Activity Time: D. sabina has been shown to feed almost continuously throughout both daylight and evening hours (Bradley 1996 in Maruska and Tricas 1996).
Broad Scale Cost/Benefit: Because of their venomous caudal spines, D. sabina is often considered to be a nuisance species. However, in recent years, it has become valuable as a research model in biomedical research, neurobiology and physiological research (Ritchie and Leonard 1983; Grondel and Zimmerman 1986; Snelson et al. 1988). Additionally, it is an important component of lagoonal epibenthic fish communities.
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