Fishes of the Western North Atlantic 337 



injecting the poisonous tissue from the spines of two species of Brazilian River Rays 

 {Potamotrygon) into various batrachians, birds and mammals. 



Size. The members of the family vary widely in size, the smallest being only one 

 to two feet across the disc at maturity, the largest'" six to seven feet and perhaps more. 



So far as we can learn, none of the largest ones have ever been weighed, but from 

 the relationship between size and weight for Dasyatis centroura up to five feet broad 

 (p. 357), we suspect that estimated weights reported for large Rays are likely to be 

 considerably too high. 



No information is at hand as to the rate of growth of any of the Sting Rays or 

 the number of years required by the larger species to reach full size. But at least some 

 of them are long-lived, for one is known to have "lived for 16 years in captivity in 

 Berlin."" 



Developmental Stages. The dasyatid Rays are ovoviviparous, the embryos lying 

 loose in the uterus without any physical connection with the mother. The yolk sac 

 persists throughout the greater part of embryonic life, and the yolk is taken directly 

 from it into the alimentary canal. But the chief source of nutriment for the embryo 

 is a creamy albuminous fluid secreted by vascular filaments which clothe the uterine 

 wall of the mother so closely that they appear like coarse fur. Seemingly this secretion, 

 squeezed out by the superficial musculature of the filaments, is taken in through the 

 embryonic spiracles, into which the maternal filaments have been found inserted in 

 some cases; it is absorbed along the digestive tract, and clots of it have been found within 

 the spiral valve of the embryonic intestine. '^ 



Habits. Sting Rays are seen (or caught) most commonly lying on the bottom on 

 the flats of bays, shoal lagoons and river mouths or on patches of sand between coral 

 heads, etc. Often they are partially buried in the mud or sand with only a portion of 

 the tail, eyes, and spiracles exposed. It is chiefly by excavating the bottom with their 

 pectoral fins that they obtain the worms, mollusks or crustaceans on which they feed; 

 such hollows in the sand are familiar spectacles when they are exposed at low tide in 

 regions of the tropics where Sting Rays abound, as in Australian waters. '^ They also 

 succeed in capturing small fish in greater or lesser numbers. On the other hand, they 

 often fall prey to large Sharks, and it is not unusual to find a Sting Ray spine imbedded 

 in a Shark's mouth. It has been observed that an Indian species {Dasyatis kuhlii)., while 

 buried in the sand, utilizes the inner posterior margin of the spiracle to form a pro- 

 of the terminal portion of the spine where the venomous tissue is situated, but only that of the basal part of the 

 spine, and of the tail itself beneath the free portion of the spine. 



10. Dasyatis centroura of the western Atlantic (p. 352), T>. aspera of the eastern Atlantic, and D. bre'vicaudata of New 

 Zealand and Australia. 



11. Whitley, Fish. Aust., i, 1940: 201. 



12. Knowledge of the embryonic nutrition of Sting Rays is due chiefly to a series of observations by Alcock (J. Asiat. 

 Soc. Beng., sg [2], 1890: 51; Ann. Mag. nat. Hist., [6] 9, 1892: 417); and by Alcock and Wood-Mason (Proc. 

 roy. Soc. Lond., 50, 1891: 202). See also Gill (Smithson. misc. Coll., 52, 1909: 173); and Southwell and Prashad 

 (Rec. Indian Mus., z6, 1919: 240), and Ranzi (Pubbl. Staz. zool. Napoli, 13, 1934: 39S) for a general survey, with 

 references, and for a photograph of the inner uterine wall of the Mediterranean Dasyatis 'violacea. 



13. For an excellent photograph showing the flats dotted with these hollows in North Queensland, see Whitley (Fish. 

 Aust., I, 1940: 198, fig. 224). 



