. Bulletin. Science. c _Q 05 CD 03 Q_ < 05 § = O) Q. â « o O <D â Z. Q Month Fig. 5. Mean number of rays per beach seine tow (± SD) pooled by month over the three-year sampling period (2000-2002). of secondary spine occurrence. The greatest sampling effort and sample sizes generally occurred in 2002, which may represent the best estimate of peak sec- ondary spine occurrence during this study. It is also possible that the exact timing of peak spine replacement may vary slightly from year to year, perhaps due to environmental factors such as food availability or water temperature. The A
. Bulletin. Science. c _Q 05 CD 03 Q_ < 05 § = O) Q. â « o O <D â Z. Q Month Fig. 5. Mean number of rays per beach seine tow (± SD) pooled by month over the three-year sampling period (2000-2002). of secondary spine occurrence. The greatest sampling effort and sample sizes generally occurred in 2002, which may represent the best estimate of peak sec- ondary spine occurrence during this study. It is also possible that the exact timing of peak spine replacement may vary slightly from year to year, perhaps due to environmental factors such as food availability or water temperature. The Atlantic stingray, Dasyatis sabina (Lesueur), exhibits an annual spine re- placement cycle very similar to round stingrays at Seal Beach; with secondary caudal spines first appearing in June, and all primary spines being shed by No- vember (Teaf and Lewis 1987; Amesbury and Snelson 1997). However, not all stingrays follow an annual spine replacement cycle, with several stingray species within the family Potamotrygonidae shedding their spines every six months and replacement spines alternating between the dorsal and ventral position, relative to the primary spine (Thorson et al. 1988). In contrast, we observed all secondary spines ventral and slightly posterior to the primary spine in round stingrays. These species specific differences in spine shedding periodicities may be related to dif- fering predation pressures or somatic growth rates. One individual round stingray possessed two fully developed spines in February when all other round stingrays possessed one. A similar exception was reported by Amesbury and Snelson (1997), who observed one anomalous Atlantic stingray with two spines in February, which they attributed to retarded spine growth, and another with two spines in May, which was attributed to improper molting. Al- though we are not certain why one round stingray possessed two spines in Feb- ruary, it may be that a ray with supernumerary spines underwent spine
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