►- 



z 



ISO 



US 



120 



I OS 



90 



75 



60 



45 



30 



15 



-Light transmission at different times and depths at Egmont Key 

 and Old Tampa Bay, Fla. 



TIME 

 Figure 2. — Catch of pink shrimp and mean percentage of 

 light transmitted to the bottom, Egmont Key and Old 

 Tampa Bay stations. 



size range 55 to 105 mm. (total length) showed 

 positive phototaxis at 0.3 foot-candle (3.229 

 lumens per m. 2 ). Shrimp showing a pho- 

 totactic drive (movement through an alter- 

 nating current toward a light source) ranged 

 from 27 percent to 68 percent in different 



About 45 cm. below surface. 



lots; those measuring 90 mm. showed the 

 least phototactic drive. A difference in reac- 

 tion to light in relation to size was also shown 

 by Fuss and Ogren (1966), -who observed that 

 large pink shrimp were more sensitive to 

 light than small ones, and by Eldred et al. 

 (1961), who found that larger pink shrimp 

 were caught at night than by day. In the 

 present study the size range was wide through- 

 out the 24-hour periods and varied most in 

 darkness. Slightly larger specimens were 

 caught in darkness than in daylight at Egmont 

 Key (table 4), but not in Old Tampa Bay 

 (table 5). 



Females were larger than males in all 

 samples of pink shrimp except two from 

 Egmont Key (tables 4 and 5). Similar sex 

 differences in the size of pink shrimp were 

 also noted by Eldred et al. (1961), Broad 

 (1950), Williams ( 1955a), and Saloman ( 1965). 



Larger pink shrimp were taken at lower 

 salinities (Old Tampa Bay station, table 5) 

 than at higher salinities (Egmont Key, table 

 4), but the significance of this difference is 

 questionable. The size differences could be 

 attributed also to the amount of light reach- 

 ing the bottom. Light penetration was con- 

 siderably greater at Egmont Key than in Old 

 Tampa Bay. If light controls activity of larger 

 animals more than salinity as reportedby Fuss 

 and Ogren (1966), an explanation is provided 

 for the capture of larger shrimp in the lower 

 salinity of Old Tampa Bay. Lindner and 

 Anderson (1956) took larger shrimp in lower 



Table 4. — Number, size (carapace length, millimeters), and sex of pink shrimp caught at Fgmont Key, Fla., at different hours 



