CAIN: REPRODUCTION AND RECRUITMENT OF RANGIA CUNEATA 



30 n 



Figure 17.-Bottom water tempera- 

 ture at station B in the James River, 

 Va. Values taken from VEPCO in- 

 strument tower (#6) and bottom 

 water samples. 



FEB ' MAR ' APR ' MAY 



JUN ' JUL ' AUG ' SEP ' OCT ' NOV ' DEC ' JAN ' 



showed the normal seasonal variation, with the 

 highest concentrations in the early part of the 

 year during the low-water-temperature period. 

 Low values (6-8 mg /liter) were recorded for deep 

 stations during the summer months. 



Relationship of the Reproductive 

 Cycle to Environmental Data 



At station A, gametogenesis started when the 

 water temperature was near 10°C in the spring of 

 1970 (Figure 7). Ripe clams were first observed 

 when the water temperature was 16°C and 

 spawning was first noted when the salinity was 

 between 3 and S'/oo. Gametogenesis and spawning 

 occurred through the summer period of high 

 salinities and high temperatures. The fall spawn- 

 ing started at the highest salinities of the year 

 with a definite major spawning after the salinity 

 dropped IQP/oo (Figure 7). The temperature during 

 the fall spawning was 13°-15°C. Set first appeared 

 the second week in November and occurred 

 throughout the winter months. Fall and winter 

 sets generally were accompanied by temperatures 

 below 10°C and occasionally to 1°C. Game- 

 togenesis was in progress again by the time the 

 water temperature reached 15°C. No set clams 

 were collected during the summer of 1971, 

 although the histological sections showed all 

 stages of development. Salinities and tempera- 

 tures approximated those of the previous year, 

 although salinity was more variable. Renewed 

 gametogenesis coincided with the highest 

 temperature of the summer and rising salinities. 

 Some spawning took place with the declining 

 salinity, but the major spawning occurred near 

 5Voo. 



Gametogenesis at station B started almost 2 wk 

 later than at station A in the spring of 1970, with 

 the temperature above 13°C (Figure 8). Ripe clams 

 were similarly noted nearly 3 wk later than at 

 station A. The salinity during the summer months 

 was near 5''/oo and gradually rising. There was lit- 

 tle agreement between spawning times as noted 

 in the histological sections and setting in the 

 collectors during the summer. The progression of 

 gonads in the fall from ripe to spent was clearer 

 and more defined at station B than at station A. 

 Spawning commenced when the salinity reached 

 the yearly high and peaked when the salinity fell 

 rapidly from 15 to l^/oo. Setting took place 2 wk 

 later and continued into the winter. Temperatures 

 during spawning ranged from 22° to 12°C. Fewer 

 clams were in the ripe phase throughout the 

 summer of 1971 when the water was nearly fresh. 

 More clams became ripe as the salinity increased, 

 but few set were collected all summer at this sta- 

 tion. Fall spawning commenced at 22°C and 6^/00 

 and continued until the salinity reached 

 approximately zero and the temperature dropped 

 to 17°C. Some set were collected immediately after 

 salinity decline and setting continued into 

 January 1972. 



Spawning was completed at station C by the end 

 of October 1970, after salinity had fluctuated from 

 to 5''/oo for the previous 2 mo. Setting began at 

 temperatures above 25°C and continued 

 throughout the winter at low water temperatures 

 and low salinities (Figure 9). The salinity at sta- 

 tion C remained below V/oo until the termination 

 of the study in January 1972. There was very little 

 spawning and setting at station C during the low 

 salinity period even though gametogenesis took 

 place normally. 



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