FISHERY BULLETIN: VOL. 73, NO. 2 



DISCUSSION 



The histological examination of gonads in- 

 dicates several generalizations about the 

 reproductive cycle of R. cuneata. Gametogenesis 

 began in early April and continued throughout the 

 summer months. Ripe gametes v^^ere observed 

 from May to late November. A slight spawning 

 peak w^as noted during the summer, but a major 

 spawn occurred in the fall. This was probably not a 

 second cycle because gametogenesis in most cases 

 had not terminated during the summer; instead, it 

 appeared to be a continuation of gamete develop- 

 ment at an increased rate. 



The gametogenic cycle of Rangia in high and 

 low salinities was basically the same. Temperature 

 appeared to be the more important stimulus in 

 initiating gametogenesis in the spring and 

 summer. A temperature of approximately 15°C 

 coincided with initiation of gametogenesis at all 

 stations. Gametogenesis in clams from freshwater 

 occurred at a slower rate during the spring and 

 summer with more clams in the spent phase than 

 at the other localities. 



The reproductive cycle as determined in this 

 study is similar to that reported for Rangia in 

 other geographic areas. Fairbanks (1963) indicat- 

 ed that in Lake Pontchartrain, La., ripe clams 

 could be found during March, April, May, and in 

 the late summer and fall. A prolonged spawning 

 season would seem reasonable as the rise in water 

 temperature to 15°C in the spring is nearly 2 mo 

 earlier at that location than in the James River. In 

 addition, the drop in water temperature to 15° C in 

 the fall is later. He also indicated a postspawning 

 recovery phase during midsummer. The very high 

 temperature (near 33°C) during the summer may 

 have inhibited gametogenesis, but in the James 

 River population a renewed surge occurred at the 

 high midsummer temperatures of 28°-29°C. 

 Tenore (1970) studying the macrobenthos of the 

 Pamlico River, N.C., found Rangia containing 

 mature gametes only in the fall. This observation 

 was probably based on visual inspection of the 

 viscera. The spring and summer ripening may 

 have been missed because the gonads are not 

 nearly as distended and colored as in the fall 

 ripening. Pfitzenmcyer and Drobeck (1964) 

 collected Rangia in August and September from 

 the Potomac River. Clams at this time contained 

 mature gametes, indicating that spawning was 

 imminent. 



The correlation of the environmental data to 

 gonadal conditions suggests that temperature and 

 salinity are important factors in spawning. 

 Salinity, however, was more important than 

 temperature. Clams upstream at station C 

 spawned in fall 1970 following a 5"/oo rise in 

 salinity, but failed to spawn in 1971 when the 

 salinity remained low. Spawning at station B was 

 apparently related to salinity decreases. The 

 correlation of salinity to spawning was not as clear 

 at station A. The salinity variation at this station 

 was very large over a tidal cycle and may have 

 prevented complete synchrony of spawning. 



Cain (1973) found that a salinity change was 

 necessary for Rangia spawning in the laboratory. 

 Spawning was accomplished by placing ripe clams 

 from low salinities ( < iVoo) into 5Voo, 28°C flow- 

 ing water. Evidently clams in upstream areas 

 require a rise in salinity to spawn, while down- 

 stream populations require a reduction in salinity 

 from the 10 to 15"/oo levels at which they live. 



There is little additional evidence in the litera- 

 ture on the importance of salinity to Rangia 

 spawning. Fairbanks (1963)could not induce spawn- 

 ing. Chanley (1965) induced spawning at 15 ''/oo by 

 rapidly increasing the water temperature 

 7°C and adding sperm stripped from a ripe male. 

 However, spawning was poor and he did not study 

 the survival of the eggs. Such strong stimuli may 

 cause premature release of immature eggs, with 

 subsequent poor fertilization and survival. The 

 only data suggesting the importance of salinity to 

 Rangia spawning is that from the Bureau of Sport 

 Fisheries and Wildlife (1965) in connection with a 

 study on the food habits of ducks in Back Bay, Va., 

 and Currituck Sound, N.C. These two bodies of 

 water normally have a salinity of less than l^/oo. 

 During that study, a storm forced ocean water into 

 the bay and raised the salinity to about 4.5''/(».This 

 intrusion of salt water must have caused spawning 

 and successful setting since the following year 

 nearly 9% of the diet of dabbling and diving ducks 

 consisted of small Rangia, an amount estimated to 

 be 83,000 lb (dry weight) for the year. During the 

 previous 3 yr no Rangia were consumed by these 

 ducks. 



Two periods of setting occurred in the James 

 River. The first was in early and midsummer, 

 which coincided with the beginning of spawning 

 as inferred from tissue sections. The second period 

 was much longer, with a greater number of 

 collected set, and took place in the late fall and 

 winter, coinciding with the increased percentage 



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