FISHERY BULLETIN: VOL. 73, NO. 2 



also of small coal particles. They indicated that the 

 larvae move upstream by selectively swimming in 

 more saline water associated with the flood tide. 

 They further indicated that Korringa's (1952) idea 

 of passive transport could not be denied, as the 

 coal particles also had a net upstream motion. 

 Rangia could be carried into upstream areas both 

 by selective swimming or by passive transport 

 under low flow conditions, and a series of dry years 

 would allow set to progressively move upriver. Set 

 from one year should be able to spawn the next 

 year and certainly within 2 yr. Gonads occurred in 

 clams 14 mm long, a length easily reached by the 

 end of the first year (Fairbanks 1963; Wolfe and 

 Petteway 1968). Rapid early growth and a rela- 

 tively short larval life (above 20°C) should allow 

 for the fast spread of set into areas uninhabited by 

 adults. As Rangia has an 8-yr average life span 

 (Fairbanks 1963) and a maximum life of 14 yr 

 (Wolfe and Petteway 1968), recruitment to the 

 population could occur at fairly long intervals. The 

 Virginia Division of Water Resources 

 (Anonymous 1970a) statistically predicts that low 

 flows of less than 1,000 cfs for 7 days will occur at 

 5-yr intervals. This situation could allow minimum 

 recruitment to maintain the upstream population, 

 assuming good survival of set and adults in this 

 region. 



The downstream extent of Rangia could be de- 

 termined by the adults approaching their high- 

 salinity limit. This may not be the case as the 

 larvae can survive higher salinities than normally 

 occur at the downstream limit— and the adults may 

 also do so. The downstream extent likely 

 represents a multifactor barrier involving 

 biological competition for space and food, and 

 increased predation of the set. 



Recruitment to the downstream population was 

 at a low level but more regular, with more set 

 collected in the summer months than at the up- 

 stream stations (Table 1). Probably the best 

 recruitment would be expected in the population 

 at station B near the lower middle of the habitat 

 range. The fall set there was high and fairly con- 

 sistent over the 2-yr period. Averaged over many 

 years, this segment would likely receive more set, 

 as this part of the estuary usually has an annual 

 salinity change from fresh to S^/oo. 



General Discussion 



The adults can utilize the high levels of detritus 

 in this oligohaline sector (Darnell 1958) and con- 



vert it into growth and reproductive materials. 

 Rangia is ripe for at least 7 mo of the year so it can 

 spawn whenever favorable changes in salinity 

 allow successful reproduction. Although adults are 

 euryhaline, embryos are much more sensitive. 

 Spawning at a salinity near 5°/oo allows for the 

 survival of the sensitive stages to the more eury- 

 topic later larval stages. The increased tolerance 

 of the larvae permits good survival during its more 

 stressful pelagic existence. 



The planktonic existence of Rangia larvae is 

 greatly extended by low temperatures. Thorson 

 (1946) indicated that prolonged low temperatures 

 exposed larvae to increased mortality from 

 disease, starvation, predation, dispersion, and en- 

 vironmental stress. Rangia larvae evidently are 

 well adapted to a prolonged exposure because 

 many set were collected during the coldest 

 months. This increase in dispersal may allow 

 Rangia to consume the unexploited resources of 

 the species-poor environment in low salinity. 

 Increased dispersal may also provide genetic in- 

 terchange between populations distributed over a 

 relatively extended habitat in the estuary. Pfit- 

 zenmeyer and Drobeck (1964) found the rate of 

 increase of Rangia over a 4-yr period in the Po- 

 tomac River to be very great. Pfitzenmeyer (1970) 

 also recorded this clam in the upper Chesapeake 

 Bay when its numbers increased from to 10,000 

 per square meter in 2 yr. 



Spawning of Rangia apparently is controlled by 

 salinity change. The mechanism of this control, 

 however, has not been examined. The exogenous 

 factor of salinity may activate an endogenous 

 control system of osmoregulation and serve as a 

 signal to induce synchronous spawning of the 

 population. The concept of "critical salinity" 

 reviewed by Khlebovich (1969) appears operative 

 for Rangia which spawns near 5'Voo. Khlebovich 

 concluded that the salinity range of 5-8''/oo is a 

 faunal boundary defining the distribution of 

 marine and freshwater species. Characteristic 

 differences in physiological performances (adap- 

 tation, growth, activity and, especially, os- 

 moregulation) are revealed at this salinity range. 



The latitudinal distribution of Rangia is impor- 

 tant because of its spread northward in the last 

 decade. In the present study, gametogenesis and 

 spawning were observed to occur over a wide 

 range of temperatures. Although larval growth 

 was best at high temperatures, survival and 

 growth apparently take place even at low 

 temperatures. Consequently, it appears that the 



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