Atlantic rangia, continued 



detritus. Fairbanks (1963) noted substantial shell 

 erosion of rangia along the north shore of Lake 

 Pontchartrain, due to the presence of carbonic acids 

 produced by carbon dioxide reacting with high concen- 

 trations of organic matter. 



Physical/Chemical Characteristics 

 Temperature: Optimum conditions for embryos stud- 

 ied in the laboratory are 18°-29°C (Cain 1973). The 

 planktonic existence of larvae is greatly extended by 

 low temperatures; larvae at survive 8° to 32°C, and 

 growth is fastest at 20° to 32°C (Cain 1 973, Cain 1 974, 

 LaSalle and de la Cruz 1985). Temperatures above 

 35°C are known to be lethal to larvae. Survival has 

 been observed at temperatures as high as 40°C for 

 small and medium sized animals acclimated to sum- 

 mer conditions (Lane 1986). The upper lethal limit 

 (LT50) for large individuals was 38°C. A temperature 

 of 36°C will begin causing mortalities after 3 days. 



Salinity: Embryos and larvae cannot tolerate pure fresh 

 water (0% o ) (Cain 1972, Cain 1973, Cain 1974). Opti- 

 mal salinities for embryos range from 6 to 10% o , with 

 eggs surviving as low as 2%o. Larvae survive in 

 salinities ranging from 2 to 20% o , and growth is fastest 

 at 10 to 20% o . Juvenile and adult Atlantic rangia can 

 tolerate a wide range of salinities, generally from to 

 25%o, and have reported to be capable of living in fresh 

 water (<0.3%o) for a period of at least 7 months (Hopkins 

 and Andrews 1970) by osmoregulating with inorganic 

 and intracellular free amino acids to control cell vol- 

 umes (Anderson 1 975, Otto and Pierce 1 981 ). Uptake 

 of osmotically active glycine from the environment 

 increases as salinity increases, and when salinities 

 drop below 5%o, the glycine is rapidly converted into 

 protein. Spawning becomes physiologically impos- 

 sible if salinities are <1%o or >15%o for long periods 

 (Otto and Pierce 1981). 



Dissolved Oxygen (DO): This species is tolerant of 

 temporary anoxic conditions (LaSalle and de la Cruz 

 1985, Lane 1986). Individuals have survived a maxi- 

 mum of 6.5 days in waters with ppm oxygen; how- 

 ever, they are intolerant of exposure to air. 



Movements and Migrations : Planktonic egg and larval 

 stages may be transported by tidal and river currents. 

 Larvae are presumed to be negatively phototropic and 

 are expected to be associated with the bottom of 

 shallow bay margins. Juveniles and adults are seden- 

 tary with only the posterior end and siphons slightly 

 exposed, and limited capability of vertical movement 

 through the sediments. Captive specimens have been 

 observed to only move toward the sediment surface 

 when covered by sand (Fairbanks 1963). Attached 

 organisms (barnacles, mussels and algae) indicate a 

 stationary position for long periods of time (Fairbanks 



1963, LaSalle and de la Cruz 1985). Although juve- 

 niles and adults do not migrate, they are easily trans- 

 ported by shifting water currents because of their small 

 mass (LaSalle and de la Cruz 1985). 



Reproduction 



Mode : Reproduction is primarily sexual with separate 

 sexes (gonochoristic), but there are rare cases of 

 hermaphroditism (Olsen 1976b). Fertilization is exter- 

 nal with the gametes released directly into the water. 



Spawning : The initiation of gametogenesis in the spring 

 and early summer is typically triggered by a rise in 

 water temperature to approximately 10°-16°C (Cain 

 1 975, Jovanovich and Marion 1 985). Fairbanks (1 963) 

 identified two distinct periods of spawning per year in 

 Louisiana; a spring spawn (March-May) and a less 

 intense period from late summer to November. In most 

 areas Rangia spawn from March to May and late 

 summer to November, but it may be continuous from 

 March to November. Wolfe and Petteway (1 968) found 

 spawning to occur from July to November with a peak 

 in September in North Carolina. Ripe gametes have 

 been reported July through November in Florida (Olsen 

 1976b) and from early summer through October with 

 fall peaks in Alabama (Jovanovich and Marion 1985). 

 Heavy spawning is associated with a rapid increase or 

 decrease in salinity of approximately 5%o (Cain 1 975). 

 Spawning has also been stimulated in the laboratory at 

 other temperatures and salinities by adjusting water 

 conditions and introducing male gametes (Chanley 

 1965, Cain 1973). Gametes are released through the 

 exhalent siphon by both sexes (Sundberg and Kennedy 

 1992). 



Fecundity : There is little available information on fe- 

 cundity of Atlantic rangia (LaSalle and de la Cruz 

 1985). 



Growth and Development 



Egg Size and Embryonic Development Egg develop- 

 ment is oviparous. In laboratory studies, fertilized eggs 

 (69 urn) have developed into ciliated blastulae 3 hours 

 after fertilization (AF), and into pelagic trochophore 

 larvae by 1 2 hours AF at 23° to 26°C (Fairbanks 1 963, 

 Sundberg and Kennedy 1992). A similar study by 

 Fairbanks (1963) described these developmental 

 stages as occurring in older larvae than Sundberg and 

 Kennedy (1 992) despite their being reared at the same 

 temperature. This may have been due to his use of 

 stripped eggs and sperm instead of naturally spawned 

 gametes (Sundberg and Kennedy 1992). 



Age and Size of Larvae : The length of the larval period 

 is dependent on temperature and food, but generally is 

 short lived (Fairbanks 1963). In a laboratory study, 

 trochophore larvae developed to the veliger stage (93 



34 



