Atlantic rangia, continued 



(im) in 8 hours. Shelled larvae develop within 24 hours 

 of fertilization (Chanley 1965, Sundbergand Kennedy 

 1 992). Larval sizes range from 75-203 urn depending 

 on the specific stage. These stages are extremely 

 fragile and may not be picked up in normal larval 

 sampling efforts. 



Juvenile Size Range : In laboratory studies, larval settle- 

 ment and metamorphosis to the juvenile stage oc- 

 curred after 6 or 7 days at a size of 175-180 |im 

 (Chanley 1965, Sundberg and Kennedy 1992, 

 Sundberg and Kennedy 1993). Field studies, how- 

 ever, indicate a size at settlement of 300-400 urn 

 (Fairbanks 1963, Cain 1975). Growth of juveniles is 

 1 5-20 mm in the first year, 5-9 in the second and 4-5 in 

 the third year (Fairbanks 1963). The growth rate of 

 Atlantic rangia can be significantly inhibited by sus- 

 pended solids above the substratum, and suspended 

 solids tend to influence growth more so than the actual 

 substrate (Fairbanks 1963). 



Age and Size of Adults : Size at sexual maturity ranges 

 from 1 4 mm (Cain 1 972) to 24 mm (Fairbanks 1 963) in 

 length, and is reached in 2-3 years (Fairbanks 1963). 

 A maximum length of 7 cm has been recorded, and 

 sizes to 5 cm are common (Fischer 1 978). A confirmed 

 life span for this species has not been determined 

 (LaSalle and de la Cruz 1985). Estimates range from 

 4-5 years to a maximum of 15 years. 



Food and Feeding 



Trophic Mode : This species is a nonselective filter 

 feeder. It controls food movement with the gill palps 

 and ciliary currents over the gills (Darnell 1 958, Olsen 

 1976a, LaSalle and de la Cruz 1985). 



catfish, blue catfish (Ictalurus furcatus), freshwater 

 drum {Aplodinotus grunniens), spot, Atlantic croaker, 

 black drum, sheepshead, pinfish, striped blenny 

 (Chasmodesbosquianus), southern flounder, and sand 

 seatrout. Invertebrate predators include white shrimp, 

 Ohio shrimp (Macrobrachium ohione), blue crab, Har- 

 ris mud crab (Rhithropanopeus harrisit), moon snails 

 (Po//n/cesspecies), and oyster drill {Thais haemastoma) 

 (Darnell 1958, Tarverand Dugas 1973, Levine 1980, 

 LaSalle and de la Cruz 1 985). A potential predator of 

 Atlantic rangia larvae are ctenophores, such as 

 Mnemiopsis, which sometimes are abundant in estua- 

 rine waters (LaSalle and de la Cruz 1985). 



Factors Influencing Populations Winter kills in the 

 northern portion of the Atlantic rangia's range indicate 

 that it has reached the limit of its temperature tolerance 

 there (LaSalle and de la Cruz 1 985). Sporocysts and 

 cercarial larvae, intermediate trematode stages of the 

 fish intestinal parasite Cercaria rangiae, have been 

 described from Rangia'm Galveston Bay, Texas (Wardle 

 1 983); sporocysts concentrate in the gonadal tissue of 

 the clam causing castration. Anthropogenic changes 

 in river discharge patterns can result in flow regimes 

 that can either enhance Rangia populations or cause 

 their declines (Harrel 1993). Channelization of rivers 

 may result in saltwater intrusions that produce favor- 

 able brackish water conditions in what was once a 

 freshwater habitat. Increased reservoir discharges 

 into a river can flush saltwater from an estuary, reduc- 

 ing Rangia abundance. Waste discharge into rivers 

 can create toxic or anoxic conditions that also ad- 

 versely affect Rangia. 



Personal communications 



Food Items : Food of the Atlantic rangia consists of 

 diatoms, algae and detritus, with detritus comprising 

 the greatest portion (Darnell 1 958, Olsen 1 976a, LaSalle 

 andde la Cruz 1985). 



Harrel, Richard C. Lamar Univ., Dept. Biology, Beau- 

 mont, TX. 



References 



Biological Interactions 



Predation : Atlantic rangia are preyed upon by fish, 

 crustaceans, molluscs, and ducks (LaSalle and de la 

 Cruz 1985). This species appears to be important to 

 the diet of the migratory ducks, such as lesser scaup 

 duck (Aythya affinis), greater scaup duck (Aythya 

 mania), ring-neck duck (Aythya collaris), American 

 black duck (Anas rubripes), mallard (Anas 

 platyrhynchos), and the ruddy duck (Oxyura 

 jamaicensis), and may be replaced in their diet under 

 more saline conditions by the dwarf surfclam (Mulinia 

 lateralis) (Tarver and Dugas 1973, LaSalle and de la 

 Cruz 1985). Fishes that are known to prey on rangia 

 include Atlantic stingray (Dasyatis sabina), spotted gar 

 (Lepisosteus oculatus), alligator gar (L. spatula), 

 longnose gar (L. osseus), gizzard shad, hardhead 



Anderson, J. W. 1975. The uptake and incorporation 

 of glycine by the gills of Rangia cuneata (Mollusca: 

 Bivalvia) in response to variations in salinity and so- 

 dium. In Vernberg, F.J. (ed.), Physiological ecology of 

 estuarine organisms, p. 239-257. Univ. South Carolina 

 Press, Columbia, SC. 



Andrews, J. 1981. Texas shells, a field guide. Univer- 

 sity of Texas Press, Austin, TX, 175 p. 



Arndt, R.H. 1976. The shell dredging industry of the 

 gulf coast region. In Bouma, A. (ed.), Shell Dredging 

 and its Influence on Gulf Coast Environments, Gulf 

 Publishing Co., Houston, TX, p. 13-48. 



35 



