Atlantic rangia, continued 



Table 5.03. Relative abundance of Atlantic rangia in 

 31 Gulf of Mexico estuaries (from Volume I). 



Relative abundance: 



# Highly abundant 



® Abundant 



O Common 



V Rare 



blank Not present 



na No data available 



Life stage: 



A - Adults 

 S - Spawning 

 J - Juveniles 

 L - Larvae 

 E - Eggs 



the Atlantic coast, the Atlantic rangia is found from 

 Chesapeake and Delaware Bays southward to Indian 

 River, Florida. In the Gulf of Mexico, the Atlantic rangia 

 is found from southwestern Florida to Texas, and to 

 Alvarado, Veracruz, Mexico (Hopkins and Andrews 

 1970, Andrews 1981, Godcharles and Jaap 1973, 

 Fischer 1978, Fritz et al. 1990). 



Within Study Area : Along the U.S. Gulf coast, this 

 species is found from the Corpus Christi Bay area to 

 southwestern Florida, and is concentrated in brackish 

 waters of Louisiana, particularly around Lake 

 Pontchartrain, Maurepas, and Vermilion Bay (Table 

 5.03) (Tarver 1972, Tarverand Dugas 1973, Andrews 

 1 981 , LaSalle and de la Cruz 1 985). It is not common 

 in the south Florida and south Texas estuaries, which 

 have relatively high salinities (Nelson et al. 1992). 



Life Mode 



Eggs and larvae are known to have a brief planktonic 

 and pelagic existence (Fairbanks 1 963, LaSalle and de 

 la Cruz 1985). Juveniles and adults are semi-sessile 

 estuarine benthic infauna capable of burrowing through 

 sediments, and they typically have only a small portion 

 of the shell protruding from the substrate. Juveniles 

 and adults are generally restricted to shallower water 

 along bay margins, presumably due to the concentra- 

 tion of free-swimming larvae by wave action where the 

 metamorphosis to a benthic existence occurs. 



Habitat 



Ty pe: All stages are found in river-influenced brackish 

 water (riverine-oligohaline) and in subtidal oligohaline 

 to polyhaline estuarine waters. This clam prefers a 

 combination of low salinity, high turbidity, and a sub- 

 strate of sand, mud and vegetation (LaSalle and de la 

 Cruz 1985). 



Substrate : Juvenile and adult stages occur in soft 

 sediments of sand and mud (Tarver 1 973, Godcharles 

 and Jaap 1973, LaSalle and de la Cruz 1985). Larger 

 sized Atlantic rangia tend to inhabit sandy bottom 

 areas, suggesting that larger sized particles trap more 

 food; sandy substrates facilitate burrowing, and excre- 

 tory products do not accumulate (Tarver and Dugas 

 1972). Sandy sediments of high organic content and 

 phosphate are more favorable for growth and survivor- 

 ship than silt/clay sediments that are also high in 

 organic matter and phosphate (Tenore et al. 1968). 

 There is also evidence that larvae settle preferentially 

 in sandy versus silty substrate, and that they prefer 

 substrate with some organic content (Sundberg and 

 Kennedy 1993). In the Trinity River delta, Texas, 

 Rangia isfound in soft mud-clay-silt substrates (Baldauf 

 1970). The sediments that Rangia resides in can 

 result in shell erosion and ultimate mortality because of 

 the presence of acids formed in the breakdown of 



33 



