Hard clam, continued 



estuarine food web (Eversole 1987). 



Range 



Overall : Mercenaria campechiensisoccurs from Cape 

 May, NJ, to the Yucatan Peninsula, most abundantly 

 on Florida's Gulf coast. Populations inhabiting the 

 muddier environments of the northern Gulf of Mexico 

 are now recognized under the subspecific name M. 

 campechiensls texana (Dillon and Manzi 1989b). 

 Mercenaria mercenaria naturally ranges from Prince 

 Edward Island, Canada, to the Atlantic coast of Florida, 

 intertidally and subtidally to 1 5 m in estuaries and bays. 

 It generally inhabits shallower waters of lower salinity 

 than M. campechiensis. A hybrid zone between the 

 two species occurs in the Indian River lagoon on 

 Florida's Atlantic coast (Dillon and Manzi 1989a, Bert 

 et al. 1 993, Bert and Arnold 1 995). Although probably 

 not native to the Gulf of Mexico, M. mercenaria may 

 have been locally introduced by aquaculture interests 

 (Dillon pers. comm.). Populations of hard clams have 

 also been introduced to the British Isles, parts of 

 France, and California (Taylor and Saloman 1968, 

 Abbott 1 974, Kunneke and Palik 1 984, Eversole 1 987). 



The most reliable physical character distinguishingM. 

 mercenaria from M. campechiensis through most of 

 their range is the strength of the ridges on their shells. 

 M. mercenaria typically has thin, easily-eroded ridges, 

 best adapted for life in silty mud. M. campechiensishas 

 thick, resistant ridges, that seem adapted for coarse 

 substrates, especially carbonate sands. A subspecies 

 M. campechiensis texanahas been described from the 

 northern Gulf of Mexico, which unlike typical M. 

 campechiensis, has thin ridges (Dillon pers. comm.). 

 This makes sense, as the northern Gulf contains 

 substantial areas of silty mud substrate. However, 

 these clams are considered a subspecies of M. 

 campechiensis, in spite of their external morphological 

 similarities to M. mercenaria. 



Within Study Area : Within U.S. estuaries of the Gulf of 

 Mexico, M. campechiensis is found from south Florida 

 to Texas. Hard clams are widely distributed, but not 

 generally abundant in the nearshore waters of the Gulf 

 coast states (Table 5.04). 



Life Mode 



Hard clam eggs and early larval stages are planktonic. 

 The last larval stage (plantigrade) is semi-benthic 

 alternating between swimming and crawling in search 

 of a suitable settlement site. Juveniles and adults are 

 semi-sessile benthic infauna capable of burrowing 

 through sediments (Eversole 1987). 



Habitat 



Type : All life stages are estuarine or marine. Hard 

 clams usually occur in dense groups in coastal bays, 



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