FISHERY BULLETIN: VOL. 81. NO. ■» 



determine annual shell increments were from 

 experimental growth lots established by Loesch and 

 Haven (1973). Monitored growth periods of hard 

 clams in these lots, as long as 1 3 yr, are the longest of 

 any bivalve shell growth study in the literature. 



MATERIALS AND METHODS 



Sources of Mercenaria mercenaria 



Mercenaria mercenaria were obtained from three 

 sources for use in this study: l)Four long-term 

 experimental growth lots, 2) two short-term 

 experimental growth lots, and 3) natural population 

 or wild stock. Long-term studies lasting a maximum 

 of 13 yr were initiated in 1967 and 1969 at four sub- 

 tidal locations in the lower James and York Rivers 

 [Table 1; see also Loesch and Haven (1973) for a de- 

 scription of long-term growth lots]. Hard clams in 

 each group were numbered individually (using an 

 indelible ink pen) and measured (shell length — 

 greatest distance along the anterior- posterior axis — 

 to the nearest 0.1 mm) prior to placement directly in 

 the substrate by scuba-equipped divers. As many 

 hard clams as possible were retrieved, measured, and 

 replanted at the lot location each fall through 1972. 

 From fall 1972 to the dates of final collection be- 

 tween 1976 and 1980, each group remained in the 

 substrate continuously. Shell height (greatest dis- 

 tance from umbo to ventral edge) was not measured 

 from 1967 to 1972. After final collection, however, 

 shell length measurements obtained each fall were 

 used to identify growth rings on the shell exterior of 

 each hard clam. Shell height was measured at each of 

 these growth rings to yield a size-time relationship 

 along the height axis, along which valves were cut for 

 microstructural analyses. 



Short-term growth studies began on 16 October 



TABLE 1.— Experimental groups of Mercenaria mercenaria used in 

 long-term and short-term studies. 



1979 and continued for 20 mo (Table 1). Age 2 + M. 

 mercenaria were obtained from the Virginia Institute 

 of Marine Science hatchery on Virginia's Eastern 

 Shore (Castagna and Kraeuter 1977). Each 

 individual was numbered, measured (shell length 

 and height) , and transplanted to a subtidal location in 

 the York River. Collections of four hard clams each 

 were made from this T series group at approximately 

 monthly intervals. Shell growth did not resume until 

 April 1980, probably due to the combined effects of 

 salinity difference between the Eastern Shore (28-30 

 ppt) and York River (16- 18 ppt) and low winter water 

 temperatures. Because of this, the exact date of 

 growth resumption in spring 1980 was unknown. 



The TI series was composed of T series hard clams 

 in which a growth cessation mark was induced in 

 spring 1980 (Table 1). This was used as a baseline for 

 determining the periodicity of formation of prismatic 

 microgrowth increments. Growth cessation marks in 

 shell microstructure were induced by the thermal 

 shock method of Richardson et al. ( 1 97 9) . On 29 May 

 1980, 16 T series hard clams were collected, 

 measured, renumbered, and placed in a moist 

 incubator at 4°C for 24 h to disrupt shell growth. TI 

 series hard clams were replanted on 30 May 1980 in a 

 segregated area of the T series location. Three TI 

 series hard clams were collected and measured on 

 22 June, 18 July, 8 August, and 13 September 1980. 



Mercenaria mercenaria from the natural population 

 of lower Chesapeake Bay and its tributaries (N = 24) 

 were collected during winter, spring, and summer of 

 1978 and 1980. Shell height and length of each hard 

 clam were measured. 



Preparation of Acetate Peels 



Acetate peels of polished and etched radial shell 

 surfaces were prepared from single valves of each 

 experimental and wild hard clam according to the 

 methods of Stewart and Taylor (1965) and Pannella 

 and MacClintock (1968). Valves were cleaned and 

 air-dried for several days prior to being embedded in 

 liquid casting plastic 6 and cut from ventral edge to 

 umbo along the height axis with a geological saw. One 

 of the sectional surfaces was ground and polished 

 with optical quality grits and cerium oxide on glass 

 plates and a cloth-covered disc polisher. Polished 

 surfaces were etched in either 1% or 5% HC1 for 20- 

 60 s and dried completely. Clear acetate sheets 

 (0.003-in thick) were carefully melted on each etched 



6 American Handicrafts, Inc., Fort Worth, Texas (reference to trade 

 names does not imply endorsement by the National Marine Fish- 

 eries Service, NOAA). 



698 



