HARD CLAM, MERCENARIA MERCENARIA: 

 SHELL GROWTH PATTERNS IN CHESAPEAKE BA 1 



1.2 



Lowell W. Fritz' and Dexter S. Haven 4 



ABSTRACT 



Dark bands in the middle homogenous layer of Mercenaria mercenaria shells, formed each summer and early 

 fall in lower Chesapeake Bay experimental and wild populations, were used to determine age. Distinct 

 growth cessation marks caused by low winter water temperatures were present in some annual increments, 

 but were not formed each year by each individual This was due primarily to differences among age groups in 

 seasonal band formation, Mercenaria mercenaria younger than 8 years tended to form light bands in fall and 

 spring, which were bisected by distinct winter growth cessation marks. Older individuals tended to form light 

 bands only in spring; thus, winter growth cessation marks were masked by dark bands deposited from sum- 

 mer through winter. These results differ fromM. mercenaria shell growth patterns found elsewhere along its 

 range, suggesting that time of annulus formation varies with latitude. 



One microgrowth increment in the prismatic layer was formed during each solar day of activity (growth). 

 From a 106-day monitored growth experiment in summer 1980, the slope of the regression describing the 

 relationship between the number of increments formed ( il and days {X) was not significantly different from 

 1.00 (t s = 1.23, P> 0.20, r= 0.98). Inactive periods, represented by growth cessation marks, became longer 

 and/or more frequent with increasing age and length of monitored growth periods. Both factors, increasing 

 age and length of monitored growth periods, contributed to decreased increment-to-day ratios. 



There has been considerable research on the 

 periodicity of line, band, zone, and increment forma- 

 tion in bivalve shell microstructure since Barker's 

 (1964) initial description (see Lutz and Rhoads 

 1980). Annual shell increments have been identified 

 in shells of many species, including Arctica islandica 

 (Thompson et al. 1980), Mya arenaria (MacDonald 

 and Thomas 1980), Spisula solidissima (Jones et al. 

 1978), and Geukensia demissa (Lutz 1977; Lutz and 

 Rhoads 1978 5 ; Lutz and Castagna 1980). In shells of 

 the hard clam, Mercenaria mercenaria (Linneaus, 

 1758), annual increments have been described in two 

 distinct ways: 1) Regions of narrow and wide mi- 

 crogrowth increments in the outer prismatic layer 

 resulting from seasonal changes in growth rate (Pan- 

 nella and MacClintock 1968; Rhoads and Pannella 

 1970) and 2) a single pair of translucent and opaque 



'Contribution No. 1015. Virginia Institute of Marine Science, 

 Gloucester Point, VA 23062. 



: Derived from a thesis by L. W. Fritz, submitted to The College of 

 William and Mary in partial fulfillment of the requirements for the 

 Master of Arts degree. 



'Virginia Institute of Marine Science, School of Marine Science, 

 The College of William and Mary, Gloucester Point, Va.; Present 

 address: Rutgers University Oyster Research Laboratory, P.O. Box 

 587, Port Norris, NJ 08349. 



■•Virginia Institute of Marine Science, School of Marine Science, 

 The College of William and Mary, Gloucester Point, VA 23062. 



5 Lutz, R. A., and D. C. Rhoads. 1 978. Shell structure of the Atlantic 

 ribbed mussel, Geukensia demissa (Dillwyn): A re-evaluation. 

 Bull. Am. Malacol. Union 44th Annu. Meet., p. 13-17. 



Manuscript accepted February 1983. 

 FISHERY BULLETIN: VOL." 81, NO. 4. 1983. 



zones in the middle homogenous layer (as viewed in 

 thin radial section; Clark 1979). However, these 

 definitions are not mutually exclusive, since translu- 

 cent zones are associated with narrower microgrowth 

 increments (or slower growth rates) than opaque 

 zones (Clark 1979). 



The season of slow shell growth by M. mercenaria 

 varies along its latitudinal range (Gulf of St. Law- 

 rence to Gulf of Mexico; Franz and Merrill 1980). In 

 the north- central part of its range (Connecticut, 

 Massachusetts, and New Jersey), reduced growth 

 rates and growth cessations occur during winter, mi- 

 crogrowth increments being between 2 and 1 00 times 

 narrower than those formed in summer (Pannella and 

 MacClintock 1968; Rhoads and Pannella 1970; Ken- 

 nish and Olsson 1975). Conversely, M. mercenaria in 

 the southern part of its range (Georgia) grow slowly in 

 summer and early fall when translucent zone forma- 

 tion occurs (Clark 1979). Mercenaria mercenaria in 

 Georgia may also grow throughout winter, since no 

 winter growth cessation marks have been observed in 

 shell microstructure (Clark 1979). Thus, latitudinal 

 variation may preclude the universal application of 

 defined annual shell increments to all populations 

 along the range of the hard clam. Shell growth pat- 

 terns of local populations must be analyzed to deter- 

 mine its unique features. 



There have been no previous stiudies of microstruc- 

 tural shell growth patterns of M. mercenaria in lower 

 Chesapeake Bay. Hard clams used in this study to 



697 



