THE SEASONAL CYCLE OF GONADAL DEVELOPMENT IN 

 ARCTICA ISLAND1CA FROM THE SOUTHERN NEW ENGLAND SHELF 1 



Roger Mann 2 



ABSTRACT 



The seasonal cycle of gonadal development of the ocean quahog, A rctica itlandica, on the Southern 

 New England Shelf was investigated by collectingadultclams at regular intervalsfrom September 

 1978 to May 1980 from a 36-50 m depth transect, preparing histological sections of the gonadal tis- 

 sue, and examining these microscopically for stages of development. Hydrographic measurements 

 made concurrently with the clam collections included temperature, conductivity, dissolved oxygen, 

 and pH. Morphologically ripe specimens were present from March through October, but predomi- 

 nated from May through September. A prolonged spawning period from May through November is 

 indicated, spawning being most intense from August through November. Multiple annual spawn- 

 ings at both the individual and population level were evident. After an assessment of the hydro- 

 graphic conditions in the area it was hypothesized that larval survival is probably greatest during 

 the months of October and November, which is the time of the breakdown of the intense seasonal 

 thermocline and before the onset of low winter seawater temperatures. 



Ocean quahog, Arctica islandica (= Cyprina 

 islandica), is a large pelecypod that occurs in 

 European waters from the White Sea to Spain 

 (Jensen 1902; Loven 1929; Zatsepin and Filatova 

 1961; Punin 1978) and in American coastal wa- 

 ters from Newfoundland to Cape Hatteras (Nicol 

 1951; Merrill and Ropes 1969; Ropes 1978). The 

 species supports an active fishery in the Middle 

 Atlantic region and has been the subject of much 

 recent study (Murawski and Serchuk 1979; 

 Thompson, Jones, and Dreibelbis 1980; Thomp- 

 son, Jones, and Ropes 1980; Ropes and Murawski 

 1980). In the Middle Atlantic region the greatest 

 concentrations of A. islandica are found in 

 depths of 25-61 m with the mean depth of occur- 

 rence increasing from 39 m off Long Island to 52 

 m off Virginia and North Carolina (Merrill and 

 Ropes 1969; Ropes 1978). 



The seasonal temperature structure of the 

 waters of the Middle Atlantic region was first 

 comprehensively described by Bigelow (1933) 

 and has subsequently been the subject of many 

 investigations and reviews (Walford and Wick- 

 lund 1968; Colton and Stoddard 1973; Bumpus 

 1973; Beardsley et al. 1976; Williams and Gods- 

 hall 1977). Two important features are evident: 

 An intense summer thermocline that builds in 

 May and persists until September, and a "pool" 



■Contribution No. 4715 from Woods Hole Oceanographic In- 

 stitution. 



2 Woods Hole Oceanographic Institution, Woods Hole, MA 

 02543. 



of cold water (annual temperature range 2°- 

 13°C), surrounded on both the inshore and off- 

 shore sides by warmer water, that develops on 

 the continental shelf below the thermocline dur- 

 ing the spring, summer, and early fall months 

 (Ketchum and Corwin 1964; Bowman 1977). The 

 cold pool of bottom water in the summer months 

 overlies much of the depth range occupied by A. 

 islandica. Maximum water temperatures on the 

 sea floor in the depth range occupied by A. islan- 

 dica occur in September and October (Bigelow 

 1933), and a strong relationship exists between 

 the 16°C bottom isotherm for October and the in- 

 shore distribution limit of A. islandica (Bigelow 

 1933, figs. 49, 60; Merrill and Ropes 1969, fig. 2). 

 Loosanoff (1953) described the reproductive 

 cycle of A. islandica based upon specimens col- 

 lected regularly from commercial catches at 

 Point Judith, R.I., from March to November (a 

 complete annual cycle was examined but not re- 

 ported). Following histological preparation and 

 microscopic examination of the specimens, 

 Loosanoff concluded that histological "Spawn- 

 ing begins near the end of June or early in July 

 when the water temperature is approximately 

 13.5°C." The conclusion was based on tempera- 

 ture data inferred from earlier observations by 

 Merriman and Warfel (1948). Loosanoff (1953) 

 also concluded that spawning continued through 

 August, and that approximately 50% of A. islan- 

 dica examined were totally spent by early Octo- 

 ber. The larvae of A. islandica have been reared 



Manuscript accepted September 1981. 

 FISHERY BULLETIN: VOL. 80, NO. 2. 1982. 



315 



