BROUSSEAU: MYA REPRODUCTION AND RECRUITMENT 



collected during the spawning season in order to 

 insure sufficient numbers of "ripe" females for fe- 

 cundity studies. A total of 2,480 clams were 

 examined of which 11% were immature, leaving 

 2,206 mature clams that were used in the analysis 

 of the reproductive cycle. 



The samples were returned to the laboratory 

 where they were kept at 0°C for not more than 3 

 days before being dissected. Each clam was num- 

 bered and its maximum length ( ±0.1 mm) deter- 

 mined. The visceral mass (gonad, liver, and 

 gastrointestinal tract) was taken out and fixed in 

 10% buffered Formalin^ (Humason 1967). The 

 displacement volume of each visceral mass was 

 taken to determine its size. The amount of gonadal 

 tissue present was determined after sectioning by 

 the planimetry method described below. The fixed 

 mass was then dehydrated in alcohol, embedded in 

 paraffin, sectioned at 8 fxm, and stained in Harris' 

 hematoxylin and eosin. Each clam was classified 

 with respect to gonad development and the 

 number in each developmental stage was recorded 

 for both sexes. 



Previous studies on the gonadal cycles of Mya 

 arenaria have divided the developmental se- 

 quence into five morphologically distinct phases: 

 inactive, active ripe, spawning, and spent (Ropes 

 and Stickney 1965; Porter 1974). Since semantic 

 problems arise with this usage, several terms are 

 redefined for use here! The term "indifferent" is 

 preferred to "inactive" to describe low levels of 

 oogenic and spermiogenic activity. As pointed out 

 by Keck et al. ( 1975) in work on hard clam gonadal 

 cycles, the term "inactive" is biologically in- 

 accurate since it implies a "static condition where 

 absolutely no morphological or biochemical activ- 

 ity is proceeding." The term "developing" is used 

 when describing the onset of gametogenesis since 

 it can be argued that ripe and partially spawned 

 gonads are active in the sense that gametogenic 

 activity continues at a reduced level. Developing, 

 ripe, and partially spawned stages are collectively 

 termed "active," whereas spent and indifferent 

 stages are termed "inactive." This distinction aids 

 in defining peaks of spawning within the annual 

 cycle. 



Recognition of the five phases of gonadal condi- 

 tion was based on the same characteristics as 

 those used by other investigators (Ropes and 

 Stickney 1965; Porter 1974). 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



The number of oocytes present in each female 

 gonad was determined in the following manner. 

 Using an ocular grid, triplicate counts were made 

 of the number of oocytes present per 0.49 mm^ of 

 gonad for each female reported in a ripe condition. 

 This area was then multiplied by the mean oocyte 

 diameter (0.65 mm) in order to determine oocyte 

 densities on a cubic basis. An estimate of the total 

 number of oocytes in the gonad could then be cal- 

 culated on the basis of gonad size. Analysis of 

 variance confirmed that the number of oocytes per 

 unit volume was constant throughout the ripened 

 gonad (P^0.05). 



Mean oocyte diameter was determined for a rep- 

 resentative sample of ripe females, selected at 

 random from each of the reported spawning 

 periods. Twenty oocytes per clam were measured 

 using an ocular micrometer. Only those oocytes 

 which were spherical in shape and ready for re- 

 lease were selected for measurement. 



The relationship between the size of the ripe 

 female gonads and the volume of the total visceral 

 mass was determined as follows. Entire viscera 

 from 17 ripe females (53-76 mm shell length) were 

 sectioned at 12 /u,m. Next, 18 sections from each 

 individual were chosen at random, mounted on a 

 Plexiglas base and fitted into a 35-mm slide projec- 

 tor and the projected tissue outlines were traced. A 

 planimeter was used to estimate the percentage of 

 gonad tissue present. A correction factor repre- 

 senting the proportion of gonad in the total vis- 

 ceral mass was used in estimating the total 

 number of oocytes per individual (0.763 ±0.21, 

 95% C.I.). 



Photographs of representative stages of the 

 female reproductive cycle were taken with a light 

 microscope at 160 x and 100 x magnification using 

 a 35-mm camera. High contrast, Panatomic X 

 ASA32 film was used. 



Densities of juvenile M. arenaria were tab- 

 ulated from the monthly samplings of the tidal flat 

 during October 1973, from May to November 

 1974, and in November 1975. At each sampling 

 period, 12 random samples (0.11 m^, 20 cm deep) 

 were taken along a 90-m transect from mean low 

 water shoreward to the marsh scarp. Samples 

 were wet seived in the field (2-mm mesh) and the 

 size-frequency distribution of the clams was de- 

 termined. Cohorts in the population were isolated 

 by the probability paper method (Harding 1949; 

 Cassie 1954). 



157 



