FISHERY BULLETIN: VOL. 80. NO. 2 



to metamorphosis by Landers (1976) and Lutz 

 et al. (in press). Landers (1976) reported that 

 fertilization and early cleavage were obtained 

 at 10°, 15°, and 20°C; however, embryos only 

 survived to the veliger stage at the two lower 

 temperatures, and to metamorphosis at 10°- 

 12°C. The cultures reared by Lutz et al. (in press) 

 were maintained to metamorphosis at tempera- 

 tures ranging from 9° to 13°C; but none of 

 these investigators defined the maximum tem- 

 perature at which metamorphosis could be 

 effected. 



After reviewing data on seasonal water tem- 

 perature structure in the Middle Atlantic Bight 

 and the reproductive biology of A. islandica re- 

 ported by Loosanoff ( 1953) and Landers (1976). 

 certain inconsistencies were evident. It has long 

 been suspected that bivalve larvae can partially 

 control their position in the water column by 

 swimming (Carriker 1961; Wood and Hargis 

 1971; Cragg and Gruffydd 1975). If A. islandica 

 spawn in July, then larvae swimming upwards 

 to the regions of highest primary productivity, 

 and hence phytoplankton food, would encounter 

 both an intense thermocline at 20-30 m depths 

 and surface temperatures in excess of 20°C. Both 

 temperature conditions would be either deleteri- 

 ous to growth or even lethal according to Landers 

 (1976). Therefore, it would appear appropriate to 

 hypothesize that spawning in October or Novem- 

 ber would be more congenial to larval survival 

 because, after the fall thermocline breakdown 

 and subsequent vertical mixing of the water col- 

 umn, vertical movement of the larvae would not 

 be limited by an intense thermocline. Further- 

 more, any temperature stratification that did 

 exist at this time would have widely spaced verti- 

 cal isotherms and thus form only weak barriers 

 to horizontal dispersion. 



A need was evident to simultaneously assess 

 the reproductive cycle of the adult A. ishindica 

 and hydrographic conditions affecting it and 

 larval survival. This study describes the game- 

 togenic cycle of adult A. islandica, based on 

 microscopic examination of histological prepa- 

 rations from individuals collected regularly 

 from several depths over a 2-yr period, and con- 

 current physical data collected during the same 

 period. 



METHODS AND MATERIALS 



Fourteen collections of A. islandica were 

 made at intervals of 4-8 wk from September 1978 



to May 1980 at depths ranging from 27 to 50 m in 

 the vicinity of Block Island, R.I. Stations at27-30 

 m depth (Station A) were north and east of Block 

 Island (lat. 41°19'N, long. 71°34'W and lat. 41° 

 13'N, long. 71°32'W, respectively). Stations at 

 36, 42, and 48-50 m (Stations B-D, respectively) 

 were on a transect directed due south at long. 71° 

 31'W at lat. 41°11'N, 41°03'N, and 41°01'N, re- 

 spectively. Specimens were collected with a com- 

 mercial hydraulic clamdredge(blade width 1.54 

 m, pump pressure 5.63-7.0 kg/cm 2 , 7.5 cm diame- 

 ter ring size; tows of 5-min duration) during the 

 period September 1978-August 1979 and with a 

 nonhydraulic clam dredge (blade width 0.62 m; 

 5.0 cm diameter ring size; tows of 20-30 min dur- 

 ation) during September 1979-June 1980. Both 

 dredges were selective for clams larger than the 

 diameter of the rings. 



The clams were opened on board the vessel and 

 either the soft tissues removed whole, or a section 

 of tissue approximately 1 cm 2 excised from the 

 surface of the midventral region. The tissues 

 were preserved in Bouins fixative for 24-48 h, 

 rinsed in water for 6 h, and stored in 70% ethanol. 

 Histological preparation of tissue sections in- 

 cluded embedding in paraffin, sectioning at 7 ju, 

 staining with Delafield's hematoxylin, and coun- 

 terstaining with eosin Y by the procedure of 

 Humason (1962). A minimum of 15 specimens 

 was examined from the midventral samples col- 

 lected on each collection date. An additional five 

 specimens of whole animals from each collection 

 date were examined in tissue excised from each 

 of the dorsal, midventral, and ventral regions in 

 order to assess the uniformity of development 

 throughout the gonadal tissue. Slide prepara- 

 tions were examined microscopically for evi- 

 dence of gametogenesis and spawning (Holland 

 1972), and each was classified into one of five 

 categories of gonadal condition, by the criteria of 

 Holland and Chew (1974) as follows: 



Early active: 



Male: Many follicles: spermatogonia and 

 spermatocytes numerous, no sperma- 

 tozoa. 



Female: Oogonia arising from stem cells along 

 the follicle; no free oocytes. Nuclei 

 stain darker than cytoplasm. 

 Late active: 



Male: Follicles contain predominantly 

 spermatids and spermatozoa. 



Female: Both free and attached oocytes pres- 

 ent. Oocytes have nuclei that stain 



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