FISHERY BULLETIN: VOL. 80, NO. 2 



The precise nature of the spawning stimulus to 

 A. islandica remains open to discussion. Loosa- 

 noff (1953) suggested that spawning was initi- 

 ated at a water temperature of approximately 

 13.5°C; however, the data of Figures 1 and 6 indi- 

 cate that absolute temperature per se is probably 

 not the ultimate spawning stimulus. Further- 

 more, laboratory experiments to induce spawn- 

 ing by temperature shock alone have proved both 

 inconsistent and usually unsuccessful (Loosanoff 

 1953; Landers 1976; Lutzetal. in press). Indeed, 

 A. islandica has proven to be a very difficult 

 species to spawn in laboratory systems. It also 

 fails to respond to salinity and pH changes, the 

 addition of suspension of sex products (Loosanoff 

 1953; Landers 1976), and the more recent meth- 

 od of Morse et al. (1977) involving exposure to 

 alkaline seawater (pH = 9.1) and hydrogen per- 

 oxide (range of concentrations 2.5 — 5 X 10" 3 M) 

 (Lutz et al. in press). While these methods of 

 stimulating spawning have generally been very 

 successful with many intertidal and shallow 

 water species (Loosanoff and Davis 1963; Morse 

 et al. 1977) which experience short-term (e.g., 

 tidal) environmental fluctuations, their inappli- 

 cability to A. islandica is, perhaps, not surpris- 

 ing considering the fact that the deep, infaunal 

 habitat of the species is comparatively well 

 damped from short-term environmental fluctua- 

 tions. Spawning occurred from May through 

 November in field populations of A. islandica, 

 and was heaviest during late August to October 

 and at the time of the fall thermocline break- 

 down. Changes in bottom temperature coinci- 

 dental with spawning occurred, but the rate and 

 magnitude were small (Fig. 1). Clarke 4 recorded 

 a prolonged spawning season for A. islandica. 

 His studies of A. islandica, which were collected 

 from a similar temperature regime to the pres- 

 ent study in depths of 20 m off Seabrook, N.H., 

 indicated some spawning from June through 

 October with the greatest intensity from August 

 to October. The prolonged nature of the spawn- 

 ing season in field populations reinforces the 

 conclusion that while a specific, absolute temper- 

 ature may be an important spawning stimulus, it 

 is probably effective only in conjunction with 

 changes in other stimuli, such as increases in 

 percentage saturation of oxygen, pH, and food 

 availability. 



Spawning stimuli other than temperature 



have been reported by Ansell et al. (1978), who 

 found a close correlation in the Clyde Sea area 

 between an abrupt increase in bottom dissolved 

 oxygen levels following a seasonal thermocline 

 breakdown and spawning activity in the infaun- 

 al bivalve Nuculana minuta. 



The fate of larval A. islandica spawned prior 

 to the thermocline breakdown also remains open 

 to discussion. The observations of Landers 

 (1976), Wood and Hargis (1971), and Cragg and 

 Gruffydd (1975) suggest that larvae in the early 

 stages of development swim upwards and that 

 substantial larval mortalities are probable from 

 early spawnings, since, at least, temperatures 

 too hot for survival would be encountered. To 

 complete development successfully larvae 

 spawned in June would have to remain below an 

 intense thermocline through which little mixing 

 occurs. This appears improbable. The inference 

 is that a period exists during which the survival 

 of larvae is limited by hydrographic events and 

 that the larvae of A. islandica do not freely move 

 throughout the entire depth of the water column 

 until after the breakdown of the summer thermo- 

 cline. Furthermore, the low winter water tem- 

 peratures recorded in the Middle Atlantic Bight 

 may also effectively depress continued develop- 

 ment of larval stages spawned late in the fall 

 months. Therefore the period during which the 

 larvae of A. islandica survive to metamorphosis 

 may be considerably shorter (approximately 2 

 mo, October and November) than that during 

 which the adults are capable of spawning (7 mo, 

 May to November). 



ACKNOWLEDGMENTS 



It is a pleasure to thank Rodman E. Taylor, Jr. 

 for his continued and enthusiastic assistance 

 throughout this program, John W. Ropes for 

 critical reading of the manuscript, and Elaine 

 M. Lynch for editorial assistance. Special thanks 

 are also due to R. Ferrara of the F.V. Albert 

 Quito and A. D. Colburn of the RV Asterias for 

 much patience during field operations. This 

 work was supported by NOAA Office of Sea 

 Grant Contracts No. 04-8-MO1-149 and NA 

 79AA-D-00102, and the Andrew W. Mellon 

 Foundation. 



LITERATURE CITED 



4 P. Clarke, Benthic Biologist, Normandeau Associates, Bed- 

 ford, NH 03102, pers. commun. May 1978. 



Ansell, A. I). 



1974. Seasonal changes in biochemical composition of 



324 



