LOUGH: TEMPERATURE-SALINITY EFFECTS ON BIVALVE LARVAE 



for optimum growi:h were significantly different 

 than those required for optimum survival. 



Analysis of the combined 6- to 8-day survival 

 and growth indicated that the interacting effect 

 of temperature and salinity and the linear effect 

 of temperature were the more important variables 

 explaining the data, although only 30.4% of 

 the variance was explained by the combined 

 polynomial. Optimum temperature and salinity 

 conditions (80% contour) for maximizing both 

 larval survival and grovd;h to 8 to 10 days of 

 age were predicted between 20° and 26°C and 

 23 and 32%o. 



DISCUSSION 



Despite the fact that the adults of the three 

 species studied are euryhaline to varying degrees, 

 their early embryos and larvae have a com- 

 paratively narrow salinity range. Early larvae 

 of Mercenaria mercenaria appear to be much 

 more tolerant to high temperatures than the 

 other two species, but require essentially oceanic 

 salinities. 



The older larvae, having been reared from 

 fertilization to the veliger stage at optimum 

 conditions, now appear to have a generally 

 greater tolerance to both temperature and salin- 

 ity. The late larvae of C. virginica appear to 

 tolerate a higher temperature range than the 

 early larvae while Mulinia lateralis late larvae 

 seem to tolerate best temperatures at the lower 

 end of its range. Late larvae of Mercenaria 

 mercenaria are able to tolerate low salinities 

 somewhat better than the early larvae, but their 

 temperature range is quite restricted. The 

 observed progressive change in their temperature- 

 salinity tolerance with time approaches the 

 range normally tolerated by the adults. This 

 same progressive change was observed for the 

 larvae of Adula californiensis by Lough and 

 Gonor (1973a, b). 



The range of temperature-salinity conditions 

 estimated for maximum growth was significantly 

 different from that estimated for maximum sur- 

 vival of the same late stage larvae. Maximum 

 predicted growi;h occurred at higher temperatures 

 and at somewhat higher salinities than those 

 for maximum survival for all three species 

 studied. All three species showed a significant 

 temperature-salinity interaction effect for 

 growth. Growth, classically, is positively corre- 



lated with temperature up to some limit; how- 

 ever, the role of salinity appears to complicate 

 the temperature effect. 



The combining of late larval survival and 

 growth to maximize both responses seems in- 

 tuitively pleasing as one would expect a compro- 

 mise situation in nature. An organism probably 

 can operate most effectively when it is in a set 

 of environmental conditions which maximize all 

 its biological responses. It has been shown by 

 Lough and Gonor (1973a, b) that temperatures 

 for maximum growth response may be an ab- 

 normal stress environment which ultimately 

 results in high mortality. Similarly, low tempera- 

 tures may be suitable for larval survival but 

 not necessarily highly productive for recruit- 

 ment and growth to the adult population. Al- 

 though the optimum temperatures and salinities 

 usually can be estimated from the raw data, 

 the statistical techniques used in this study allow 

 one to define and interpret an organism's response 

 to a matrix of environmental factors and to 

 determine whether the response(s) between 

 stages of development or sampling intervals 

 is significantly different. 



INFERENCES 



Tolerance studies of various stages or at various 

 times in the life history of an organism are 

 especially important to pollution studies. Dif- 

 ferent stages of crab larvae have been shown 

 to have different temperature-salinity tolerances 

 of ecological significance (Costlow et al. 1960, 

 1962, 1966). This study demonstrates that dif- 

 ferent periods in the life of bivalve larvae also 

 differ in their tolerance to temperature and 

 salinity. The determination of water quality 

 standards based on only one stage in the life 

 of an organism is not realistic. All stages of 

 development are important, particularly when 

 the synergistic effect of a pollutant is studied. 

 Davis and Hidu (1969) found it was necessary 

 to evaluate the effects of pesticides on all stages 

 of clam and oyster larvae as their tolerances 

 are markedly different. 



The field of aquaculture also may benefit from 

 these tolerance studies. Based on this study a 

 long-term experimental program should be under- 

 taken to maximize both survival and growth 

 recognizing that different stages of an organism 

 may have different optimum conditions. Possibly, 



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