FISHERY BULLETIN: VOL. 72. NO. 4 



that was approximately linear, varying by 2.3° 

 to 2.7°C from one column of test tubes to the next. 

 Each of the 11 columns in the block represented 

 a different temperature level (Tables 1-3). Tem- 

 peratures rarely varied more than ± 0.3°C either 

 within a test tube or from one test tube to another 

 in a column. The eight rows represented different 

 time exposures (1, 5, 10, 30, 60, 120, 180, 360 min). 

 There were a total of 88 different temperature- 

 time combinations or treatments. 



Twenty-six milliliters of water placed in each 

 test tube were brought to stable temperature 

 levels. Four milliliters of water containing the 

 appropriate developmental stages were injected 

 into each test tube to give a concentration of about 

 9 to 12 animals/ml. We inoculated the 11 test 

 tubes in any row simultaneously, using an 

 apparatus holding 11 syringes whose plungers 

 were depressed together (see Figure 1, Kennedy 

 et al., 1974). 



Eighty-eight plastic beakers, each holding 

 about 340 ml of sea water, were placed in an 8 x 11 

 matrix in a water bath at 25° to 26°C. When 

 a time period in the block ended, we removed the 

 appropriate row of 11 test tubes at once and 

 washed the contents of each test tube into its 

 corresponding beaker. Survivors were incubated 

 in the beakers for 19 h (trochophores) or 23 h 

 (cleavage stages, straight-hinge) after the experi- 

 ments ended. Preliminary experiments indicated 

 that this allowed surviving cleavage stages and 

 trochophores to develop to the straight-hinge 

 stage and bacteria to decompose dead individuals. 

 It also allowed bacteria to decompose the meat of 

 dead straight-hinge larvae. 



At the end of the incubation period, the animals 

 in each beaker were preserved in 1% buffered 

 Formalin."* Numbers of straight-hinge larvae that 

 were alive or dead at the end of an experiment 

 were counted for each treatment. Indications 

 of death included an empty shell or decomposing 

 meats within a shell. For each experiment, we 

 used 10 control test tubes held at room tempera- 

 ture, with the experimental animals treated to all 

 handling described except exposure in the block. 

 Three experiments were made on each of the three 

 developmental stages. 



Temperature of the injected water was about 



^Reference to trade names does not imply endorsement by 

 the National Marine Fisheries Service, NOAA. 



23° to 25°C. This altered the temperature in the 

 test tubes at the cold and warm ends of the block 

 for a short period of time after injection. We made 

 approximate corrections for these changes 

 (Kennedy et al., 1974), and the corrected temper- 

 atures are noted in Tables 1 to 3. No corrections 

 were made for periods longer than 10 min. 



In a separate experiment, we measured oxygen 

 levels in the test tubes over a 6-h period using 

 straight-hinge larvae of M. mercenaria . Num- 

 bers of larvae were similar to numbers used in 

 temperature-time experiments. Over the 

 temperature range of 18° to 43°C, dissolved 

 oxygen levels remained near saturation, with 

 almost no change during the 6 h. We concluded 

 that there would be no stress from low oxygen 

 levels (Morrison, 1971) so we did not aerate during 

 the experiments. 



Multiple regression analyses of percentage 

 mortality on temperature and time were 

 calculated by a UNIVAC 1108 using a BMD02R 

 stepwise regression program, version of 2 May 

 1966, from the Health Sciences Computing 

 Facility, University of California at Los Angeles. 



Davis and Calabrese (1964) indicated that 

 accuracy in experiments involving sampling and 

 handling bivalve larvae is about ± 10%. Thus, 

 differences of less than 207c in percentage mor- 

 tality from one treatment to another may not be 

 meaningful. 



RESULTS 



There was a direct relationship of mortality 

 with temperature increase and, at higher 

 temperatures, with increase in time exposure 

 (Tables 1-3; Figures 1-3). As the animals aged, 

 temperature tolerance increased, with cleavage 

 stages most sensitive to higher temperature and 

 straight-hinge larvae least sensitive (Tables 

 1-3; Figures 1-3). 



The general mortality patterns for the tripli- 

 cated experiments at each developmental stage 

 were scrutinized and judged to be similar so the 

 data were combined. Over the (approximately) 

 20° to 26°C interval (columns 2 to 4 in the block), 

 survival was high at each temperature level. The 

 average number of straight-hinge larvae found 

 alive in each of these columns and in the controls 

 were compared, with no significant differences 

 found (P > 0.05). Therefore there was no unusual 



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