100 



80 



_l 60 - 

 < 



i 40h 



20 - 



WINTER 



• NO MUD PROVIDED 

 O MUD PROVIDED 



SUMMER 



50 



Figure 11-2. Survival of adult N. obsoletus exposed to 

 "1.0" ppm in the presence or absence of sediment. 



NOTE: Winter exposures were initiated 10/22/76 (50 snails/treatment). Sunn- 

 mer exposures were initiated 7/12/77 (25 snails/treatnnent). Control mortal- 

 ities less than 5 percent. Mean oil hydrocarbon concentrations ± s.d. (N) were: 

 1.25 ppm ±0.34 (25), winter; 0.70 ppm ±0.27 (23), summer. 



Larval mortality of N. obsoletus was high in four to eight day experiments 

 at "1.0" ppm, and relatively low at lesser concentrations (Table 11-1). 

 Substantial batch variability in larval tolerance was observed (Figure 11-3). 

 Whereas 50 percent mortality was recorded at "1.0" ppm after three-days in 

 experiment "A", the 50 percent level was not exceeded until day eight in a 

 second experiment using a different hatch of larvae. Indeed, no mortality 

 occurred until day four in experiment "B". 



All C. inoratus larvae exposed to "1.0" ppm died witliin four days. At 

 "0.1" ppm and "0.01" ppm, about 33 percent of the larvae were still living 

 after three weeks (Figure 11-4). Survival to the megalops stage, attained after 

 25-28 days, was: control, 58%; "0.01" ppm, 34%; "0.1" ppm, 30%; "1.0" 

 ppm, 0%. 



140 



