100 
• NO MUD PROVIDED 
O MUD PROVIDED 
DAY 
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). Sum¬ 
mer exposures were initiated 7/12/77 (25 snails/treatment). 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. irroratus larvae exposed to “1.0” ppm died within 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 
