from dripper arms entered each funnel at a rate of 

 1.0 1/min. A Teflon delivery tube carried the 

 experimental solutions to the funnel from a buret 

 calibrated to deliver 20 ml in 15 s. 



To obtain a dilution factor for estimating the 

 effective concentration of experimental solutions 

 within a testing chamber, seawater solutions of 

 ^"*C-naphthalene (sp. act. 3.6 mCi/mmole, Amer- 

 sham-Searle Corporation) were introduced and 

 samples taken at timed intervals from the mid- 

 point of the chamber and counted for radioactivity 

 by liquid scintillation spectrometry. The chamber 

 contained a crab model displacing 701 ml, a 

 volume typical of the crabs tested. The maxi- 

 mum concentration in the chamber occurred 45 s 

 after ^^C-naphthalene was added and was 0.0188 

 (± 0.0058 SDi times the concentration of the 

 introduced solution. This dilution factor did not 

 differ significantly from that found by Pearson 

 et al. (1979) using a visible dye. 



Approximately 24 h before testing, crabs were 

 transferred to the testing chambers from the 

 holding tanks where they had been fed an ad 

 libitum diet of the blue mussel, Mytilus edulis. 

 Because, in preliminary experiments, tidal phase 

 was found to influence chemosensory responses 

 (Pearson et al. 1979), testing was synchronized to 

 begin and end within either a rising or falling tide. 

 The seawater for the test dilutions and control was 

 drawn and filtered 1 h after a tidal change. Testing 

 then began as soon as possible and stopped before 

 the next tidal change. 



Each day a maximum of 40 crabs were presented 

 individually with 20 ml of either one of nine 

 dilutions of naphthalene stock solution, one of 

 eight dilutions of WSF, or a control of filtered 

 seawater. Molting and mating crabs were not 

 tested. The order in which individual crabs were 

 watched and the choice of experimental solution 

 were randomized except that active crabs and ones 

 with retracted antennules were passed over. The 

 observer did not know the identity of any test 

 solution. Individual crabs were observed for 1.0 

 min prior to introduction of the experimental 

 solution, and their antennular flicking rate and 

 other behavior recorded. The flicking rate of 

 one antennule was measured using a hand-held 

 counter. The solution was then introduced, and 

 the observations continued for 1.0 min after the 

 beginning of solution addition. The behavior 

 was scored with the criteria used by Pearson 

 etal.(1979). 



To be scored as detecting an experimental 

 solution, a crab had to exhibit an abrupt change 

 in the orientation of the antennules within 30 s 

 after solution introduction, and the ratio of the 

 antennular flicking rate for 1.0 min after solution 

 introduction to that for 1.0 min before had to 

 be 1.50 or above. This value was determined 

 previously by Pearson et al. (1979) from observa- 

 tions of crabs in the testing apparatus without 

 any solutions present. Because 1.50 was the 

 95th percentile of these antennular flicking rate 

 ratios, the a priori probability that a flicking rate 

 ratio >1.50 represented a spontaneous increase 

 rather than a reaction to the experimental solu- 

 tion was <5%. 



Results 



Composition of the WSF 



The monoaromatic hydrocarbons by far dom- 

 inated the WSF (Table 1) and composed 99.19^ of 

 the total hydrocarbons measured. The remaining 

 aromatic hydrocarbons, mostly the naphthalenes, 

 were present at concentrations 100 times less than 

 that of the monoaromatics. The hydrocarbons 

 partitioned into the WSF from the crude oil in 

 proportion to their solubility in seawater (Clark 

 and MacLeod 1977; Bean et al. 1978). 



Table l. — Composition of the water soluble fraction of Prudhoe 

 Bay crude oil. Sample size was 3 for the di- and triaromatics and 

 6 for the monoaromatics. 



Fraction 



mg liter 



Total alkanes 



Naphthalene 



Total methylnaphthalenes 



Total dimethylnaphthalenes 



Phenanthrene 



Methylphenanthrene 



Dimethylphenanthrene 



Total polynuclear aromatics 



Benzene 



Toluene 



Ethylbenzene 



m- plusp-Xylene 



o-Xylene 



Total tnmethyl benzenes 



Total monoaromatics 



Total hydrocarbons measured 



< 0.001 

 .0851 i 

 .0766 = 

 .0269 = 

 .0006- 

 <.0001 

 <.0001 

 .1892 = 



10002:0 



6.74-0 



.30 = 



1.12 = 



1.12 = 



.46 = 



19.75 = 



19 94 



0.0088 

 00080 

 00015 

 0.0004 



:0.0175 



29 



42 



02 



06 



08 



12 



86 



Detection Thresholds 



Whereas Dungeness crabs detected both naph- 

 thalene and the WSF of Prudhoe Bay crude oil, the 

 crabs detected the complex WSF mixture more 

 readily and consistently Because the percentage 

 of crabs detecting naphthalene varied widely over 



823 



