PEARSON ET AL.: CHEMOSENSORY ANTENNULAR FLICKING RESPONSE 



(Pearson et al. 1979). The onset of food searching 

 was indicated when a crab probed the substrate 

 with its chelae or exhibited the capture response 

 described by Pearson et al. (1979). 



To examine recovery of detection ability, we 

 stopped the flow of oil-contaminated water after 

 the first presentation of FDCE. Clean seawater 

 then entered the chambers at 0.9 1/min. After 24 h 

 and 48 h both exposed and control crabs were 

 again presented with experimental solutions and 

 their behavior observed and scored. 



Because the first experiment indicated rapid 

 recovery, we wished to see if such recovery was 

 quick as 1 h and, therefore, repeated the exposure 

 phase of the first experiment. Instead of present- 

 ing FDCE with oil-contaminated water still pres- 

 ent, we turned off the contaminated water and 

 presented the FDCE 1 h later. The start and finish 

 of exposure for individual crabs was staggered to 

 achieve this 1-h clearance of oil from the chambers. 



Statistical Analysis 



The experiments were run until 28-33 crabs had 

 been tested under each experimental condition. 

 The numbers of crabs detecting and not detecting 

 the various experimental solutions were totaled 

 for exposed and control conditions. Although data 

 is presented as the percentage of crabs detecting 

 the FDCE, chi-square analysis was done on 2 x 2 

 contingency tables of the number of crabs detect- 

 ing or not detecting under control or exposed 

 conditions. Data for crabs showing chelae probing 

 were treated similarly. 



RESULTS 



Hydrocarbon Concentrations 



During the first experiment, where the clam 

 extract was presented in the presence of oil- 

 contaminated seawater, the total hydrocarbon 

 concentrations by IR analyses were 0.27 ( ±0.04) 

 ppm (n = 22) during the 24-h exposure and 

 0.013 (±0.004) ppm in = 6) 24 h after the oil- 

 contaminated water was stopped. During the 

 second experiment, where the clam extract was 

 presented 1 h after stopping the oil-contaminated 

 seawater, the total hydrocarbon concentration by 

 IR averaged 0.34 ( ±0.07) ppm in = 10). Also, in 

 the second experiment after 1 h the concentration 

 of monoaromatic hydrocarbons (Table 1) fell to 

 0.008 times the exposure level. 



Table L— The concentrations (parts per billion) of mono- 

 aromatic hydrocarbon.s in the testing chambers. Determined 

 by helium gas partitioning, n = 4. 



Impairment and Recovery of 

 Chemosensory Detection 



After 24-h exposure to and still in the presence 

 of oil-contaminated seawater, the percentage of 

 exposed crabs detecting the clam extract was 

 about half that of control crabs (Table 2). In 

 contrast, the percentage of crabs probing with 

 chelae did not differ significantly between control 

 and exposed conditions (Table 3). Of the exposed 

 crabs that probed the substrate with their chelae 

 after presentation of 10 "2 gFDCE/1, 48% (n = 25) 

 did so without the normally preceding increase in 

 the antennular flicking rate to above the criterion 

 ratio of 1.50. One control crab (3%, n = 31) probed 

 with the chelae without the normal increase in 

 the antennular flicking rate. For crabs showing 

 chelae probing, the antennular flicking rate ratio 

 was significantly higher for control individuals 

 (median = 2.36, range = 1.46-17.50, n = 31) than 

 for exposed individuals (median = 1.56, range 

 = 0.76-6.82, n = 25; median test, x^ = 919. 

 df = 1, P = 0.998). Previously, no Dungeness 

 crab in = 89) presented with high levels of clam 

 extract (10"^ to 10 ~^ g/1) showed chelae probing 

 without first increasing the antennular flicking 

 rate (Pearson et al. 1979). 



Recovery of detection ability occurred rapidly. 

 In the first experiment the percentage of crabs 

 detecting FDCE at both levels did not differ 

 between control and exposed conditions for both 

 24 h and 48 h (Table 2). In the second experiment, 

 where the FDCE was presented 1 h after the flow 

 of oil-contaminated seawater was stopped, again 

 the percentage of crabs detecting did not differ 

 significantly between control and exposed condi- 

 tions (Table 4). 



Whereas the antennular response to the clam 

 extract was reduced under exposure and recovered 

 after return to clean water, the basic rate of 



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