themselves; when the concentration was reduced to I ppm, the righting response was 

 measurably delayed. Modifications in the speed and pattern of attempts to escape 

 were also observed. Crabs exposed to I to 50 ppm of dieldrin developed sluggish 

 movements. At the highest concentrations, the normal response, that of raising the 

 major cheliped, frequently caused the crabs to fall over backwards, after which they 

 were unable to right themselves. The ability of adult crabs to coordinate properly in 

 both the righting response and in coordinated escape behavior is essential for sur- 

 vival, because both responses enable the crab to avoid predation and dehydration. 

 The response of planktonic organisms to light is affected by trace amounts of 

 several pesticides ( Forward and Costlow, 1 976). Sublethal concentrations of metho- 

 prene, a synthetic juvenile hormone mimic, had no effect upon either swimming 

 speed or phototaxis of larvae of the mudcrab, Rhithropanopeus harrisii. Hydro- 

 prene, another juvenile hormone mimic, had no significant effect upon swimming 

 speed during the first three zoeal stages but produced a marked increase in swimming 

 speed in the stage-four zoea. Phototaxis was almost entirely unaltered by exposure 

 to hydroprene; however, stage-three zoea maintained at the highest concentration of 

 the compound (0.1 ppm) demonstrated a significantly increased level of positive 

 phototaxis. Whereas this level was higher than that observed for the control larvae, it 

 was significantly lower than that observed for the larvae tested in the acetone 

 control, the carrier employed to maintain hydroprene in solution (Forward and 

 Costlow. 1978). A third compound. Dimilin®. produced a pronounced effect on the 

 larvae of Rhithropanopeus.harri.sii, even when sublethal levels of the compound were 

 used. Although swimming speeds were generally accelerated, this response varied 

 considerablv from one larval stage to the other. Phototaxis was unaffected within the 

 first three zoeal stages; in stage four, however, although swimming patterns were 

 normal, phototactic response was drastically altered. Swimming and phototactic 

 responses enable the organism to avoid predators and adverse salinity and tempera- 

 ture conditions. Phototaxis probably aids the organism in maintaining the proper 

 vertical position for horizontal migrations (Bousfield. 1955). Any variation in these 

 basic behavioral patterns could therefore impair survival even though mortality may 

 not be attributed directly to the compounds in question. The results of these studies 

 suggest that this approach may be a most useful bioassay to determine the presence 

 of extremely small amounts of pollutants in estuarine waters. 



Sublethal effects of methoxychlor have also been observed at the physiological 

 level. Caldwell ( 1974) describes the way in which methoxychlor affects osmotic and 

 ionic regulation in two species of adult crabs. Cancer magister and Hemigrapsus 

 nucius. Sublethal levels resulted in a decreased resistance in tolerance of the adult 

 crab to reduced salinity and also resulted in partial inhibition of the gill NaK mg 

 ATPases in Cancer magister. The author, however, could not demonstrate that 

 osmotic regulation in Hemigrapsus nudus or osmotic and ionic regulation in Cancer 

 magister were significantly impaired by these treatments. 



The duration of individual larval stages, long known to be affected by temperature 

 and salinity, has been further shown to be affected by the presence of certain com- 

 pounds in the water column. Increased concentrations of Kepone, methoxychlor, 

 and malathion resulted in reduced molting rates in the mud crab Rhithropanopeus 

 harrisii and the blue crab Callinectessapidus (Bookhout et al., 1976, 1980; Bookhout 

 and Monroe, 1977). Whereas Dimilin® was toxic during larval stages of several 

 species of estuarine crab (Rhithropanopeus harrisii and Sesarma reticulatum), it did 

 not affect the duration of the larval development (Christiansen et al., 1978). At con- 

 centrations of 1.0 to 5.0 ppb, the duration of zoeal development of the mud crab 

 Rhithropanopeus harrisii and the blue crab Callinectes sapidus was increased as the 

 concentrations of methoxychlor were increased (Bookhout et al., 1976). 



Morphological abnormalities or the appearance of extra or supernumerary larval 

 stages have also been observed. In the stone crab, Menippe mercenaria, exposure to 

 mirex caused no significant increase in duration of developmental stages, but the 

 percentage of extra sixth zoeal stages increased as the concentration of mirex in- 



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