Salinity perception by larvae of the crabRhithropanopeus harrisii(gould)

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Abstract
The mechanism of salinity perception was measured in Stage I zoeae of the estuarine crab Rhithropanopeus harrisii. The behavioral assay for perception of a salinity decrease was the loss of positive phototaxis while the reversal in geotactic sign from positive to negative was used for studying a salinity increase. A change in sodium chloride concentration is the primary environmental cue signalling a salinity change. However, neither sodium nor chloride per se is required for detection, since other salts such as lithium nitrate, can substitute for sodium chloride. The minor ions of seawater (calcium, magnesium, potassium and sulfate) have a modulating effect upon larval behavior, but play no role in salinity perception. The exact mechanism for perception of an increase and a decrease in salinity is different. Substitution experiments indicate that in the perception of a salinity decrease, only salts having a cation similar in size to sodium can mimic salinity changes whereas in the detection of a salinity increase, cation size is not an important parameter. The stimulation effectiveness of anions is correlated with neither size nor valence. Though osmolality is probably not involved in perceiving a decreased salinity, it may serve a role in the detection of a salinity increase. A charge in pH has no effect upon perception of a salinity decrease, but does affect perception of a salinity increase. A classical salt receptor appears to be involved in sensing a salinity decrease, while a different type of receptor is used for detecting an increase in salinity.