Chemical Signal-to-Noise Detection by Spiny Lobsters

The smallest difference in concentration detected between a chemical stimulus and background is called the threshold of "just noticeable difference" (jnd). Measurements of jnd thresholds have been made extensively in psychophysical research on olfactory and taste perception by terrestrial mammals, but not on chemoreception by marine organisms. Marine organisms live in a persistently noisy chemical environment, because stimulatory compounds are often components of the background in seawater. Measurements of jnd thresholds, therefore, should be especially appropriate in the marine environment and were the focus of this study. Laboratory assays were used in measuring the ability of spiny lobsters, Panulirus interruptus, to detect a glycine stimulus against a background concentration of glycine. Glycine was chosen as stimulant because it is a major component of dissolved organic matter (DOM) in seawater, is abundant in prey tissues, and is excitatory to the appetitive feeding phase of P. interruptus. Chemical determinations of glycine in seawater were made by reverse-phase, high-performance liquid chromatography. The jnd threshold that was estimated in this study for glycine detection by lobsters was about 2-8% above the background concentration of glycine in seawater. This threshold is slightly lower than ones demonstrated for odorant detection by humans and other terrestrial animals. Consequently, the olfactory sense of lobsters appears to be well constructed to detect subtle changes between concentrations of stimulus and background, a facility that may be important in the ecology of this animal.