Accuracy of localization of a chemical stimulus in flowing and stagnant water by the nurse shark,Ginglymostoma cirratum

Summary

The locomotor response of juvenileGinglymostoma cirratum, the nurse shark, to six covert, discrete sources of an attractant chemical stimulus (fresh shrimp extract) was monitored in a tank (5×5×0.5 m) by means of a square matrix of 1936 photocells embedded in the floor and an on line computer. The experiments were done in flowing (1.17 cm/s) and stagnant water in order to determine the effects of flow on the accuracy of localization and to clarify some of the locomotor mechanisms involved. The response was quantified by a temporal and spatial analysis of activity, distance travelled, mean velocity, mean turn size, step length between turns and its direction vector, frequency of turning, the time spent by the animal in the quadrants of the tank, the mean direction vector of the steps and computer plots of locomotor pathways. For controls, the locomotion was monitored in the absence of chemical stimulation and with a placebo (seawater) replacing the shrimp extract. Five animals were used in 22 experiments in flowing, 20 in stagnant water. The relationship between each of the above-mentioned locomotor variables and the position of the discrete source of stimulation was examined by regression analyses and compared quantitatively between flowing and stagnant water. Localization is very precise in flowing water and is dependent on the gradient of the stimulus in the medium. Downstream sites of stimulation are better localized than upstream ones. In stagnant water only generalized localization occurs. The flowing water provides the direction vector for precise localization.