Reversal of Phototaxis in the Larvæ of Polydora pulchra, Carazzi (Polychæta, Spionidæ)

Abstract

REVERSAL of phototaxis has been noted in a number of animals and attributed to fright, hunger and food and other chemical stimuli. The only case known to me in which the direction of phototaxis is determined by the salinity of the environment is provided by the larvæ of Polydora pulchra, Carazzi, previously recorded¹ from the plaice ponds at the Marine Biological Station, Port Erin. The larvæ, which ranged in size from 0.5 mm. to 1.5 mm. were all positively phototactic in solutions of salinity 32.6 per thousand and above, and all negatively phototactic in solutions of salinity 32.5 per thousand and below. Salinities below 30.0 per thousand were lethal within 24 hr. In any given salinity the larvæ maintained the same orientation indefinitely or until death, and the behaviour of specimens kept overnight at salinity 40.0 per thousand was afterwards indistinguishable from those taken from pond water (salinity about 34.0 per thousand). For any given salinity, the reaction was the same over a wide range of light intensity from 3 watts to 100 watts at a distance of 200 cm. and over a range of temperature 0.0°–32.0° C.