Experiments on the Influence of Light on the Behaviour of Polyzoan Larvae

Abstract

1. An apparatus is described for investigating the photic responses of marine larvae. A parallel beam of light is projected along horizontal Perspex troughs containing the larvae, while the use of a water bath prevents their being subjected to a rise in temperature. 2. Using a light beam of 5000 lux at normal summer sea temperatures, plotting the positions of the larvae in the troughs at regular intervals has revealed four types of behaviour: I. Larvae at first all photopositive, later all becoming photonegative; for example, Celleporella hyalina. II. Larvae at first all photopositive, only a small proportion of them later becoming photonegative; for example, Bugula neritina. III. Larvae photopositive throughout the length of their natant phase; for example, Flustrellidra hispida. IV. Larvae showing no response at all to directional illumination; for example, Alcyonidium polyoum. 3. It has been shown that the reversal of the sign of reaction to light in Celleporella hyalina larvae is dependent on the passage of time; it does not require their exposure to a certain amount of light for its inception. 4. When disks cut from Fucus serratus fronds were placed in the troughs as a substrate for settlement, the majority of Celleporella hyalina larvae developed a photonegative reaction before they metamorphosed. 5. Horizontally and vertically placed panels in the Menai Straits received settlement of polyzoans on all surfaces free from sediment and dense algal cover. These adverse factors would have masked any selection based on the behavioural responses of the larvae. 6. Laboratory experiments with opaque panels suspended vertically, obliquely and horizontally have shown that Celleporella hyalina larvae settle on the side away from the light. Thus they normally settle on the undersurfaces of horizontal and oblique panels, but this pattern is reversed if the panels are lit from below. Settlement on panels in the sea would therefore seem to depend on the reactions of larvae to light as well as on the unfavourability of upward-facing surfaces. 7. The nature of the larval responses is discussed.