Observations on the behaviour of the planulae larvae ofClava squamata

Abstract

The preoccupation of many workers in recent years with the detailed analysis of the settlement behaviour of the larvae of numerous sedentary invertebrates has contributed significantly to our understanding of the various factors influencing the secondary orientation reactions of animals as formulated by Fraenkel & Gunn. Investigations with the planula larvae of hydroids along these lines has hitherto been largely ignored. The intertidal hydroidClava squamataMüller has a distribution restricted to the larger fucoids in the littoral zone and during the summer months liberates its planulae in large numbers.Detailed examination of the distribution of colonies indicated that the planulae probably possess clearly defined patterns of behaviour which assists them in locating substrates suitable for colonization. The planulae which are released in response to light are normally found to be photopositive on inert surfaces and to settle photopositively. OnAscophyllum, however, the normal substrate in nature, the older larvae are photonegative. The crawling activity of the planulae is considerably influenced by the texture and contour of surfaces. On rough and pitted surfaces activity is considerably reduced indicating a thigmokinetic response of the larvae. Water currents influence behaviour, planulae orientating positively to the direction of flow. The possible significance of these patterns of behaviour in the substrate selection and distribution of the species is discussed.