Adsorbed layers: the stimulus to settlement in barnacles

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Abstract
The factor responsible for inducing settlement in cyprids ofBalanus balanoides(L.) can be fractionated by ammonium sulphate, and gives positive reactions to a series of tests identifying it with arthropodin, the water soluble protein fraction of arthropod cuticle. Solutions of the settling factor have little effect on the behaviour of cyprids exploring a surface, and do not promote settlement. Slate surfaces previously soaked in a dilute seawater extract of the settling factor can be distinguished by cyprids from freshly cleaned slates, even when cyprids and both surfaces are freshly immersed in the same extract that was previously used to soak the first set of slates. It is argued from this that the cyprids must respond to a specific molecular configuration manifested by the protein only when physically or chemically bound to a surface. Since in nature cyprids respond to a surface consisting of protein tanned by the natural polyphenols of the epicuticle (sclerotin), we consider that in our laboratory experiments with adsorbed layers of soluble arthropodin they respond to the same molecular configuration at the surface. Such recognition of specific molecular groupings of an insoluble material involves a truly contact chemical sense, which we have called a ‘tactile chemical sense’. This facility may exist in other animals. The settlement behaviour of barnacle cyprids is analyzed in terms of instinctive behaviour.