Observations on the Orientation of Some Species of Barnacles

Abstract

There are in the literature a number of conflicting statements regarding the effect of various stimuli on the cypris stage of barnacle larvae, before and at the time of metamorphosis, and little experimental work has been done to elucidate the type of reactions involved. Visscher (1928), working with Balanus amphitrite, B. improvisus and Chthamalus fragilis, stated that during the earlier part of their free-swimming stage the cypris larvae were positively phototropic, but later the reaction to light became erratic, and at the time of attachment was decidedly negatively phototropic. He further stated that in a settlement experiment they orientated with the anterior end (i.e. with the paired eyes) away from the light source. However, MacDougall (I943) working with two of the above species (Balanus improvisus and Chthamalus fragilis) and also Balanus eburneus, and Pyefinch (I948) working with B. balanoides and B. crenatus, both state that the cypris larvae retain their positive reaction to light up to the time of settlement. There also appears to be some disagreement regarding the orientation at, and subsequent to, metamorphosis of several species of barnacles. Moore (1935), in considering B. balanoides, has suggested that if a barnacle is settling in a current, then at the time of settlement it is likely to be orientated with the current. A general orientation at right angles to the current was supposed to be due to rotation after settlement. This rotation was considered to be a response to the orientation of the beating cirri in relation to the current; it was suggested that feeding would be more effective with the animal orientated across the current.