Early changes in the shell form of Balanus balanoides (L.)

The shell shape of isolated growing individuals of Balanus balanoides (L.) changes considerably during early growth but by the time it reaches 3 mm in length its shape is almost the same as that of the adult, and thereafter it grows uniformly in all dimensions. The early metamorphosed barnacle is extremely squat with nearly vertical parietes and a relatively large aperture. The base, initially elliptical, becomes more circular and later develops ribbed borders. The carinolateral compartments are absent in the newly metamorphosed barnacle; their development and expansion is probably the main factor in the change in shape of the base. The apex of the carina, initially the lowest, becomes the highest point of the shell. The apex of the rostrum becomes relatively depressed during growth. The angle between the parietes and the base lies between 60° and 80° in newly metamorphosed individuals and drops to 35–45° in the adult.We suggest that the significance of these changes, if considered in conjunction with the orientation to current, is to maximize adhesion and minimize the drag of water currents on the shell, and to direct the water flow into the cirral net.