Dimorphism and Size Distribution in Velella and Physalia

Abstract

WOODCOCK¹ attributed right- and left-handedness in Physalia, the Portuguese man-of-war, to a selective advantage in avoiding entrapment in windrows of Sargassum weed and floating debris. This selective advantage was presumed to be due to differences in sailing patterns through convection cells in the surface water of the northern and southern hemispheres. The absence of Sargassum from the South Atlantic and the paucity of debris in the barren, blue waters in which Physalia is characteristically found would seem to invalidate this hypothesis. On the other hand Woodcock's arguments concerning food concentration by the convection cells and sailing patterns are more convincing. Woodcock² continued to favour the hypothesis that there are significant differences in abundance of the two forms between the northern and southern hemispheres. Other authors^3,4 have commented on this problem; however, none has pointed out that the more extensive literature on Velella, long known to be dimorphic⁵, shows no statistically reliable difference between the abundance of the two forms in the northern and southern hemispheres.