Shape Variation in Hydractiniid Hydroids

Colonies of hydractiniid hydroids consist of feeding polyps connected by a common gastrovascular system. The gastrovascular system consists of stolons, which enclose gastrovascular canals. Stolons may be fused into a stolonal mat or extend from the periphery of the colony. Hydractinia forms a stolonal mat early in colony development; Podocoryne, on the other hand, does not. To facilitate comparisons of these taxa, we propose a simple shape metric, perimeter/{radical}area, and show that this measure: (1) correlates closely with relative amounts of peripheral stolon and stolonal mat structures in Hydractinia, (2) permits analyses of within- and between-species variation of growth morphology in Podocoryne and Hydractinia, and (3) allows quantitative analysis of breeding studies of Hydractinia, both before and after stolonal mat formation in the progeny.