The hydroids Clava multicornis and Clava squamata

Abstract

There is much variation in the form of colony assumed by the common hydroid Clava, and two forms were for long distinguished as separate species, a diffuse one found on stones and rocks in the lower part of the tidal zone and in shallow subtidal depths, with scattered polyps arising from an open hydrorhizal network of creeping stolons, and a clustered one, living on fucoid and other seaweeds in the tidal zone, with densely clustered polyps borne on a compact hydrorhizal base of very short anastomosing stolons. Johnston (1847) did not separate these forms and recognized one species only, named by priority Clava multicornis (Forskal, 1775). It was largely on the authoritative works of Hincks (1868) and Allman (1871–2) that the two forms became accepted as distinct species, the name Clava multicornis (Forskål) being adopted for the diffuse one (though Hincks admitted uncertainty in ascribing Forskål name to this) and Clava squamata (Müller, 1776) for the clustered one. More recently certain authors have considered them to be forms of one species, designated by priority Clava multicornis, but others have continued to treat them as separate species, and the question of their specific identity or distinctness cannot be said to have been resolved. Broch (1916, pp. 38–40), from a study of material from various localities, concluded that there are all transitional conditions between the two forms, rendering specific distinction impossible, and that they are evidently ecologically conditioned forms of one species. He did not, however, adduce any experimental or other evidence in support of his belief.