MOLECULAR EVIDENCE FOR MULTIPLE EPISODES OF PEDOMORPHOSIS IN THE FAMILY HYDRACTINIIDAE

Reduction of complex life cycles in the Hydrozoa is commonly achieved by the heterochronic reduction of the medusa from a free-living to a sessile stage. Two competing traditions of hydroid taxonomy dating from the last century disagree about whether the degree of medusa reduction is an appropriate generic character. A phylogeny based on a fragment of the sequence encoding mitochondrial large ribosomal subunit RNA is constructed for thirteen representatives of three hydractiniid genera, each defined by its degree of medusa reduction. The minimum length trees found by parsimony and distance methods are not consistent with a monophyletic origin for all three genera. Tree topologies constrained to hold the genera Stylactus and Hydractinia monophyletic were shown by a statistical test to be significantly worse than the minimum length trees. These results support the hypothesis that medusa reduction has happened repeatedly in the family Hydractiniidae, thereby calling into question the tradition of hydroid taxonomy which defines genera based on degree of medusa reduction.