Zoologica1 JOU~M~ o f fhe Linnean Society (1991), 101: 189-199. With 3 figures The life cycle of Hydrichthys mirus (Cnidaria: Hydrozoa: Anthomedusae: Pandeidae) FERDINANDO BOERO*, JEAN BOUILLONf AND CINZIA GRAVILI* Station Biologique Leopold 111, Laing Island, Papua ~ Y e w Guinea, *Dipartimento di Biologia, Universitd di Lecce, 1-73100 Lecce, Italy and vaboratoire de Zoologie, Université Libre de Bruxelles, Av. F.D. Roosevelt 50, B-1050 Bruxelles, Belgium Receiued Aprii 1990, accepted for publicafion fuly 1990 The previously unknown life cycle of the parasitic hydroid Hydrichthys mirus is described. The aduli medusa has 5-6 tentacles and couid be rcferred to Leuckarfiara. Another species of Hydrichthys has previously been shown to have a Sfomotoca-like medusa, characterized by the possession of two tentacles. It is proposed that Hydrichthys originated from the Leuckarfiara lineage and that, through paedornorphosis in at least one species, the rncdusa retained the two-tentacle state of the newly- released medusa of al1 pandeids, thus becorning referable to Sfomotoca. It is suggested that Sìomofoca has hitherto constituted a polyphyletic taxon, ernbracing parallel lineages of pandeids, each retaining juvenile features. An additional genus, Larsonia, is introduced to accornrnodate species with Sfomofoca-like medusae and Hydrichfhys-like hydroids. KEY W0RDS:--Cnidaria - parasitism - pacdornorphosis - evolution. CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . 189 Materiali and rnethods . . . . . . . . . . . . . . . . 190 Results . . . . . . . . . . . . . . . . . . . . 190 Description of the hydroid . . . . . . . . . . . . . . 190 Description of the medusa . . . . . . . . . . . . . . 192 Behaviour . . . . . . . . . . . . . . . . . . 195 Taxonomic status . . . . . . . . . . . . . . . . . 196 Evolution of Hydrichfhys and related genera . . . . . . . . . . 197 Conclusion . . . . . . . . . . . . . . . . . . . 198 Larsonia gen. nov. . . . . . . . . . . . . . . . . 198 Acknowledgements . . . . . . . . . . . . . . . . . 199 References . . . . . . . . . . . . . . . . . . . 199 INTRODUCTION The genus Hydrichthys was proposed by Fewkes (1887) to accommodate a hydroid parasitic on fish. Its morphological features are quite different from those of al1 other known hydroids: colony completely deprived of pensare, forming an encrusting mat; gastrozooids with no tentacles; usually having branched gonozooids bearing clusters of medusa buds. The medusa, however, is a typical pandeid and Fewkes (1887) reared it to an immature four-tentacle stage. On the basis of its newly-released medusa Hydrichthys is referred to the 189 0024-4082/91/020189 + 11 S03.00/0 1991 The Linnean Society oi'London