Taxonomy and nomenclature in the widespread and well-known sea anemone Metridium de Blainville, 1824

Metridium senile (Linneaus, 1761) is simultaneously one of the most familiar sea anemones in temperate shallow water and a species embroiled in taxonomic and nomenclatural confusion. Their large body size, characteristic ruff of tentacles, and ability to thrive on artificial and natural substrates make these animals conspicuous and common on rocky ledges, pilings, wharfs, shipwrecks, and bivalve reefs. Their reported range encompasses all temperate habitats in the northern hemisphere plus a handful of locales in the temperate southern hemisphere that have typically been characterized as anthropogenic introductions. Understanding the range of M. senile has been confounded by confusion about its circumscription relative to its congeners and inconsistency in the names used. Here, we summarize the taxonomy of the genus and clarify the status of M. senile. Based on results of recent phylogenetic studies, we consider Metridium to have two valid species: M. farcimen (Brandt, 1835) and M. senile. We recognize M. senile as the valid, appropriate name for all populations of Metridium in the Atlantic and for some of the populations in the Pacific. We consider M. dianthus (Ellis, 1767), M. exile Hand 1955, M. huanghaiense Pei, 1998, and M. sinensis Pei, 1998 to be junior synonyms of M. senile. We clarify the differences between M. senile and M. farcimen, and comment on the status of M. canum Stuckey, 1914.