Polyps fishing on a crab: novel association between Hydrichthella epigorgia (Hydrozoa, Ptilocodiidae) and Achaeus (Crustacea, Inachidae)

Abstract

Hydrozoans are a ubiquitous component of the marine benthos which may occur associated with a wide range of taxa. Some of these associations have been recently reviewed, while others remain mostly unexplored, such as the case of hydrozoans and brachyuran crabs. The present study describes a novel association between the ptilocodiid Hydrichthella epigorgia and spider crabs of the genus Achaeus. Three complementary approaches have been followed in order to determine the prevalence and specificity of this interaction: (1) in situ observations, (2) assessment of independent observations from image repositories, and (3) analyses of specimens kept in museum collections. The present contribution constitutes the first report of H. epigorgia in the Red Sea and the first evidence of a Ptilocodiidae epibiont on a crustacean. The prevalence obtained is the highest ever reported for a hydroid–crab association (96%), which suggests specific ecological interactions between both species. The Achaeus crabs are reported using H. epigorgia polyps as fishing rods to capture zooplankton, with pereiopods 2–3 acting as “fishing legs” and harboring a much larger amount of dactylozooids than other pereiopods. Different morphotypes are described for H. epigorgia in relation to the substrate used, and the validity of the representatives of the genus is discussed. Further experimental studies will clarify whether the Achaeus–H. epigorgia association may represent a novel case of mutualism in hydroid–crab epibiosis.