Marine Biodiversity

, Volume 49, Issue 1, pp 11–23 | Cite as

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

  • Joan J. Soto ÀngelEmail author
  • Luis Martell
  • Ferran Palero
Original Paper


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 AchaeusH. epigorgia association may represent a novel case of mutualism in hydroid–crab epibiosis.


Behavior Coral reef Decorator crab Epibiosis Red Sea Symbiosis 



We are deeply indebted to Alex Mustard, Anne Frijsinger and Mat Vestjens, Borut Furlan, Cigdem Cooper, David Hall, Gustav Paulay (Florida Museum of Natural History, Gainesville), Herbert Meyrl, Ilan Ben Tov, Jamie Craggs, Ole Johan Brett, Pawel Borowka, Rafi Amar, Richard Aspinall and Steve Jones for kindly providing us with high-quality photographs of living specimens. Thanks are due also to two anonymous reviewers for their thorough revisions and useful suggestions. FP acknowledges the project CHALLENGEN (CTM2013-48163) of the Spanish Government and a post-doctoral contract funded by the Beatriu de Pinos Programme of the Generalitat de Catalunya.

Supplementary material

12526_2017_746_MOESM1_ESM.xlsx (24 kb)
ESM 1 Worldwide records of Hydrichthella epigorgia Stechow, 1909 (XLSX 24 kb)


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Copyright information

© Senckenberg Gesellschaft für Naturforschung and Springer-Verlag GmbH Germany 2017

Authors and Affiliations

  1. 1.Laboratory of Marine Biology, Department of ZoologyUniversity of ValenciaBurjassotSpain
  2. 2.University Museum of Bergen, Natural History Collections, University of BergenBergenNorway
  3. 3.Centre d’Estudis Avançats de Blanes (CEAB-CSIC)BlanesSpain
  4. 4.INRA, Univ. Nice Sophia Antipolis, CNRS, UMR 1355-7254 Institut Sophia AgrobiotechSophia AntipolisFrance
  5. 5.Department of Life SciencesThe Natural History MuseumLondonUK

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