Associations between pycnogonids and hydroids from the Buenos Aires littoral zone, with observations on the semi-parasitic life cycle of Tanystylum orbiculare (Ammotheiidae)*

Abundance and seasonality of Tanystylum orbiculare Wilson, 1878 populations (Pycnogonida; Ammotheidae) associated with Sarsia sarsii (Loven, 1836) and Tubularia crocea (Agassiz, 1862) (Hydrozoa; Anthomedusae) are analysed and the semi-parasitic life cycle of this species is described. In the analysed population, the first larval stages were found exclusively as parasites of S. sarsii . During the attachment to the hydranths, protonymphon larvae of T. orbiculare feed by sucking the hydranths of the cnidarian. Change of host is not obligatory; juveniles and adults were found both on S. sarsii as on T. crocea, and adults were also observed on other invertebrates. In the Mar del Plata intertidal, Tanystylum orbiculare was the most common pycnogonid species on colonies of T. crocea and S. sarsii. Anoplodactylus petiolatus was less abundant on both species and A. assimilis was found only on T. crocea. Endeis spinosa is absent on hydroids from the Mar del Plata intertidal zone but ectoparasitic larvae and adults were found on Obelia longissima colonies from Mar del Plata harbour together with adults of T. orbiculare and A. petiolatus . Endoparasitic larvae of A. petiolatus were found associated with colonies of Bougainvillidae.

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