ORIGINAL PAPER The life cycle of Ortholinea auratae (Myxozoa: Ortholineidae) involves an actinospore of the triactinomyxon morphotype infecting a marine oligochaete Luis F. Rangel 1,2 & Sónia Rocha 2,3 & Ricardo Castro 1,2 & Ricardo Severino 2 & Graça Casal 2,4 & Carlos Azevedo 2,3,5 & Francisca Cavaleiro 2 & Maria J. Santos 1,2 Received: 1 April 2015 /Accepted: 7 April 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Actinospores released from the marine oligochaete Limnodriloides agnes inhabiting a Southern Portuguese fish farm are molecularly recognized as developmental stages of the life cycle of Ortholinea auratae, a myxosporean parasite that infects the urinary bladder of Sparus aurata. The molec- ular analysis of the 18S rRNA gene reveals a similarity of 99.9 to 100 % of the actinospores analyzed to the myxospores of O. auratae. The actinospores belong to the triactinomyxon morphotype and occur in groups of eight within pansporocysts that develop in the intestinal epithelium of the oligochaete host. This is the first record of a myxosporean using an oligochaete as its invertebrate host in the marine environment. Keywords Myxozoa . Ortholinea auratae . Sparus aurata . Limnodriloides agnes . Life cycle . Triactinomyxon . Oligochaeta Introduction Since Wolf and Markiw (1984) revealed the involvement of an invertebrate host in the life cycle of Myxobolus cerebralis, several other studies have aimed to clarify the complete life cycles of myxosporean parasites. Initially, these studies relied solely in experimental infections. This methodology is, how- ever, laborious and time consuming (Székely et al. 2014), and the reliability of its results has been discredited by the use of molecular tools (Holzer et al. 2004; Marton and Eszterbauer 2011). Nowadays, both the description of myxosporean para- sites and the understanding of its complex life cycles rely on molecular methodologies (Eszterbauer et al. 2006; Caffara et al. 2009; Karlsbakk and Køie 2012; Køie et al. 2013; Székely et al. 2014). Although there are about 2310 myxosporean parasites de- scribed (Lom and Dyková 2006; Morris 2012), only about 40 have their complete life cycle elucidated (Lom and Dyková 2006; Yokoyama et al. 2012; Székely et al. 2014). Of these, six correspond to myxosporean parasites inhabiting the ma- rine environment, all of which use polychaetes as their inver- tebrate hosts: Ellipsomyxa gobii Køie, 2003 and E. mugilis (Sitjà-Bobadilla and Álvarez-Pellitero, 1993) infect Nereis spp. (Køie et al. 2004; Rangel et al. 2009); Gadimyxa atlantica Køie, Karlsbakk and Nylund, 2007 infects species of Spirorbis (Køie et al. 2007); Ceratomyxa auerbachi (Noble, 1950) infects Chone infundibuliformis Krøyer 1856 (Køie et al. 2008); Sigmomyxa sphaerica Karlsbakk and Køie 2012 infects Nereis pelagica Linnaeus, 1758 (Karlsbakk and Køie 2012); and Parvicapsula spp. infect Hydroides norvegicus Gunnerus, 1768 (Køie et al. 2013). Species of the myxosporean genus Ortholinea are mainly parasites of the kidneys, urinary ducts, and urinary bladder of fish hosts, mostly marine. Although they infect fish species belonging to several taxonomic orders, there are more * Luis F. Rangel luisfiliperangel@sapo.pt 1 Department of Biology, Faculty of Sciences, University of Porto, Rua do Campo Alegre, s/n, Edifício FC4, 4169-007 Porto, Portugal 2 Interdisciplinary Centre of Marine and Environmental Research (CIIMAR/CIMAR), University of Porto, Rua dos Bragas 289, 4050- 123 Porto, Portugal 3 Laboratory of Cell Biology, Institute of Biomedical Sciences Abel Salazar (ICBAS), University of Porto, Rua Jorge Viterbo Ferreira n 228, 4050-313 Porto, Portugal 4 Department of Sciences, High Institute of Health Sciences-North, CESPU, Rua Central da Gandra, 1317, 4585-116 Gandra, Portugal 5 Zoology Department, College of Sciences, King Saud University, 11451 Riyadh, Saudi Arabia Parasitol Res DOI 10.1007/s00436-015-4472-5