Ten new types of sphaeractinomyxon actinospores are morphologically and molecularly described from the coelomic cavity of two marine oligochaete hosts, Hrabě, 1967 and Tubificoides pseudogaster (Dahl, 1960), from Aveiro 2 estuary, Portugal. The smallest sphaeractinomyxon type measured 17 µm (length) × 19 µm (width) × 19 µm (apical diameter), whereas the largest type measured 61 µm × 76 µm × 80 µm. While considering the ten types of sphaeractinomyxon, it was found that the number of spores developing inside pansporocysts varied between one, two, four and eight. The total prevalence of infection was of 19% for the two host species, with a maximum recorded for spring and summer (25-26%). While considering each type of sphaeractinomyxon individually, it was found that the prevalence values ranged between 0.3 and 1.7%. All described sphaeractinomyxons were most similar to mugilids infecting Myxobolus species. Limnodriloides agnesThe validity of the tetraspora and endocapsa collective group names is discussed.

Seasonal occurrence of metazoan ectoparasites is described for the first time in marine European flounder, Platichthys flesus (L.). The parasitofauna, in this study monitored during 1 year, was found to be similar to that previously recorded for flounder. Moreover, specimens of Caligus sp. Mü ller, 1785 and Lepeophtheirus pectoralis (Copepoda : Caligidae), Acanthochondria cornuta (Copepoda : Chondracanthidae), Holobomolochus confusus (Copepoda : Bomolochidae) and Nerocila orbignyi (Isopoda : Cymothoidae), and also, a praniza larva (Isopoda : Gnathiidae), were isolated. From these, L. pectoralis and A. cornuta were the dominant parasites in all samples of flounder, while Caligus sp., H. confusus, N. orbignyi and the gnathiid praniza seemed to infect the flounder only occasionally. As far as the seasonality of infections is concerned, it differed considerably from that described for estuarine environments. Indeed, both prevalence and abundance of L. pectoralis and A. cornuta reached significant peaks in the summer, whereas the literature identifies the autumn as the season of maximum infection on estuarine flounder. Thus, the former period seems more favourable for the occurrence of epizooties of L. pectoralis and A. cornuta in flounder culturing systems running on seawater and operated in the studied or similar environments.

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 molecular 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.

Ortholinea labracis n. sp. is described and its life cycle is inferred from a Southern Portuguese fish farm, with basis on microscopic and molecular procedures. This myxosporean parasite infects the urinary bladder of the European seabass Dicentrarchus labrax and the intestinal epithelium of a marine oligochaete of the genus Tectidrilus. Myxospores subspherical in valvular view and ellipsoidal in sutural view measuring 7.6 ± 0.3 (6.8-8.7) μm in length, 7.2 ± 0.2 (6.7-7.7) μm in width and 6.5 ± 0.4 (5.8-7.7) μm in thickness. Two polar capsules, 3.0 ± 0.2 (2.6-3.4) μm long and 2.4 ± 0.1 (2.0-2.9) μm wide, located at the same level, but with divergent orientation and opening to opposite sides of the suture line. Sequencing of the SSU rRNA gene revealed a similarity of 100% between the analysed myxospores and triactinomyxon actinospores. The phylogenetic setting of O. labracis n. sp. shows subgrouping in correlation with tissue tropism, but identifies this parasite as another exception to the main division of Myxosporea into the main freshwater and marine lineages.

The morphology, ultrastructure, genetics, and morphometrics of a species of Diplostomum von Nordmann 1832 (Digenea: Diplostomidae), isolated from the European flounder (Platichthys flesus (L.)) caught off the northwest coast of Portugal, are characterized. The metacercarial stage was found unencysted in the lens capsule of the eye. Light microscopical observations revealed the existence of some variability in specimen shape and size, with two morphotypes, referred to as "round" and "long", being apparent. Scanning electron microscopy revealed a smooth, unarmed tegument, with the lappet region being the most irregular and porose. Both the oral and ventral suckers were provided with a series of papillae, which presented very distinctive ultrastructural features and were particularly conspicuous in the case of the ventral sucker. The two morphotypes detected were found to have 100% genetic correspondence in the 18S + ITS1 + 5.8S region of the rDNA. Since the genetic data for this metacercaria differed from those of the species of Diplostomum available in GenBank, a description of a new genotype (accession number GQ370809) is provided. The molecular phylogenetic analyses, in conjunction with principal components and cluster analyses based on morphometric data, revealed the existence of consistent differences between the Diplostomum sp. metacercariae from flounder compared with Diplostomum spathaceum, Diplostomum mergi, Diplostomum pseudospathaceum, and Diplostomum paracaudum. The latter of these species was found to be the most similar to the present material. Our results do not support an evolutionary separation of the European and North American species of Diplostomum.