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Spinitectus inermis (Zeder, 1800) is a rare specific nematode of the European eel, Anguilla anguilla (L.), that ranges widely in Europe (North, Baltic and Mediterranean Seas basins) but occurs only locally, particularly in lakes, fishponds and in the lowland reaches of rivers (Moravec F. 1977: Folia Parasitol. 24: 85; Moravec F. 1979: Acta Soc. Zool. Bohemoslov. 43: 35-42; Moravec F. 1994: Parasitic Nematodes of Freshwater Fishes of Europe. Academia, Prague, 473 pp.). This nematode has been studied since the beginning of the 19 century but its morphology was little known until the redescription by Moravec (1979, op. cit.) plus additional data revealed by scanning electron microscopy (Moravec F. 1996: Parasite 3: 169-174). The knowledge of the biology of this nematode is also scarce. Recently, however, it was found that nymphs of several species of mayflies (Ephemeroptera) were suitable experimental intermediate hosts of this nematode (Saraiva A.M., Moravec F., Pereira A., Cruz C. 2002: Folia Parasitol. 49: 118-126). Since S. inermis is an abundant parasite of eels in the Sousa River, Portugal, its seasonal occurrence and maturation were followed and the results are presented herein. The population dynamics of S. inermis was followed in eels from the lowest reaches of the Sousa River, a tributary of the Douro River, from the locality of Senhora do Salto, situated about 20 km east of Porto, northern Portugal. Monthly samples were taken by electrofishing from November 1998 to October 1999 (total sample size 326; mean body length of fish 22.0 ± 4.42 cm). Fish were examined for parasites and the recovered nematodes were washed in physiological saline (0.9%), fixed in hot 70% ethanol, stored in 70% ethanol and cleared in glycerol for light microscopical examination for sex and maturity stage determination. The occurrence of nematodes, sex ratio and stages of development of females (gravid females with mature eggs / females in other developmental stages) throughout the year were compared by Chi-square test (Siegel S., Castellan N.J. Jr. 1989: Nonparametric Statistics for the Behavioral Sciences. McGraw-Hill Book Company, New York, 399 pp.). The prevalence and mean intensity of infection of S. inermis in eels during the studied period is shown in Fig. 1. S. inermis was present throughout the study period. Significant difference in the occurrence of this parasite (χ = 32.81, d.f. = 11, p < 0.05) was found during the year. Prevalence reached high values (30-57%) from November to February and low values (10% and 13%) in June and July. Intensity ranged from 1 to 26. The high values of mean intensity occurred in January (9) and June (14), with the minimum value (2) in December. Adults (males and females) of S. inermis occurred in eels Fig. 1. Monthly changes in the prevalence and intensity of Spinitectus inermis in eels from the Sousa River from November 1998 to October 1999. Fish sample size in parentheses.

Obseravtions on the occurrence and maturation of Spinitectus inermis (Nematoda: Cystidicolidae) in the Sousa River, Portugal