Abstract:The acanthocephalan Echinorhynchus salmonis Müller, 1784 is a common parasite of salmonid fish, but it has rarely been reported from an intermediate host. Samples of benthic amphipods, Monoporeia affinis (Lindström), were taken from multiple, deep sites (usually below 70 m) in the Gulf of Bothnia over the course of more than a decade and examined for acanthocephalans. Overall, only 0.44% of 23 296 amphipods were infected, all with just a single worm. This prevalence is consistent with several previous reports of acanthocephalans in deep-water, benthic amphipods, but it appears low compared to that often reported for acanthocephalan species infecting littoral amphipods. Parasite occurrence did not exhibit a clear regional pattern (i.e. northern vs southern sites) nor did it have any relationship with site depth. At sites sampled over multiple years, parasite abundance was consistently low (mostly < 0.01), though two spikes in abundance (over 0.06) were also observed, indicating that infection can be substantially higher at particular times or in particular places. The median density of E. salmonis in samples containing the parasite was estimated as 8.4 cystacanths per m 2 .

The acanthocephalan Echinorhynchus bothniensis Zdzitowiecki and Valtonen, 1987 differs from most other species in the genus Echinorhynchus Zoega in Müller, 1776 by infecting mysids (order Mysida) instead of amphipods (order Amphipoda) as intermediate hosts. Here we report on the occurrence of E. bothniensis in mysids (Mysis segerstralei Audzijonytė et Väinölä) and in its fish definitive hosts in a high Arctic lake. Out of 15 907 sampled mysids, 4.8% were infected with a mean intensity of 1.05 worms (range 1-5), although there was notable variation between samples taken in different years and sites. Larger mysids appear more likely to be infected. Of five fish species sampled, charr,Salvelinus alpinus (Linnaeus), and a benthic-feeding whitefish morph, Coregonus lavaretus (Linnaeus), were the most heavily infected (mean abundances of 80 and 15, respectively). The adult parasite population in fish exhibited a female-biased sex ratio (1.78 : 1). Although E. bothniensis is rather unique in infecting mysids, many aspects of its natural history mirror that of other acanthocephalan species.Keywords: body size, Echinorhynchidae, intermediate host, Mysis, sex ratio, thorny-headed worms Mysids (order Mysida) are known to be the intermediate hosts of some trophically-transmitted helminths (Smith and Lankester 1979, Prychitko and Nero 1983, Marcogliese and Burt 1993, including the acanthocephalans Echinorhynchus bothniensis Zdzitowiecki et Valtonen, 1987 and E. leidyi Van Cleave, 1924. These acanthocephalans differ from their close relatives by using mysids and not amphipods as intermediate hosts (Schmidt 1985). Valtonen et al. (1983) found that amphipods could be infected with eggs from female acanthocephalans from Baltic cod, Gadus morhua Linnaeus, which were later identified as E. gadi Zoega in Müller, 1776, whereas experimental infection of amphipods with eggs of acanthocephalans from smelt, Osmerus eperlanus (Linnaeus), failed.This led Zdzitowiecki and Valtonen (1987) to describe the acanthocephalans from smelt in the northern part of the Bothnian Bay as a new species, E. bothniensis. Using allozymes, Väinölä et al. (1994) confirmed that E. bothniensis was genetically distinct from other species of Echinorhynchus Zoega in Müller, 1776 in northern Europe. Valtonen and Crompton (1990) found E. bothniensis in 14 fish species, although it matured in only six of these, most commonly smelt and burbot, Lota lota (Linnaeus). Here, we report the occurrence of E. bothniensis in its intermediate and definitive hosts in a high Arctic lake, Lake Pulmankijärvi, with the goal of describing the natural history of this species and comparing it to other acanthocephalan species. A cursory description of some of these data were reported by Aura et al. (1990) in a conference abstract. MATERIALS AND METHODS Study siteLake Pulmankijärvi is a high Arctic, oligotrophic lake on the border between Finland and Norway (69°59'N; 28°00'E). Its size is 12.1 km 2 , with a mean and maximum depth of 6.7 and 34 m, respectively. Waters flow to...