Transmission of the sealworm, Pseudoterranova decipiens (Krabbe), from invertebrates to fish in an enclosed brackish pond

The life cycle and transmission dynamics of sealworm, Pseudoterranova decipiens (Krabbe), were studied in Wallace Lake, an enclosed brackish pond on Sable Island, Nova Scotia. Abundance of larval sealworm was determined in amphipods (Gammarus spp.), mysids (Neomysis americana S.L. Smith), and threespine (Gasterosteus aculeatus Linnaeus) and fourspine (Apeltes quadracus Mitchill) sticklebacks. Sealworm larvae were more numerous in mysids (0.001-0.004 worms/host) than in amphipods (0-0.0005 worms/host), and P. decipiens abundance in mysids and fourspine stickleback was positively correlated. Amphipods outnumbered mysids in three depth zones in the lake, but density of infected mysids (1.43/m(2)) was much higher than that of infected amphipods (0.22/m(2)) in the vegetated zone where fish were found. Threespine and fourspine stickleback fed mainly on copepods (98% and 57% of diets, respectively), and less frequently on amphipods (1% and 23% of diets, respectively) and mysids (1% and 19% of diets, respectively). Both sticklebacks fed preferentially on mysids (Murdoch's index of preference = 1.49) over amphipods (Murdoch's index = 0.67). In separate feeding experiments, fourspine stickleback ingested 1.3 (+/-1.2) amphipods or 7.7 (+/-2.0) mysids per day. Calculated transmission rates suggest that these fish acquire sealworm from eating infected mysids at a rate of about 1 per month, compared to 1 every 8 yr from amphipods. Abundance of larval sealworm in N. americana on Sable Island Bank (0.002 worms/host) in January 1995 was similar to that in Wallace Lake. Results of this study demonstrate that mysids are more important than amphipods in transmission of sealworm to small benthophagous fishes in an isolated, landlocked brackish-water environment.