Seasonal population characteristics and gut contents of the invasive mysid, Hemimysis anomala, in Milwaukee Harbor, Lake Michigan

Abstract

The invasive mysid, Hemimysis anomala, is abundant along harbor breakwalls in Lake Michigan, forming large swarms, but we know little of its population dynamics or diet in these novel environments. We examined abundance, body length, and sex/developmental stage in animals collected using lighted funnel traps in Milwaukee Harbor, WI, from summer 2021 to spring 2022. We applied novel immunochemical analysis to examine gut contents using purified polyclonal antisera raised against nine putative zooplankton prey (Bosmina longirostris, Bythotrephes longimanus, Cercopagis pengoi, Daphnia galataea mendotae, Daphnia pulex, Dreissena veliger larvae, Keratella cochlearis, Leptodiaptomus ashlandii, and Limnocalanus macrurus). Population dynamics differed from those described in other Great Lakes; mid-late summer populations were dominated by juveniles (< 6 mm), and small (∼7 mm) adult males (80–90 %) with few reproductive females (5–10 %). Males did not die after reproducing and adults grew through winter (0.011–0.015 mm d⁻¹), reaching 10–12 mm by early spring. Immunochemical gut analyses showed Hemimysis to be generalist feeders, consuming all nine species tested. Most trapped individuals (58 %) had empty guts, suggesting that Hemimysis cleared their guts before recovery. No differences in diet were seen among sexes/developmental stages, or across seasons. We also compared Hemimysis directly sampled from pelagic swarms, to those recovered from benthic traps. There were no differences in size or sex/developmental stage of individuals between traps and swarm populations, refuting the idea that swarming serves a reproductive function, but there were differences in prey found in guts between individuals in swarms and from traps, suggesting swarming is associated with feeding.