Date of Award

May 2022

Degree Type

Thesis

Degree Name

Master of Science

Department

Biological Sciences

First Advisor

John A Berges

Committee Members

John Janssen, Rafael L Rodríguez-Sevilla

Keywords

gut content, Hemimysis, immunochemical methods, invasive species

Abstract

Aquatic invasive species, typically introduced in ballast water, are a concern in many ecosystems. In the Laurentian Great Lakes, the Ponto-Caspian mysid, Hemimysis anomala, has established and is especially abundant in harbor breakwall environments in Lake Michigan, forming large swarms. Predicting the effects of the invader depend on whether Hemimysis is competing for zooplanktonic prey or exploiting other benthic resources. To understand population dynamics (seasonality, size distribution, sex ratios, abundance, etc.) and food web position, weekly to monthly sampling of breakwall environments was conducted using lighted funnel traps in Milwaukee Harbor, WI. In addition to time series sampling, we also sampled from within a swarm to compare individuals in traps to individuals in the swarms. Preserved samples were quantified, body size measured, and sexes determined. In addition, subsamples of at least 6 females, males, and juveniles from each collection were stored at -70°C for gut content analysis using purified antibodies raised against potential prey species (Bosmina longirostris, Bythotrephes longimanus, Cercopagis pengoi, Daphnia mendotae, Daphnia pulex, Keratella cochlearis, Leptodiaptomus ashlandii, Limnocalanus macrurus, and veliger larvae of Dreissena mussels). Samples from mid to late summer indicate a population dominated by juveniles (< 6mm with no obvious secondary sexual characteristics), the majority (80-90%) of adults being males, and very few sexually reproductive females (5-10%). Adult sizes during this period were smaller (average 7 mm, maximum 8.5 mm) than those in winter months or reported in other Great Lakes studies. Late fall and winter samples indicate lower overall abundances composed predominantly of females (55-75%) with fewer males (15-25%) and rarer juveniles (~5%). Individuals were also typically larger in winter samples (average 10 mm, maximum 12 mm). These data suggest that Hemimysis populations in Milwaukee Harbor breakwall environmental show quite distinct characteristics from those described in other Great Lake ecosystems. Gut content revealed that Hemimysis appear to be generalist feeders and all prey items were found in their guts. many individuals (39.7%) had empty guts, most likely due to the bias of the traps with time for Hemimysis to clear their gut before the traps are pulled. Additionally, in the guts that were not empty, the type and abundance of a particular prey item show that there appear to be no differences in dietary differences between sexes, or between adults and juveniles (measured as a proxy of length). There was also no difference in size of individuals present in the swarm, however individuals in the swarm were less likely to consume D. pulex and B. longimanus, but more likely to consume C. pengoi. These results support only the hypotheses related to population dynamics, and do not support the hypotheses related to dietary differences.

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