Midwater and epibenthic behaviors of Mysis relicta Loven: observations from the Johnson-Sea-Link II submersible in Lake Superior and from a remotely operated vehicle in northern Lake Michigan

Submersible dives provided an opportunity to observe the opossumshrimp. Mysis relicta, in Lake Superior (64°54'N, 67°09'W,depth = 280 m). Observations included probable mating, midwaterand benthic responses, and several swimming modes. While matingthe male and female were joined in a ventral to ventral positionby interlaced thoracic appendages. During the evening ascentor predawn descent mysids actively swam vertically upwards anddownwards using their thoracic appendages which beat metachronally.When escaping from the bottom, mysids thrust their abdomensdownwards and rapidly accelerate directly forwards. The midwaterresponse, significantly slower than the benthic response, wasa jerky laterally undulatory movement which propelled the animalat a 45° angle from the forward motion. In a cruising modeMysis swims with its periopods, parallel to the bottom, at approximatespeeds of 2–5 cm^–1. A remotely operated vehiclewas employed to observe mysids at close range at the sediment–waterinterface at deep-water and nearshore stations in northern LakeMichigan (44°34'N, 87°7'W, depth = 100 m, Algoma, Wisconsinharbor, depth = 20 m) during May 23–25, 1987. At the 20m inshore station mysids occurred in significant numbers duringthe day in very bright light. Mysid swimming behavior on thebottom at deep-water stations significantly fashioned the sedimentlandscape, a potentially important form of superficial bioturbation.