What are the mysterious pea-size gelatinous globules along the shoreline of Lake Superior?

Remnants from living organism baffles swimmers and beach goers.

Native zooplankton in Lake Superior called Holopedium gibberum in a handful of sand. Photo credit: Minnesota Sea Grant

Native zooplankton in Lake Superior called Holopedium gibberum in a handful of sand. Photo credit: Minnesota Sea Grant

Michigan Sea Grant and Michigan State University Extension receive numerous questions during August and early September from curious swimmers and beachgoers on Lake Superior. These questions are often regarding the appearance of small round gelatinous globules in the water along the shoreline. Some making these observations think that these are pollutants related to plastics, but the fact is these are remnants from living organisms. In some years, these observations are more numerous than others. These pea-size gelatinous globules are a natural occurrence and are remnants of a native zooplankton in Lake Superior called Holopedium gibberum.

Adult Holopedium live in a large pea-size gelatinous mantle with their legs sticking out allowing them to swim in the water as they feed. It is an omnivorous filter feeder that consumes several species of planktonic algae as it swims upside down. The gelatinous mantle is larger than the body, thus increasing it volume by nearly eight times and helps protect Holopedium from predation. The gelatinous mantle may also help reduce its density and thus retard its rate of sinking.

Like other zooplankton in Lake Superior, they migrate toward the surface at or near sunset and return to deeper water during daylight. As part of the Holopedium life cycle, they leave their protective gelatinous mantle. As a result, these gelatinous mantles are then blown toward shore as they float near the surface. Swimmers usually are the first to encounter these pea-size gelatinous mantles, as they feel them in the water before they see them because these gelatinous mantles are difficult to see.

Holopedium gibberum are usually found in low numbers in all the Great Lakes including many inland lakes. They are also found in northern arctic lakes. Holopedium are absent in winter and spring. Their abundance increases during the summer with maximum population peaks occurring between June and October. As is seen in other zooplankton, they can reproduce both sexually and asexually. Females hatch from overwintering, resting eggs, to produce a sexual generation in the fall that will mate to produce the next generation of overwintering resting eggs.

Why are Holopedium gibberum more abundant in some years compared to others? It may be related to an invasive zooplankton that entered the Great Lakes years ago. This invasive zooplankton is the spiny waterflea (Bythotrephes longimanus), which preys on zooplankton in the Great Lakes. Since Holopedium is of large size when in its gelatinous mantle, the spiny waterflea preys on other smaller zooplankton. This then eliminates competition for Holopedium from other zooplankton for food, thus allowing its populations to grow in greater numbers.

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