Freshwater Jellyfish

Freshwater Jellyfish

Did you know that some British Columbia lakes have jellyfish? Freshwater jellyfish (Craspedacusta sp.), are an invasive species, originating from the Yangtze catchment area in China, that have been around for a few decades. They are now found worldwide (except Antarctica) and have been recorded sporadically in BC since 1990 in a number of lakes near Victoria, Vancouver, Nanaimo, and further north on both Vancouver Island and the Lower Mainland.

Freshwater jellyfish have a varied life cycle, comprising three primary stages: egg, polyp, and medusa. Two kinds of larvae and a cyst stage also form. Some life cycle stages grow at the lake bottom on submerged wood and stones and are often cryptic and rarely recorded. Eggs are produced by female medusae. If fertilized, each egg hatches into a tiny, flat larva called a planula. It swims for a few days among zooplankton before settling down on underwater detritus. The planula then becomes a polyp, a small cylindrical or tube-shaped organism with a mouth and tentacles at one end (complete with stinging nematocysts to capture prey), and the opposite end attached to the substrate. Asexual reproduction results in the formation of buds, which eventually become polyps and form a polyp colony.

Occasionally, jellyfish polyps form a detachable bud. Each of these side buds develops into a tiny, cigar-shaped larva called a frustule. The frustule frees itself from its parent polyp and either crawls a few inches away or is carried off by water flow. It then settles down to become a polyp itself and produce a new polyp colony.

When full grown, the medusa has a nearly transparent body, called a bell, that can be up to 30mm across (but usually smaller) and dangles long, hair like tentacles that we all associate with jellyfish. Sometimes just female medusae form and sometimes just males. Only rarely in North America and Europe do both male and female medusae appear together.

The medusae live for only a few weeks, release eggs, and die. The polyps can live from spring until fall, and then in winter they contract and become podocysts in order to survive cold temperatures. Transport of podocysts by aquatic plants, animals, or birds could contribute to the spread of freshwater jellyfish. When the weather warms, the podocysts develop into polyps to continue the cycle.

Both polyps and medusae feed on zooplankton. The polyps eat protozoans, rotifers, copepods, and cladocerans. The medusae can use cnidocytes in their tentacles to sting these same critters and capture even larger prey, such as water mites and insect midge larvae. Only rarely do they stun newly hatched fish fry and they are completely harmless to humans.

There is much more to be learned about the freshwater jellyfish in BC including their distribution, development, and role in local food webs. Researchers need your assistance with spotting them. If you have seen them somewhere in BC freshwater systems, please contact Florian Lüeskow (University of British Columbia, Department of Earth, Ocean, and Atmospheric Sciences) at flueskow@eoas.ubc.ca or 236-818-6221.

Sources:
Florian Lüeskow, M.Sc, Ph.D. candidate
LakeLife, BCLSS Newsletter Volume 12, Issue 3 (2009)
Terry L. Peard, Ph.D www.freshwaterjellyfish.org

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