Fresh-water Jellyfish in Maine
by Mathew Scott, Fishery Biologist
This article is from the Spring, 1963 issue of "Maine Fish and Game" magazine.
Any sportsman who does a double-take at seeing what looks like a jellyfish in his favorite Maine lake or pond can stop doubting. These animals do occur in Maine. And he needn't worry about being stung, for these fresh-water jellyfish are harmless to all but the tiny organisms they eat.
There have been many reports and references on the fresh-water jellyfish over the past eighty years. It was first found in England in 1880; and since 1908, it has been reported from about thirty of the United States. Previous to this article, it had apparently not been reported from northern New England. Many observations indicate that it is found in several Maine lakes although collections of the jellyfish have been made only from Androscoggin Lake, in Kennebec and Androscoggin counties.
Zoologists call it Craspedacusta sowerbyi, but more commonly it is referred to as the fresh-water jellyfish, or medusa. The jellyfish is an invertebrate, or an "animal without a backbone," and belongs to a group called Coelenterata, which includes animals having a sac-like digestive tract and tentacles with stinging cells. Nearly all of these animals are salt-water species, and the ones shown here are the only fresh-water forms in North America.
The fresh-water jellyfish is a very small animal measuring about one inch in diameter. The outer edge of its bell or hood bears several hundred tentacles of different lengths and sizes. These are used by the jellyfish in capturing its food, which is mostly small plankton organisms. These tiny organisms are entangled in the tentacles and paralyzed by the stinging cells.
Every so often a "population explosion" occurs, and then the jellyfish are found in large numbers in offshore areas. They are moved by the wave action of the water and by their own rather feeble swimming movements. The activity of the jellyfish is an up and down swimming motion by the contraction and expansion of the bell-like hood.
In some years, these creatures are found in abundance, but later they appear to be absent. These occasional eruptions occur when the temperature and water conditions are suitable for such a population outbreak. Skin divers report that large clouds of the animals sometimes occur from the surface down to about twenty feet of water.
There are two stages in the life history of this creature. The first stage is the medusa, which has both male and female individuals. They secrete their sperm and eggs into the water, fertilization occurs, and the eggs fall to the bottom. From them develops a second form called the hydroid.
This is a stage in the life cycle when the animal lives on the bottom and moves very little. The hydroid stage develops and eventually gives rise to the free-swimming medusa stage. At this point, the life history has made one cycle and a new population of medusae occurs.