Jellyfish in our Lakes
(September 2008)
Freshwater jellyfish can grow bigger than this, but only to the size of a quarter.
Photo Source: US Geological Survey.
Though seldom encountered, freshwater jellyfish (Craspedacusta sowerbyi) do exist and actually inhabit some of
New Hampshire’s lakes! In fact, freshwater jellyfish have been documented in most of the states (the exceptions are
Montana, Wyoming, the Dakotas, and Alaska), and it is a global species found on five of the seven continents (not Africa
or Antarctica).
Freshwater jellyfish are not commonly seen because: they are tiny and typically grow only up to the size of a quarter; their
occurrence is somewhat inexplicably random since a swarm of jellyfish, called a “bloom,” may be seen in a lake one year
and then never seen in the same lake again, and; the species does not always look like an obvious jellyfish, so you cannot
easily determine if it is present in a lake or not.
To be more specific, jellyfish have two phases in their life cycle: the “polyp” phase and the “medusa” phase. The jellyfish
that we normally think of as a free-swimming, tentacled, and bell-shaped animal is called the “medusa.” But, like many
members of the Hydrozoa (a class of animals that includes Portuguese-Man-of-War and fire coral), freshwater jellyfish
are mostly sedentary beings, attached to underwater rocks, plants, or downed wood in their “polyp” phase, where this
microscopic form of a jellyfish looks more like an itty-bitty plant stalk than an animal. The polyps can asexually
reproduce, meaning they bud out new polyps and form colonies, stinging and capturing tiny invertebrates for food (they
are too small to sting humans, so don’t worry). The polyps become dormant in the winter, able to survive just enough to
possibly spring to new life the following warm season. It is thought that in this stage of dormancy, the resting jellyfish
polyps are able to hitch a free ride on stocked fish, the feet of waterfowl, or on plants that are moved around. In this way,
the jellyfish can get transported to other waterbodies where a new colony of jellyfish polyps can form. Actually, the
United States Geological Survey contends that freshwater jellyfish originated in China, catching a ride around the world
on ornamental aquatic plants.
How does one phase of the jellyfish’s life lead to the other? The polyps sprout out a little bud that becomes the medusa
form of the jellyfish. It is this more familiar form of a gracefully swimming, gelatinous, alien-like creature that is able to
reproduce sexually. Jellyfish eggs grow into a kind of larva and then into polyps to begin their life cycle anew. However,
New Hampshire Lakes Association
603.226.0299
[email protected]
www.nhlakes.org
jellyfish medusae in a single lake are often either all male or all female. This means it is very likely that a “bloom” of
jellyfish is made up of genetically-identical clones that cannot make fertile eggs. Sexual reproduction, the cultivator of
biological diversity, has never been confirmed in the wild.
So far, there are approximately 25 lakes in New Hampshire that have had freshwater jellyfish sightings, so this means you
might be the first to see them in another lake! If you see what you suspect is a freshwater jellyfish, contact the New
Hampshire Department of Environmental Services with information about the sighting, including the lake/pond name,
town, date, plus any other pertinent information (email [email protected] or call (603) 271-0698.)
To learn more about freshwater jellyfish, visit freshwaterjellyfish.org/history.html.
A microscopic “polyp” budding a “medusa,” the mobile form of a jellyfish.
Photo Source: US Geological Survey.
New Hampshire Lakes Association
603.226.0299
[email protected]
www.nhlakes.org