freshwater jellyfishDid you know there are JELLYFISH in Bays Mountain Lake?  Today our Nature Study class visited Bays Mountain in Kingsport, Tennessee, to learn about freshwater jellyfish.  We even convinced a good-natured naturalist to take us on a barge ride around the lake to investigate!  How in the world did they get there, you ask?  Read on…..

OBJECT LESSON:

Although we call them freshwater jellyfish, some argue that the Craspedacusta sowerbii is more closely related to the Hydra family than a “true” jellyfish, so many naturalists and scientists just call them Freshwater Jellies.  The main difference between jellies and “true” jellyfish is the presence of a velum — a think circular membrane around the cap that helps propel the jelly forward.  Freshwater jellies are transparent, gelatinous, umbrella-shaped creatures with a whorl of stringy tentacles around the edge of their bodies.  Microscopic barbs called nematocysts run along the tentacles to help capture food and protect the jelly from predators.  Luckily for us, jellies are quite small — adults are the size of a quarter — and their stings can’t even penetrate our skin.

Jellies have no head or skeleton, and contain no special organs for respiration or excretion.  Their bodies are 99% water.  Their large, flat reproductive organs are the only parts of the freshwater jelly that are not translucent, and this makes them easy to spot on sunny days when they tend to surface in large groups called “blooms.”  They eat tiny microscopic animals called zooplankton that are found floating in the water.  When they come into contact with prey, stinging cells in the jelly’s tentacles paralyze the organisms and then sway about moving the captured prey its mouth which is in the middle of the underside.  The bell (body or umbrella part) of the jelly goes through several contractions to move the prey into the mouth and digestive cavity.  The primary predators of freshwater jellies are crayfish and turtles.

Freshwater jellies have a multi-stage life cycle that includes two forms — the polyp and the medusa.  The medusa form is more familiar, and it is during this adult stage that jellies reproduce sexually and fertilized eggs that develop into larvae detach from the medusa and drift away.  This larvae form, now called polyps, will also reproduce — but it will do so asexually by dividing from one another.  This is called “budding,” and the buds will then develop into adult medusa and the cycle repeats.  During the winter the polyps contract and become “resting bodies” that are capable of surviving the cold temperatures.  They are circular and fairly flat with no hooks or burrs, but there does seem to be some adhesive property which allows them to become attached to surfaces.  Some scientists believe that the resting bodies are one way in which the jellyfish are transported — on aquatic plants, by aquatic animals, or on the feet of birds.  When conditions become favorable and temperatures rise, the resting bodies develop into polyps and the life cycle continues.

Freshwater jellyfish can be found all over the world, but are not native to the United States.  There are two schools of thought on their origins.  Some believe they originated in the upper Yangtze River basin in China, and were first observed in the western world in water lily ponds in London’s Regents Park in the late 1800’s — having presumably arrived as polyps on the plants imported from China.  Others say they originated in South America and made their way to the US in the bilge water of ships.  However they got here, jellies spread rapidly and are now found all over the US in every state except North and South Dakota, Montana, Wyoming, Alaska and Hawaii.  Why might this be?  (Too cold in most of these places, and Hawaii just doesn’t have enough fresh water)

I asked the class how jellies might have gotten into Bays Mountain Lake.  They can enter a body of water in many ways — in the water of bait buckets, through flooding from another site, on boat propellers.  But we decided the most probable answer may be that they were carried in on the feet of migrating birds during the “resting body” stage of development.

The appearance of jellies is sporadic and unpredictable.  Often they will appear in a body of water in large numbers even though they were never reported there before.  The following year they may be absent and may not reappear for several years.  It’s also possible for jellies to appear once and never appear in that body of water again.  In some lakes they appear almost every single year.  Why this difference?  What triggers the appearance of medusa in certain places and at certain times?  Temperature seems to play some role in triggering the medusa stage, which is why there is a freshwater jellyfish season, typically from August to September when water temperatures are highest.

FIELD WORK:

Searching for jellyfish can be a fruitless venture, but the best time of year is NOW, and they are most likely to be found in calm bodies of water, which they prefer to rocky, fast-flowing rivers and streams.  After walking across the dam and peering into the deepest part of the lake (which is what jellies usually prefer) we were unable to locate a single specimen.  So Ranger Bob (sans the ponytail and facial hair he has sported for years) graciously treated us to a long ride on the barge so we could explore further.  While we didn’t encounter any jellies (none of the naturalists we spoke with have seen them this year), we did see beaver dams, water lilies and lots of dragonflies.  Definitely time well spent!

WRAP-UP: