Early in June, the St. Johns Riverkeeper reported someone seeing “jellyfish” in the St. Johns River near Switzerland Point and sent a picture to support the observation. The “jellyfish” was actually a Comb Jelly, one of the prettiest and most delicate marine creatures that we see in the ocean and the St. Johns River estuary. They are about the size of a walnut, hence their nick name “Sea Walnut”.
The surprise was that it was so far upstream in the St. Johns River. Comb jellies are marine, and their preferred habitat is high salinity. We measure sea water in parts per thousand (ppt) of salt with full strength being 35 ppt. Switzerland Point was 10 ppt the next day, so it was relatively high for that section of the river. Low rainfall in the upper basin coupled with a strong on shore wind had pushed salt water well up stream. They have never been seen in fresh water.
Comb jellies are similar to jellyfish but in a different phylum called Ctenophora or comb bearing.
They are oval-shaped animals with eight rows of tiny comb-like plates of cilia or ctenes, that beat in synchrony as they move through the water. As they swim, the ctenes refract light to produce a shimmering rainbow effect. They are fascinating to observe under a microscope.
A comb jelly is a simple creature with few specialized organs. Most jellies can detect chemical traces in the water that allow them to locate food. These simple animals are omnivore predators that can eat phytoplankton and zooplankton and even other ctenophores. In addition to the combs, some species have two tentacles used for feeding. They do not have stinging cells or nematocysts like jellyfish, but have sticky cells called colloblasts that they use to capture prey. Videos showing them feeding are fascinating to watch as they elegantly spin to catch and capture their food.
They also are equipped with a gravity-sensitive structure, called a statocyst, that provides orientation in the water. A small stone-like statolith sits on 8 tufts of cilia-like nerve endings. Each tuft stimulates the row of combs to beat. So as long as the statolith is pressing on the appropriate tuft, the ciliated combs continue to beat and the ctenophore swims along in an upright orientation. But when waves or something else causes the comb jelly to tilt to one side, the statocysts no longer make contact with that row or rows of combs, they stop beating and the organism slowly rights itself.
One of the more spectacular traits is the bioluminescence of comb jellies. When disturbed at night, the sea walnuts glow soft green. You can pass a dip net through the water from a dock on a dark night and watch the comb jellies shine in the water. We often capture them in nets, return them to the lab and go into a dark room to watch them shimmer.
Such a beautiful and fragile creature here in the St. Johns River. So simple, yet elegant and complex at the same time. And they are 95 percent water.
Glad you asked River Life
Is there any truth to the rumor that you are retiring? Thank you for the question. I will be stepping down as Executive Director of the Jacksonville University Marine Science Research Institute effective August 1st, 2023. I will remain at JU for another year on sabbatical and in an advisory role while I see if I can turn “River Life” into a book along with a few other projects. Stay tuned, I plan to continue writing River Life for the foreseeable future.
River Life runs the first Tuesday of the month in The Florida Times-Union. E-mail A. Quinton White, executive director of Jacksonville University’s Marine Science Research Institute, with questions about our waterways at qwhite@ju.edu. For more on the MSRI, visit ju.edu/msri.
