Ecology

(8/2019) Fresh water jellyfish have long been a topic of controversy throughout Pennsylvania and Maryland. Many have shared stories and rumors about their existence in our local ponds and lakes, but are they real? Do the stories hold any truths?

I remember the summer days I spent as a child, going to my neighbors’ pond to look for the penny sized jelly fish. I would share my hunting stories at school, but no one would believe me, not even the teacher!

The answer is yes! Freshwater jellyfish, or Craspedacusta sowerbyi, do exist in our ponds, lakes and even some of our rivers! According to the USGS, their presence in Pennsylvania has been recorded as far back as 1957. It is thought that they were originally introduced to the United States in the late 1800’s by the transportation of water hyacinth, or other aquatic vegetation, from its native region in China. Once in the country, polyps and resting bodies were probably translocated accidentally from water body to water body via water fowl, stocked fish and aquatic vegetation.

Today, they can be found in every continent, excluding Antarctica, and almost every state in the United States. Its unnatural wide-spread presence throughout the globe classifies them as an exotic invasive. An exotic invasive is a plant or animal that isn’t naturally found, or native, to a specific area, that usually causes harm to the native ecosystem and balance of the local environment. You may have heard of a few exotic invasive species who have made their way through our area or are about to, such as insects like the spotted lantern fly, emerald ash borer, the brown marmorated stink bug (the brown ones, not the green!), the gypsy moth, the hemlock wooly adelgid, the Japanese beetle and the Asian longhorn beetle. As well as plants and trees like poison hemlock, tree-of-heaven or Ailanthus, Japanese stilt grass, mile-a-minute, kudzu, Japanese honeysuckle, Japanese barberry, English ivy and to most people’s surprise some species of Rubus or raspberries!

But the real question is, are fresh water jellyfish harmful to our native species like so many other exotic invasives are? This question arises because, depending on the situation, exotic species are sometimes intentionally introduced to an area for beneficial reasons. For instance, a tiny, parasitic Asian wasp has been introduced to the east coast to combat the brown marmorated stink bug. Before their release, extensive research was done to assure they wouldn’t harm any of our native stink bugs, who are closely related to the invasive brown marmorated stink bug, or throw any other species off balance. Sometimes entomologists (insect scientists/specialists) are lucky enough to find native insects, diseases or fungi that can combat exotic invasives, but unfortunately that isn’t always the case.

The USDA recognizes that freshwater jellyfish pose no great threat to the ecosystems of the waters that they inhabit, but more research is still being conducted. In fact, they can seen as an "indicator species," because they prefer to live in clean water, thus indicating a clean body of water. Scientists came to this conclusion for several reasons. The first being that their populations are controlled by crayfish! Crayfish are considered their only "important predator" as they feed on the hydromedusa phase. The second reasoning being that they feed on the plentiful zooplankton and copepods. In some cases, they will feed on freshly hatched striped bass or small fish eggs, but they are not considered an important predator to these small fish. They are considered "opportunistic feeders," as they will eat and digest what they can, when they can. To fully understand how they "catch" and digest their prey, it is important to comprehend their life cycle/stages and form.

According to the USDA C. sowerbyi can reproduce sexually and asexually. It all begins with a sexually fertilized egg released by a mature freshwater jellyfish, or "medusa." The egg goes through several stages until it settles and attaches to the ground, metamorphosing into a "polyp." It is here that asexual reproduction can occur, a male frustule larva can grow on the polyp, detach itself, settle in a new location, and metamorphosize new polyps. Otherwise, new female polyps can form on the original parent polyp to form a colony. Medusas are released from the polyps, which is what we see and know as the jellyfish! The medusa continues to mature and then settles to the bottom of the body of water to release eggs. Sexual reproduction between a male and a female is uncommon.

Jellyfish are most commonly related to memories of the beach. Whether it was a fun filled day poking the gelatinous tops of the unfortunate creatures that washed up on the shore, or being stung by their stingers while swimming in the salty water. But wait! Can freshwater jellyfish sting you while swimming in your favorite local pond, lake or river? The answer is no, thank goodness! Freshwater jellyfish tentacles cannot pierce our human skin. They are only poisonous to small fish and tiny macroinvertebrates. Despite lacking many common predator-like characteristics, C. sowerbyi often camouflages itself by secreting a think mucous that catches dirt and debris particles. Both the polyp and medusa stages have "stingers" or nematocysts to catch prey.

During the cold months, polyps overwinter in a special form called "podocasts." They contract into these small bundles and can remain dormant for several months. Together they produce a chitin-like coating that allows them to survive through more etreme conditions than if they were in an active form. Chitin is a polymer that makes up most of an insect’s exoskeleton and is also the primary component in fungus cell walls. Once the extreme conditions pass, the podocasts then metamorphosize into polyps to continue with their life cycle.

To reiderate, freshwater jellyfish, or Craspedacusta sowerbyi, pose no threats to humans, our ecosystem or our wildlife. If anything, they provide species diversity and serve as a food source to our local pond life! So if you don’t believe me, grab your friends, family, children, grandchildren, sister, brother, husband or wife and get out to the closest body of water and go catch yourself some freshwater jellyfish! They will remain a shocking spectacle of our waters for genertions to come.

Strawberry Hill Nature Preserve and Environmental Center is a non-profit environmental education and conservation organization located in the foothills of the Blue Ridge Mountains of south-central Pennsylvania in Fairfield, Pennsylvania. To learn more about all they offer, we encourage you to visit them at 1537 Mt Hope Rd, Fairfield, PA 17320, or visit them on-line at www.strawberryhill.org