Strange Creatures in Missouri Lakes
Freshwater Jellyfish
Craspedacusta sowerbyi, the freshwater jellyfish. Photo from USGS
Last summer, one of our field crews captured a freshwater jellyfish and brought it back to our lab. While we knew jellyfish existed in Missouri, we didn’t know how common they are. And don’t worry, they pose no threat to humans.
Freshwater jellyfish, or Craspedacusta sowerbyi, are originally from China. They were likely transported out of China via ornamental aquatic plants sometime in the late 1800s. The specimen we had in our lab was only about the size of my pinky fingernail, though they can grow to approximately an inch wide. Much of the life-cycle is spent as a polyp, attached to something in the lake. Individual polyps will often replicate themselves to form colonies. Eventually, some environmental cue (e.g. temperature of 77º F) triggers the polyps to metamorphose into medusae (plural, singular = medusa), the jellyfish form we’re familiar with. At this point the jellyfish is mobile and can move about in the water column. While in this form, freshwater jellyfish are capable of sexual reproduction, however most populations in the U.S. are comprised of all male or all female individuals, forcing them to only reproduce asexually in the polyp stage. To survive the winter, polyps will contract and harden into egg-like podocysts. When the lake water warms again, the podocysts will revert to polyp form.
If you see a freshwater jellyfish, look around for more. They often will metamorphose into medusae in large numbers, an event sometimes called a “bloom.” Once in the medusa form, they live for a few weeks and then die. In lakes where they have been observed, the free-swimming medusae do not bloom every year. In some cases it may be more than a decade between jellyfish blooms. Look for freshwater jellyfish in the summer and fall, when the lake water is warm.
More information about freshwater jellyfish available at USGS and freshwaterjellyfish.org
Bryozoans
Bryozoan colony, Jack Bay photo
The colonial bryozoan, also called “a moss animal” can appear like a slimy football in the water and is typically attached to a branch or other structure. While each organism is approximately 1 mm long (roughly the thickness of a dime) the colonies of some species can become basketball-sized or larger. While they are functionally similar to coral, bryozoans are unrelated. Each individual has tentacles that slide out of the colony mass to filter algae and protozoans from the water. Bryozoans reproduce both asexually (via budding) and sexually. Interestingly, all freshwater bryozoans are hermaphroditic, meaning they have both male and female reproductive organs.
In harsh conditions, bryozoans produce resting cells, called statoblasts, that settle to the bottom of the lake to mature when conditions for survival improve. Statoblasts have tiny hooks that can latch onto a bird or other animal and be relocated to another, potentially more favorable location. When our lakes cool in the fall bryozoan colonies will sometimes wash ashore. We get a couple of calls each year at this time from confused lake residents who have discovered a colony. Colonies are slimy, but firm to the touch and will eventually dissolve in the water. Statoblasts produced in the previous year will establish new colonies in the spring.
Unionid Mussels
Lampsilis reeveiana lure. Some mussel species use a lure that resembles a small fish or an aquatic insect to aid reproduction. Photograph by Douglas K. Hardesty, U.S. Geological Survey.
Mussels are common in streams and lakes across Missouri, and just about everyone who has spent any time outside has seen one. However, did you know that most freshwater mussel species are parasites for a portion of their lives?
The microscopic larvae of newly developed Unionoid mussels (called glochidia) are held inside the female parent until they can be transferred to a temporary host. Many mussel species have adapted techniques to bring the host closer and improve the odds of the glochidia reaching the host. At the heart of these techniques is to convince the host to eat the glochidia. This is where the world of mussels gets weird.
Some mussels release their glochidia in packets that resemble hosts’ prey items. These packets float above or dangle downstream of the parent mussel on a strand of mucus. A host eats the packet and the glochidia are delivered to the gills. Other species have developed tissue that resembles prey. For example, one species of Lampsilis mussel has section of mantle tissue that looks remarkably like a darter (stream fish) . The Lampsilis mussel will twitch this tissue to entice an attack from a largemouth bass. When the bass strikes, the Lampsilis immediately releases thousands of glochidia.
Once inside their host, the parasitic glochidia will clamp down on the gill filaments, where they will remain for days to weeks before falling off. Once off the host, the young mussels will settle to the bottom and begin the years-long process of maturing into adults.
Mussels frequently have very specific requirements of the hosts for their glochidia. Mussel populations are in trouble if their host fish is eliminated. It is estimated that 70 percent of mussel species in North America are either extinct or imperiled.