Jellyfish
Craspedacusta sowerbii
On the topic of jellyfish in general, even a small amount of research produces a sense of awe for these small creatures. According to the website oneearth.org there are about 300,00 species of jellyfish and they have existed for some 700 million years. Comprised mainly of water and with no brain, their existence is summed up by "oneearth.org" in the following text excerpt:
“Before the dinosaurs, trees, or even fungi, there were jellyfish. They are the oldest multi-organ animal, surviving all five of Earth’s mass extinction events. “
Previously on this website information concerning the freshwater jellyfish in Hotel Lake was limited to an October 2020 article in the Harbour Spiel which you can now find in the library by clicking here.
This webpage presents information on this small jellyfish called Craspedacusta sowerbii and delves into its origin, spread, and life cycle as well as an update on current research efforts to understand its impact on the ecosystem.
In BC, sightings of freshwater jellyfish in lakes have been concentrated in the Lower Mainland and Vancouver Island areas. On the Sunshine Coast, the first reports of Craspedacusta sowerbii in Hotel Lake date back to 2001.
Hotel Lake Jellyfish video taken in July 2021
The freshwater jellyfish with the scientific name, Craspedacusta sowerbii, is now thought to inhabit every continent except Antarctica. This jellyfish is indigenous to the Yangtze River Valley in China and experts theorize that it has made its way around the world on the hulls of boats, through aquarium dumping, in bait buckets and by hitching rides on stocked fish, aquatic plants, and waterfowl.
The diagram below, of Craspedacusta sowerbii, is from plos.org. PLOS is a nonprofit, open Access publisher empowering researchers to accelerate progress in science and medicine by leading a transformation in research communication.
Freshwater jellyfish have three major stages: egg, polyp, and medusa.
Except for the warmest summer months, the freshwater jellyfish exists as a polyp found underwater attached to vegetation, rocks or even tree stumps. Often existing in small colonies, they are cylindrical in shape and look much like like tiny bowling pins. These “bowling pins” have one end fixed to the ground, and the other end extending up into the water with a ring of tentacles surrounding its mouth/anus. In this phase, the jellyfish polyp has a fully developed digestive system and is able to catch prey and feed itself efficiently.
In the coldest months of the year, or in other harsh environmental conditions such as drought, polyps can shrink into tiny dome shaped bodies known as podocysts. This resting, dormant stage allows jellyfish to survive detrimental conditions for lengthy periods and rebound when circumstances become more favourable.
What we see in Hotel Lake in the warmth of July, August and September is the medusa, a whitish, translucent bell that is approximately one inch in diameter. Composed of at least 90% water the medusa have no lungs, heart or brain. Oxygen is absorbed through their thin skin. They have no blood for a heart to pump. The nerve net found just below the epidermis allows them to respond to changes in their environment. A group of jellyfish, which can be in the thousands, is called a bloom, a swarm or a smack of jellyfish.
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Sexual reproduction happens when medusae release sperm and eggs into the water. Asexual reproduction is more common and occurs when polyps “bud”. The complexity and adaptability of this life cycle are a significant part of the reason that the jellyfish has been such a survivor.
The medusa feeds on zooplankton including water fleas and copepods. To catch and digest this prey, the jellyfish has tentacles lining the rim of the bell and a large stomach structure (manubrium) hanging down from the centre of the inside of the bell with a mouth-opening and four frilly lips. The many tentacles each contain thousands of cells called cnidocytes which are the source of barbs and venom. Dangling its tentacles, the jellyfish drifts along until prey touches a tentacle.The prey is then stung, paralyzed, coiled by a tentacle and brought to the mouth. Food is taken into the mouth opening, digested in the stomach and waste is finally expelled through the same opening. This sounds like a water fleas’ worst nightmare.
So, if freshwater jellyfish eat zooplankton, what eats jellyfish? It would appear that fish avoid jellyfish. Turtles and crayfish are mentioned as the only interested predators.
The tentacle’s stinging cells are used for paralyzing very tiny prey and have not been proven to have the capacity to pierce human skin. Recently concern has been expressed for allowing freshwater jellyfish to get close to our eyes (e.g accidentally being scooped up in googles) or near open wounds as has happened in a couple of incidents. On Vancouver Island, two children needed to go the hospital and undergo a week or so of antibiotic treatment.
Science and Jellyfish
Craspedacusta sowerbii and other freshwater jellyfish hold a place of special interest for the scientific community. Jellyfish are known as an indicator species, meaning changes in their populations represent greater changes in the ecosystem. Around the world today, jellyfish are thriving in record numbers.
A local zoologist Dr. Florian L​üskow, became interested in freshwater jellyfish when they were reported in Killarney Lake near Victoria. The aim of his research is to understand the distribution and impacts of the jellyfish on lake ecosystems in B.C. An initial study analyzed polyp and medusa responses to different temperatures. You can his read his article: “Increasing Temperature Facilitates Polyp Spreading and Medusa Appearance of the Invasive Hydrozoan Craspedacusta sowerbii “ by clicking here.
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Recognizing that medusae bloom in the warmth of July, August and even September the possibility exists that climate change will affect both life cycle stages and their food web impacts. Luskow surmises that increases in temperature due to environmental warming “will favour the expansion of the species in the future to higher latitudes.”
Many researchers emphasize how much can be learned from citizen scientists who document their findings. Florian Luskow asks if you find medusae to email him at flueskow@eoas.ubc.ca. Observers are asked to share the place, time, conditions and if possible, even a picture of the jellies.
Researchers lament that many questions remain and much is not known. Many articles end with the thought that “The ecological impacts of this non-native species are unclear”. Some of the questions revolve around the concerns that non-native species with few predators can outcompete the local residents for food. If these jellyfish were to cause a major decline in planktonic algae, daphnia, and other organisms that form the basis of aquatic food chains, they might indeed have an indirect impact on populations of fish and other larger animals.
