Monitoring Monday – Freshwater Jellyfish???

Did you hear about the kid who found a jellyfish in Stumpy Meadows Reservoir, or the woman who saw some jellyfish in the San Joaquin River? Those aren’t urban myths. California does have freshwater jellyfish.

Craspedacusta sowerbii, also known as the peach blossom jellyfish or simply just freshwater jellyfish, is the only known species of freshwater jellyfish in North America.  First found in the United States in 1880, it appears to have been introduced into North America from South America or the West Indies and is originally from the Yangtze River in Southeast Asia. It has been noted in 45 US states and several Canadian provinces. Globally they have spread over much of Asia, Europe, South and North America, Australia, and some parts of Africa.

Within California freshwater jellyfish have been reported in: Aliso-San Onofre; Butte Creek; Central California Coastal; Central Coastal; Clear Creek-Sacramento River; Cow Creek; Coyote; Fresno River; Honcut Headwaters-Lower Feather; Lower American; Lower Klamath; Lower Sacramento; Middle Fork Feather; Middle San Joaquin-Lower Chowchilla; Monterey Bay; North Fork American; Paynes Creek-Sacramento River; Russian; Sacramento Headwaters; San Pablo Bay; Suisun Bay; Tulare Lake Bed; Upper Bear; Upper Cosumnes; Upper Dry; Upper Eel; Upper Merced; Upper Mokelumne; Upper Putah; Upper San Joaquin; Upper Stanislaus; Upper Tuolumne; Upper Yuba

These tiny penny-sized delicate creatures are gentle swimmers and cannot tolerate much of a current, so they usually occur in calm or standing waters. Thought to be common, C. sowerbyi are most often found in ponds, lakes, reservoirs, slow-moving rivers and quiet or sluggish pools next to flowing water. Yet freshwater jellyfish are rarely spotted. There are a few reasons for this. For one, these creatures are tiny, having a bell that only ranges from from 5 to 22 millimeters in diameter and by weight, the jellyfish is more than 94 percent water. This makes them difficult to spot unless they are clustering together in what are called “blooms”.

Secondly, we are only seeing one stage of a multi-stage life cycle. The tentacled saucers are called “medusae” and are the sexually reproductive stage of the life cycle. During other life stages, freshwater jellyfish are virtually microscopic and impossible for a casual observer to detect.

The C. sowerbyi life cycle consists of four main stages: podocysts, polyps, frustules, and medusae. They are also able to reproduce both sexually and asexually.  Medusae are the sexual stage of the life cycle and can produce eggs and sperm that fuse to form planulae, which a type of larvae that can develop into polyps or podocysts. Fertilized eggs grow into ciliated planulae (larvae), which then settle and metamorphose into the polyp form.  Polyps are sessile forms that attach to stable surfaces and feed on plankton. Polyps can reproduce asexually by budding off frustules, which are small larvae that can swim freely or settle and form new polyps. They can also produce medusae, which are the bell-shaped forms that most people recognize as jellyfish. Podocysts are microscopic resting bodies that can form from polyps or medusae under unfavorable conditions, such as low temperature, drought, or pollution. They can remain dormant for years until they encounter suitable conditions to hatch into polyps.

This dormancy can make a lake seem like jellyfish are absent and only when jellyfish blooms are they seen. These blooms are thought to be temperature dependent, requiring water of at least 25° C, and are most common in summer and fall.  Other factors that may affect hydromedusa blooms include zooplankton populations, alkalinity, and calcium carbonate.

Initially, C. sowerbyi was probably transported with ornamental aquatic plants. Considering that the polyp and podocyst stages of this animal adheres to objects such as submerged plants, it is easy to see how it could have been imported to Europe, North America, and elsewhere as a hidden passenger on ornamental aquatic plants or translocated accidentally with fish stocking. The podocyst resting stage, like tiny seeds, may also be easily transported from place to place by migrating waterfowl, or watercraft used for recreational activities. While cold water temperatures have acted as a historical check on their growth and expansion, warming temperatures around the globe appear to be helping their range expansion.

The impact of this widespread jellyfish is unclear. C. sowerbyi are not considered dangerous to humans.  Although its stings can paralyze macroinvertebrates and small fish, its small nematocysts are not likely to penetrate human skin. Like other cnidarians, C. sowerbyi is an opportunistic predator, feeding on small organisms (zooplankton, eggs, aquatic larvae) that come within its reach and could impact local biological communities by competing with native species for food.

The spread of C. sowerbyi reminds us how easy it is to accidentally transport unwanted aquatic organisms. Never dump aquarium contents, ornamental aquatic plants, or bait into native waters and when going from one body of water to another to clean, drain and dry aquatic equipment and watercraft.