Hydromedusae form the largest group of cnidarians and are generally smaller then the true jellyfish (typically only a few millimeters to centimenters at maximum size). Hydromedusae are distinquished from true jellyfish by prodcing their eggs and sperm under the bell, but on the outside of the animal, while true jellyfish produce eggs inside the gut.
Cnidarians can be found all the way from the surface to the bottom of the oceans, although most species have a specific range of depth over which they are found. Within the water column, hydromedusae they can be common, frequently being a large but variable proportion of the volume of zooplankton collected by nets. All Hydromedusae are predatory and eat smaller zooplankton. Some species have relatively few predators, while others are preyed on heavily by other cnidarians, and fish such as mackerel and chum salmon.
Like all cnidarians, they swim by contracting their bell which pushes them forward. This swimming activity whips the tentacles around the bell through the water where they may hit food. Longer tentacles are pulled along behind the animals like sticky fishing line. Once stung, prey are passed to the mouth and eaten whole.
Most hydromedusae are part of a life cycle that alternates between a bottom-dwelling "vegatative" colonial stage that will bud off medusae and a pelagic "reproductive" stage that releases either eggs or sperm. The fertilized eggs then attach to the bottom to complete the cycle. Some species have eliminated the benthic phase, and simply bud off new individuals or have their colonial phase within the plankton.
We know little for sure about how long most species live in the Arctic, but most are throught to have an annual life-cycle. There are ~70 species of cnidarians with prominent planktonic phases known to exist in the arctic.
Page Author: Russ Hopcroft
Updated: Aug 20, 2010