Fish Cousin of the Week: Moon Jelly

Moon jellies (Aurelia aurita)

Moon jelly in Puerto Rico, Photo courtesy of Brian McGauley.
Moon jelly in Puerto Rico, Photo courtesy of Brian McGauley.

Off on another tangent today, we’re going to talk about a member of the invertebrates, the moon jellyfish. Jellyfish are members of the cnidarian phylum. This phylum (remember: domain, kingdom, phylum, class, order, family, genus, species) contains jellyfish, anemones, corals and box jellies. Moon jellies are a common display jellyfish at most aquariums. Like most cnidarians, the life cycle of a moon jelly switches back between a sessile (stationary) polyp stage and a swimming medusa stage. If you want to see this in person, and get a better handle of the reproductive cycle of jellyfish, Monterey Bay Aquarium has a great exhibit with large-scale models.

Most cnidarians possess a complicated defense system of stinging cells called nematocysts. These bundles contain, essentially, a coiled spring and a poisonous dart. If you’ve ever been stung by a jellyfish, you know how painful these darts can be firsthand. Imagine being a tiny fish and getting stung by one of those. These stinging cells run all along the jellyfish’s tentacles and is how jellyfish catch their dinner. A moon jellyfish’s stomach is divided into 4 compartments. You can see it really well just after they’ve fed. At most aquariums, moon jellies are fed brine shrimp, so their stomach will show up as 4 bright pink semi-circles in the middle of their bell. Brine shrimp are the equivalent of tiny versions of restaurant shrimp, and also known as sea monkeys. (Interested in more information of brine shrimp? If so, check out my senior honors project at URI on the Time Post-Hatch Caloric Value of Aremia Salina)

It is possible to keep moon jellies at home! They need a tank by themselves (no décor either!) with no sharp edges to slice themselves or corners that they can get stuck in. Steady, and not too powerful, water flow is essential and make yourself a surface skimmer to keep their water clean! You can by brine shrimp at most fish supply stores. If you want to go ahead and start a tank, you may want to look into purchasing How to Keep Jellyfish in Aquariums by Chad Widmer. Its more geared towards large-scale aquarium display, but you can modify the instructions for home use.

Mystic Aquarium has a fun fact sheet to check out and impress your friends!

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