Spring is here and the harbor is truly coming alive—boats are returning to their moorings, people are creeping out to bask in the sunshine and the hydromedusae are blooming. This is a hallmark of spring that aquarist Jackie Anderson always looks forward to!

Jackie collects a few hydromedusae from Boston Harbor and uses the pitcher to transport them back to the holding tanks behind the scenes at the Aquarium.

Jackie is one of the people who takes care of the jellies that you see in the Aquarium. She's also the force behind one of our newest exhibits—the fleeting hydromedusae display near the Edge of the Sea tidepool touch area.

Jackie inspects the tiny hydro medusa recently collected from harbor waters

Hydromedusae belong to a group of animals that are classified as Cnidarians. Cnidarians include corals, anemones and jellyfish. Hydrozoans begin life as a stationary hydroid (more on that later). Some species remain in this stage, but many others can release free-swimming jellies known as hydromedusae.

In the free-swimming stage, they can look very similar to some of the jellies you see at the Aquarium with the bell shape and tentacles flowing beneath. To get around, these animals pulse just like a sea jelly, too. However, they rarely grow larger than your fingernail and most are nearly translucent.

See they tiny jellyfish-like swimmer in the beaker?

Jackie has been monitoring Boston Harbor for these fascinating gelatinous zooplankton and has been collecting some for display. "I'm fascinated by these animals," she says. "I want to be able to show other people that they are a part of our local ecosystem, you just have to look closely."

This book doesn't come close to identifying all the hydromedusae we encounter

One type of hydromedusa that we often see this time of year is Sarsia. They have the usual bell-shaped umbrella, with four canals and four long tentacles and a mouth-like clapper hanging below. They reach a maximum of only 18 mm high!

Sarsia tubulosa

Many hydromedusae look similar, but we believe this is a Tiaropsis

As mentioned above, the free-swimming jellies are just one part of the life cycle of hydromedusae. When they're not pulsing around the surface of the water they are anchored to rocks or pilings at the ocean's floor during the hydroid stage, long filaments studded with polyp. When the polyp is ready, the polyp breaks off into tiny hydromedusae. (Here's a helpful diagram about the life cycle of moon jellies.)

The hydrozoans (a different species from the hydromedusae above) spend much of the year in the hydroid stage, which are the little nubs along these filaments attached to the glass.

There's still a lot to learn about these tiny seasonal neighbors of Boston Harbor. Come by the Aquarium and see if you can spot these ephemeral jellies in our special exhibit. Or if you're near the shore this time of year, look closely. You might see a glint of tiny tentacle flitting across the surface.

Flat-calm water, a good eye and a trusty scoop is what you need to spot the hydromedusae in the water.