After a few fits and starts, spring is finally here and the harbor is coming alive.

That means the hydromedusae are blooming, and we’re giving visitors a chance to see them up close.

We’ve created a fleeting exhibit with hydromedusae display near the Edge of the Sea tidepool touch area.

sarsia exhibit
A visitor observes the tiny hydromedusae in their display near the Edge of the Sea tidepool touchtank.

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. 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.

sarsia swarming in exhibit
Sarsia and tiny brine shrimp churning around their exhibit

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! 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.

I'm fascinated by these animals. I want to be able to show other people that they are a part of our local ecosystem, you just have to look closely.
- Jackie Anderson, aquarist

Collecting Hydromedusae in Boston Harbor

Jackie Anderson always looks forward to the arrival of the hydromedusae. She is one of our skilled aquarists and has been monitoring Boston Harbor for these fascinating gelatinous zooplankton. Flat-calm water, a good eye, and a trusty scoop is what you need to spot the hydromedusae in the water. Here are a couple pictures of what it takes to collect these creatures in Boston Harbor. 

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.