‘Tis the season to get out and tidepool again, and the water isn’t so cold that it’s not bad if you get a little wet. There are the usual suspects along the shore like barnacles and periwinkle snails. But, if you look a little closer, you might notice the delicate wavy tentacles of hydroids.
These are the tiny larval stages of a bunch of tiny animals that spend part of their lives stuck to something and part of it drifting freely in the water. They are related to and look a bit like tiny anemones while stuck and jellyfish when they are floating. The “stuck” phase of their life is known as their polyp form and the floating phase is the medusa stage.
Hydroids, jellyfish and anemones all belong to a group of animals called Cnidarians. They have strange gelatinous bodies with tentacles that wave in the water and help them to capture their prey. They do this by using super specialized cells called cnidocytes that contain toxins that stun their prey.
In the case of some species of jellyfish, this is a strong toxin that humans can feel. It can be painful or even deadly in rare cases. But, most Cnidarians don’t have a super potent sting and touching them with your finger simply feels a little sticky. This is the case with most hydroids, although if you get enough of them together, they can be a bit sting-y.
If you’ve seen or snorkeled in a coral reef, you may have noticed the tentacles of coral polyps waving in the water. Corals are also Cnidarians and share one of the features of hydroids, which is that they often live in colonies together. Sometimes, they work together so that some of them specialize in feeding and others in either holding the colony together or reproducing. So, if you are looking for hydroids in a tidepool, you’re likely to see a little cluster of these clear-bodied tiny creatures.
The clusters of the hydroids we have in Maine are usually only an inch or two across, but other colonial hydroids can be quite giant – like Siphonophores that can be several meters across. These are worth looking up. They, like many other Cnidarians have fluorescence in their tissues so they glow in the water. This is particularly common in the deep-water species where some of the weirdest Cnidarians these live.
Tidepools aren’t the only place to find hydroids. Like many things that live in the sea, they find places to live on just about any surface. This means that you might also find them on boat bottoms, docks, or lines in the water. They are often found in clumps along with other tiny sticky animals like bryozoans and tunicates. Together, they form a kind of fuzzy, multicolored and textured coating.
The bryozoans are typically the tufty brown ones and the tunicates are the more slimy, drippy ones. Tunicates and bryozoans are all filter feeders that grab tiny bits from the water as it passes by. Certain species of tunicates can be quite a nuisance, like the invasive sea squirt, Didemnum vexillum. It is sometimes called sea snot for its orange goopy, drippy form that can weigh down gear floating in the water. All of these critters are also part of the cause of the stinkiness of docks after they are pulled for the season and left sitting in the sun.
While these tiny creatures can be a pain when they “grow” over surfaces you might otherwise wish were clean, they offer a rare window into understanding the great diversity of living things that often float around in the water without being seen. There are over 3700 species of Hydrozoans alone.
When these colonial animals settle and stick to things, we get a chance to look at them up close. And, when you see them waving their translucent tentacles in a tidepool, they look truly magical – not quite like snorkeling in a coral reef, but lovely all the same.
Comments are not available on this story.
Send questions/comments to the editors.
