I have to admit that I wasn’t all that eager to snorkel in two feet of water, particularly while pushing aside thick swaths of eelgrass, but it is awfully hard to say no to an eight year-old with a new snorkel. As often is the case, it was an experience I would not have had were it not to a bit of egging on, and that I’m so grateful I did. It was like snorkeling through a set of multicolored crayons – green eelgrass handles with tips painted in multicolored hues. Glossy coatings of orange, red, and lavender coated the tops of the fronds as they waved just beneath the surface. But what was this strange glistening stuff we were looking at? 

It is something more related to humans than you would certainly expect – it’s a tunicate. But what is a tunicate? What appears as a slimy coating on things like eelgrass and tide pool rocks is actually an animal. They’re a bit like tiny anemones in that they are small sac-like filter-feeding animals with translucent “tunics” over their tiny bodies. This tunic, however, connects to those of many others around it. In fact, the tunics connect a network of organisms that share blood vessels and circulate nutrients and liquids amongst the colony. Tunicates are also sometimes called sea squirts because they take water in one siphon, filter out plankton for their food, and then squirt the water back out through another siphon. 

The odd thing about these colorful slippery colonial animals is that they are chordates. That means that they belong to the same phylum as vertebrates even though they don’t have vertebrae. In the large animal kingdom, there are vertebrates (backbones) and invertebrates (no backbones) and there aren’t too many overlaps – except for tunicates and one other odd one – the lancelet. This is an odd fish-like animal that looks like a fish but doesn’t have a jaw or other sensory organs. All chordates have a notochord that transmits senses throughout the body. In vertebrates, sensory transmission is a complex process, whereas in lancelets and tunicates, this is very simple. It’s pretty neat to think that a similar sensory process occurs in a colorful coating on a blade of eelgrass as does in your body, but nonetheless it is pretty neat. But, tunicates have a superpower that humans don’t – they are capable of whole body regeneration, which means that they can regrow all their body tissues. Scientists have been studying how they do this to see what they can learn. 

Several different species of tunicates can be found in Maine. There is the lovely Golden star tunicate (Botryllus scholsseri) that literally looks like little golden stars nestled in a slick dark coating on the rocks. Sea pork (Aplidium constellatum) is another interesting one that looks a little like the salt pork you might put into chowder. And a sea peach (Halycynthia pyriformi) is yet another unusual one. It looks a bit like a giant heart with siphons pointing in and out. Finally, sea grapes (Molgula citrine) are one of the smallest of the bunch. They are only a couple of centimeters big and truly look like little floating grapes.  

 There are several more tunicate species that are not native to Maine but are commonly found here. In the 1990s, several species arrived on the New England coast in the ballast water of ships coming from across the ocean. They are super settlers, which means that if their larvae comes to a new shore and can find a place to stick, then they can easily make a new home. This has been the case for several species in the Gulf of Maine. One is the orange sheath tunicate (Botrylloides violaceus). It has become so common that it seems familiar along the coast – and it is one of the colors that tipped the eelgrass on our snorkeling expedition. Another tunicate species that has caused problems in recent years is Didemnum lahillei. This is a light-colored tunicate that can grow in thick formations over the surfaces of things. This has particularly been a problem on lobster traps, weighting them down with such a thick layer that they are hard to use.  

The presence of these invasive species has been increasing in recent years. Warmer water temperatures have made it easier for them to survive. The invasive tunicates also don’t have any natural predators, so they are able to reproduce and grow without limits. An increase in structures in the water has also given tunicates more places to attach and thrive. In the meantime, however, they are fascinating animals to study and, when not a nuisance, lovely to find along the shore or under the water. It is always humbling to learn that we are so closely related to something as simple as a slimy coating atop a blade of eelgrass.