Meet one of the strangest creatures in the sea. Tunicates are invertebrates, meaning they lack a backbone. Invertebrates are a very large and diverse group of organisms that include crabs, sea urchins, jellyfish, sponges, spiders, beetles, bugs and butterflies — just to name a few.
All tunicates are filter feeders. As such, they all have two siphons. One siphon takes in water, and particles are extracted as a food source. The remaining water is then expelled out a second siphon. Tunicates get their name from their “tunic,” or outer covering, that protects them from predators. The tunic is made of cellulose, a long chain of linked sugar molecules.
And tunicates are all marine organisms. There are no freshwater tunicates, but some types can be found in fairly low salinity brackish waters.
Solitary tunicates are commonly referred to as sea squirts because — well, if you squeeze them, they squirt water. Although referred to as solitary, these tunicates often form clumps, each tunicate with its own set of siphons. Solitary tunicates are frequently found on mangrove roots, but they can be found on other structure as well.
Tunicates come in many shapes, sizes and colors. Some solitary tunicates are beautifully colored in pinks and purples. These kinds are typically, although not always, found offshore in clearer waters. The ones we see most often are clear and small. Other common inshore species are larger, bumpy and globular in shape.
Colonial tunicates are made of hundreds of tiny individuals called zooids that together act as a single organism. Zooids are individual animals that form a complex group, all sharing a common set of siphons. As such, they look like glossy blobs of various colors with a rather intricate pattern (the zooids) on the surface.
In Charlotte Harbor we see lots of white, orange and black colonial tunicates. Colonial tunicates, usually the whitish ones, will often become encrusted on blades of turtle grass, the wide-bladed seagrass we see throughout the Harbor.
Pelagic tunicates are the most rarely seen types. In fact I have only seen them once. Most pelagic tunicates look like little circles linked together, but the ones I saw looked like bunny heads — hence my sea bunnies in the title. Pelagic means it drifts. These tunicates, unlike the ones previously mentioned, are not attached to anything and drift with the current in open water.
Most tunicates are hermaphrodites, meaning they possess both male and female sexual organs. Tunicates reproduce by releasing sperm into the water column. The sperm then enters another tunicate through the siphon with incoming water, where the eggs are then fertilized.
As filter feeders, tunicates benefit water quality by removing particulates and excess nutrients from the water. Good water quality is important to all marine life, and us humans too. But water quality may not be the only way tunicates benefit us.
Researchers have been studying various properties of tunicates and have learned that they may possess important anti-cancer properties. Specifically, a chemical in their body tissues is extremely active against melanoma, a potentially deadly skin cancer. But that’s not all: Scientists believe the cellulose found in tunicates could be used to help repair human muscle tissue or even grow new muscle.
One of the most fascinating things about tunicates is that they are more closely related to vertebrates, like ourselves, than to most other invertebrate animals. This is because tunicate larva (the tadpole-like swimming stage) have a nerve cord, similar to the nerve cord found inside the vertebrae of all vertebrates — including humans. That makes tunicates our closest invertebrate relatives.
Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. Contact her at firstname.lastname@example.org or 941-764-4346.