If you have even had the chance to see or feel a tunicate, you’ll know they’re weird little creatures. At first glance, the tunicate resembles a sponge, with an exhalent and inhalent opening. They’re often found in similar environments, encrusting rocks, ships, and docks. Like sponges, many species of tunicates are also colonial, colourful, and generally unassuming, which is why you may not have noticed, or even heard of, tunicates.
Despite their initially similar appearance, sponges and tunicates couldn’t be more different. For starters, if you give them a gentle touch, you can immediately feel a difference. Sponges will often be soft, porous, and somewhat flexible, whereas most tunicates feel solid, firm, and gel-like (kind of like a gel-cushion you’d buy for your shoes). If you have ever accidentally stepped on a tunicate, you might have received a shot of water back, hence why they are often called “sea-squirts”. Stick them under a microscope, or even look at them with a magnifying glass, and you should start to notice that tunicates are much more complex animals than sponges. This is because while sponges are the simplest of all animals, not even having cells organized into tissue, tunicates belong to the phylum which has the most complex animals of all: the Chordata.
For those of you that are familiar with the word “chordate”, you might be surprised. That’s because the phylum Chordata is our own phylum, including all vertebrate animals. That’s right: tunicates are your closest invertebrate relatives! Chordates include all animals which have a notochord at some point in their development. Notochords are stiffened (but flexible) rods that run along the dorsal (back) side of the animal, and provide structural support for muscles and bone. Most vertebrates only have a notochord early in their embryonic life-stages (although some adult fish still have them), and adult tunicates also lose their notochords.
Larval tunicates (called “tadpoles”) have a well-developed notochord, which allows them to be very strong swimmers. They use their notochord as a “push-pull” point for their little muscles. Tunicate tadpoles are attracted to light, non-feeding, and have a very short larval stage (it takes a lot of energy to swim so much!). Being a strong swimmer is therefore very important for finding a suitable environment in which to settle. After as little as a few hours, the tadpoles will start to swim down and try and find a place to settle. They settle “tail-up”, and the tail/notochord begins to resorb as they undergo metamorphosis.
Adult tunicates look very different than their tadpoles. The organs are near the base, and the majority of the body consists of a large feeding organ called the pharyx or “gill-basket”. Food particles are trapped as water is drawn in through the inhalent opening (atrial siphon in the diagram below), and through slits in the gill-basket. The entire body is covered in a sheath called the tunic, which is filled with an acellular jelly-like substance called tunicin (I love it when terminology makes sense).
The fossil record for tunicates is very, very poor, and specimens from the Cambrian and Carboniferous that were once considered tunicates have since been found to be organisms from other phyla (Chen et al. 2003). However, there are several well-preserved fossils of a critter called Shankouclava from the Early Cambrian of southern China that are still considered tunicates (Chen et al. 2003). Considering tunicates are the “base” of our own phylum, this find is a pretty big deal, and shows that the group has had little change over more than 500 million years! (check out their paper for some neat images!)
References and Links:
Baker, A.L. et al. 2012. Phycokey — an image based key to Algae (PS Protista), Cyanobacteria, and other aquatic objects. University of New Hampshire Center for Freshwater Biology. http://cfb.unh.edu/phycokey/phycokey.htm 16 Feb 2018.