When I recently posted photos of some tunicates (common marine animals of the coast) encrusting an old lobster trap to my website, a former student commented "kinda looks like someone sneezed, Doctor." Despite my overwhelming love for tunicates, he was right. Tunicates can be quite amorphous in shape and slimy to the touch.

When I recently posted photos of some tunicates (common marine animals of the coast) encrusting an old lobster trap to my website, a former student commented "kinda looks like someone sneezed, Doctor." Despite my overwhelming love for tunicates, he was right. Tunicates can be quite amorphous in shape and slimy to the touch.



Tunicates are animals that bridge the gap between invertebrates (do not have a backbone) and vertebrates (have a backbone). Humans are vertebrates; we have a spinal cord encased in a hard, protective vertebral column. Birds, fish, frogs, snakes are also vertebrates. The vertebrates belong to a bigger group, a Phylum, called the Chordata. All of the chordates have, at some time in their life, a notochord (a long tube that supports the body), a dorsal nerve cord, pharyngeal gill slits and a post-anal tail. In humans the notochord is replaced during development by the vertebral column, the pharyngeal gill slits become parts of the inner ear, jaw and tonsils, and the tail is lost.



Tunicates are also Chordates, they have all the above-mentioned characteristics as juveniles, called tadpole larvae because they look a lot like tadpoles (they swim using a tail, have a notochord, have gill slits).



Tunicates can be colonial or solitary. All are marine. They have sac-like bodies that are sessile (attached), in fact, they are the only sessile chordate. You will find them attached to hard surfaces, like rocks, boat bottoms or piers, or anchored in soft mud or sand. One kind, the salps, live their whole lives floating in giant rafts in the ocean. A few years ago we had an enormous colony of salps wash up on the beaches in York: they resembled jellyfish but luckily didn't sting. Swimming through them was like swimming in a plankton stew; it was very disconcerting.



An adult tunicate can best be described as a sac with an incurrent and excurrent siphon through which water enters and exits, bringing in tiny planktonic morsels of food that get filtered out of the water stream. Their common name, sea squirt, comes from their ability to squirt water out the excurrent siphon when disturbed.



How does the free-swimming tadpole larva end up as a sessile sac-like adult? My marine biology textbook ("Marine Biology" by Castro and Huber) is as enthusiastic about this process as I am. "The metamorphosis of a tadpole larva into a juvenile tunicate is nothing short of spectacular. The notochord and tail are re-absorbed, the filter sac and siphons develop, and free existence is no more."



I will never forget my first exposure to tunicates in my freshman Invertebrate Zoology class at the University of New Hampshire. The professor, Larry Harris, would take us out tide-pooling. I remember most of those trips occurring at dawn. Bleary-eyed and full of donuts and coffee, we would invade the tide pools of southern Maine and New Hampshire. Wade far enough out and you'll find tunicates encrusting the rocks, competing for space with sponges and seaweeds.



Back in the lecture hall, we learned that of all the different invertebrate phyla we had studied — sponges, sea stars, crabs and the like — we were most closely related to the tunicates, that they were in our own Phylum.



They are more closely related to us than to those other invertebrates. Ever since learning this, I have had an unexpected affection for this group. Just imagine, tunicates don't have arms or legs or faces but instead live out their lives as little filter-feeding sacs — so different from us in appearance, yet so closely related.



Sue Pike of York has worked as a researcher and a teacher in biology, marine biology and environmental science for years. She teaches at St. Thomas Aquinas High School. She may be reached at spike3@maine.rr.com.