I was recording some video of the band of gravel on the beach at the North Spit of the Coquille River a couple of weeks ago when an odd shape in the view caught my eye.
An off-white cylinder about as big around as my finger, but shorter, the shape seemed too regular for a stone or piece of driftwood. When I looked around the camera, the bumpy surface of the thing gave me my answer: A pyrosome!
Pyrosomes are colonial tunicates: “colonial” because the members of the group are individual organisms -- albeit clones.
Each bump is an individual, a “zooid,” that’s capsule-shaped and, in our species, up to about a third of an inch long. The zooids are bound together by a gelatinous “tunic” into form a hollow cylinder, each zooid is packed in head-end out. Its tunic gave the colony I found a somewhat rubbery feel.
The zooids take in sea water at their head/outside ends by beating water in with the cilia around their mouths. They filter their food from the water, then pass the cleaned water into the inside of the cylinder.
The colonies move when the water flows out the hole at one end of the hollow cylinder. Like many other pelagic (open-ocean) animals, pyrosomes move up nearer the surface at night and down to darker layers in the daytime. The finger-sized colony I found apparently would have moved about 300 feet up and down in a day/night; a large colony of the same species is reported to move up to 2,500 feet in a day/night.
Had it been nighttime -- and had the colony been still alive and in the water -- I may have witnessed the reason for the “pyro” in the name: from the Greek word for “fire,” the name refers to bright glow the animals can produce.
While pyrosomes are colonial, pelagic tunicates; the kind of tunicates more familiar to people are the “sea squirts” that live singly or in groups of separate individuals attached to rocks or docks.
Tunicates are most remarkable because of their larvae. Very differently shaped than the adults, larval tunicates look rather like tadpoles with a flexible, cartilage-like tail that houses a central nerve cord along the back -- evolutionary precursor to a spinal cord. (The overall group, in fact, is the phylum Chordata, a reference to this cord.) When larval tunicates metamorphose into adults, the tail structures are absorbed into the body.
It's that central nerve cord that makes tunicates -- and therefore pyrosomes -- more closely related to us than they are to, say, worms or crabs or seastars.
Tunicates reproduce sexually; individual members in a colony reproduce sexually to create individual larvae that will, with luck, swim away to start new colonies. Established pyrosome colonies grow by individual zooids budding to add neighbors.
There are probably fewer than ten species of pyrosomes in the Earth’s seas, but very many subspecies. The ones that we find washed ashore Oregon beaches are probably Pyrosoma atlanticum. (Despite the species name, this pyrosome is found in temperate seas world-wide.) That species may grow to colonies up to 2 feet long and 2.5 inches wide. Colonies of a giant tropical pyrosome, Pyrostremma spinosum, can reach up to 70 feet long and nearly 4 feet in diameter -- you could probably get inside a large one.
There are sea creatures that live inside pyrosomes, mostly small crustaceans, notably certain shrimp. And, of course, other sea creatures eat them, such as some fish and a few whales.
Finding the pyrosome in January was a surprise since it’s more common to see them on the beach in summer. Too, this was the only one I saw on the beach, and it’s common for them to come ashore in large numbers.
A big influx of pyrosomes off Oregon’s coast a couple of years ago was thought by some researchers to have been triggered by major changes in offshore ocean temperatures the year before. I wonder if this year will bring more of these remarkable colonies to our beaches this summer.
I’ll be sure to keep an eye out for more.