Chordate for a day

Take a cup of clean sea water from a handy shore. Shine a light through it from the side. Get a magnifying lens and look at it, cross-ways to the light. After a bit, your eyes adjust, and you start to see pin-points and streaks of light, swimming, crawling, jitter-bugging around. Many are assorted copepods and baby amphipods; you'll get to recognize them by their swimming styles. Some might be tiny shrimp, seemingly drifting until a whim takes them and they zip to another drifting spot. And some are probably free-swimming immature stages of animals you would recognize easily as adults, such as barnacles or oysters.

That may explain how new animals show up in my aquarium; I replenish the water with fresh supplies from Boundary Bay. The pink and red growth that encrusts much of my sea lettuce at the moment probably arrived as tadpole-like larvae, a couple of millimetres long.

I mentioned them a couple of weeks ago, but didn't really know what they were. Since then, I've been watching them daily, as they grew, and searching the web for a match.

Yesterday's photo: three patches of "little barrels".

At first, the new residents just looked like a smear of bumpy jelly, but as they grew, I began to see spots on top, then a rounded barrel shape, with a hole at the top. A few days ago, I noticed that the holes periodically closed and opened. In new photos, I could see a circle of ribs inside the holes.

Chain sea squirt, Violet Tunicate.

The largest mass had developed a number of larger bubbles, each with its hole at the top.

Red mass, with exit siphons.

That was a clue: I began looking at tunicates, or sea squirts. Tunicates are basically water pumps, with an intake siphon and an outflow, with a stomach in between, all contained in a tough wall, called a tunic, and encased in a layer of jelly. Some are free-swimming, but the class Ascidiacea live fixed to a base. Mine would be ascidians, then.

Ascidians may be solitaries, or live in communities; colonial ascidians. And among BC's colonials are the Botylloides violacea, which match mine. Yay!

B. violacea is an import from Japan which has established itself on both coasts of N. America, becoming a nuisance in some areas. It is sessile, growing

"... on a variety of surfaces, includ(ing) docks, boat hulls, buoys, ropes, pilings, the undersides of rocks, eelgrass (Zostera marina) blades and seaweeds. It often overgrows mussels, barnacles, encrusting bryozoans and solitary sea squirts." (Exotic Species of San Francisco Bay)

A colony is soft and smooth to the touch, like a jellyfish without the sting. Each separate colony is one colour, which may be anything from cream to red to violet to lavender; neighbouring colonies don't necessarily match.

It is a filter feeder; water passing through is strained to retain food particles.

Diagram of a typical ascidian. From U. of Manitoba. My labels and arrows.

Water enters the larynx and is filtered. Cleaned water goes directly to the outflow; anything trapped in the sieve goes to the stomach. Undigested particles enter the intestine and are removed through the outgoing siphon into the common area underneath the colony's jelly coating. From there, everything is expelled through the large "bubble" tubes.

Watching my critters through the microscope, I could see the individual animals (zooids) contract their sides every few minutes, expelling the contents of the siphon. Against a light, even with just the magnifying lens, a stream of tiny particles is visible leaving the surface of the colony; between puffs, larger crumbs flow inwards.

Out of the water, each individual collapses: there is no solid structure to hold it up.

Collapsed.

Colonial ascidians are hermaphrodites, having male and female organs in each individual, but they also reproduce asexually, by budding. The eggs of B. violaceus are retained in brood pouches, and fertilized by sperm released into the water surrounding the colony. Each egg produces a tadpole-like larva, about 2 mm. long. (Photo here.)

The larval stage, even though it lasts only a day, is fascinating. Consider: unlike most of the other invertebrates, it has the precursor of a spinal cord. But it will have no spine. Instead, like an embyro of any vertebrate (animals with backbones, including us), it has a notochord, a flexible, rod-shaped body that runs down the tail, and behind that, a neural tube, which in vertebrates becomes the central nervous system. These, together with the gill slits and a groove in the wall of the pharynx, makes it one of the Chordata. The tunicates are the sole members of the subphylum Urochordata, which means "tail chordates". (More detail - much more - here.)

The weirdest thing about this is that, after the larva finds a home and settles in, it digests its own tail, notochord and nervous system, leaving only a rudimentary clutch of nerves.

The larva is equipped to do one thing only: find a place to anchor itself for the rest of its life. It has an eye spot, which also disappears, a balance organ, the muscular tail for swimming, and about 30 "ampullae", (one biologist calls them "buds", another says "suckers") on the head. It does not eat; it has no mouth, nor siphons. It must find an appropriate location within a day, or it dies. It lands head down on the chosen spot, grabs on, and immediately begins the transformation into an adult.

I looked for larvae and didn't find any still swimming. I did find a few that were just settling in; the tail was gone, but they were still individuals; just the head of the "tadpole" and two visible buds. Under the microscope, I could see the hearts beating. This was strange, too. The blood flows one way for a while, then reverses and flows back.

Newly attached larva, and infant colony with first three zooids. Borrowed from Mike.

Young Botrylloides colony.

Once the larva is settled in, it starts to bud asexually, producing new individuals, arranged in circles or double rows. Each one is entirely self-sufficient; broken off the main colony, it can survive and continue to produce new zooids.

In the photo just above, there is a larva, recently attached. All that is visible is the central blob and a circle of ends of blood vessels. (Mike the Mad Biologist calls them "little hearts".) In the next new colony, now with six zooids, the ring of "hearts" is quite plain. (It's black because it's on the back side of the sea lettuce; all we see is its shadow.)

And below these, on the left, is a slightly larger colony, showing the basic arrangement: zooids around a central space, a circle of blood vessels around it. These zooids are now mature enough to show the central siphon, and the hint of a community cloaca (outlet tube).

A starter home. Another typical arrangement.

A crowded bit of mature colony on sea lettuce, 1 cm. across. Out of water, and therefore collapsed.

A few more curious facts about tunicates:

• Tunicates are the only animals able to create cellulose. (Actually, tunicin, a polysaccharide similar to cellulose.) This is used to stiffen the tunic.
• Tunicate blood contains high concentrations of vanadium, functioning as a pigment. Vanadium is toxic to most organisms. Some tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. 
• "Various Ascidiacea are used as food. Sea pineapple (Halocynthia roretzi) is cultivated in Japan (hoya, maboya) and Korea (meongge) and, when eaten raw, has been described by Lonely Planet as tasting like "rubber dipped in ammonia". The peculiar flavor is attributed to an unsaturated alcohol called cynthiaol." From Wikipedia. I've lost my appetite. Besides, there's vanadium in them.
• Some marine animals will eat tunicates, in spite of the vanadium. Sea stars, flatworms, and otters, among others, are mentioned as predators. In my tank, not so much. The hermit crabs ignore them completely, although they love to chew on the sea lettuce. Snails don't crawl over them. And when a stray piece falls into the tentacles of an anemone, it is promptly shaken off.
• Most tunicates are thought to live about one year as adults, but the colony may endure for decades.