I heart these zooids

Half the fun of maintaining my little salt-water aquarium comes from the surprises and mysteries it serves up every so often. Did you know clams had teeth? Some of them do, on the hinge. These are not teeth, though:

Gates on the intake siphon.

A brief history for those who haven't seen the early dishpan aquarium posts: I brought home a broken sandworm from the beach, and decided to try to keep it alive until it regenerated its head. I created an environment for it, populated with seaweed, sand, rocks, and the occasional volunteer clam and snail. And I got so interested in observing the lives of my little "family" that I have kept on with it.

I have bought nothing but a small aquarium and a pump and filter; all the residents are from the Boundary Bay/White Rock area. Every trip to the beach, I bring back a litre of clean sea water, and maybe a bit of seaweed when the crabs have eaten the old plants. I don't feed the animals, leaving them to establish their own food chain, algae to snail to crab to crab crumbs and poop to hermits and worms and back to algae again, for example. (Except that it's a lot more like a web than a chain.)

Every so often something new shows up, something I had not been aware of introducing, that could have come as microscopic immatures in the water, or on the seaweed or in sand.

These were on a bit of sea lettuce. I noticed just the flash of cream as it waved in the current, and tried to photograph it. Today I fished it out and tried again. The large patch is about 1 cm across. It is a brownish red, but there are flecks of red in the cream colonies, too.

I think -- I think -- they are Harbour Star tunicates, Botryllus schlosseri. I found a photo in ExoticsGuide that matches, down to the internal structures. I posed them against a light and saturated the colour to get a sort of X-ray:

A tunicate colony for Valentine's Day. I'll have to remember.

Each individual animal (zooid) stands upright; some look like pillars or old-fashioned hair rollers, others are more olive-shaped. (Like a stuffed olive with the filling gone.) There's a circular tube down the centre. The whole mass is covered with a sort of clear jelly; I took them out of the water, but they carried their own insulation. The new buds can be seen around the edge of the colony.

In this photo, taken under blue light, the central well is more visible, and you can see individual cells in the walls. (Click on it to see it full size.)

Colonies typically mature after 1-2 months, and reproduce in spring and early summer. ... During its reproductive period, which lasts up to 10 weeks, a colony can produce up to 8,000 eggs. The eggs are retained within the colony where they develop into tadpole-shaped larvae with 8 finger-like buds, called ampullae, on the head end. When released from the colony a larva spends less than a day in the water column before attaching head-down onto a firm surface. The settled larva metamorphoses into a zooid, which buds off others to the sides to form a colony.

...

Colonies that develop from larvae that settle in the spring live for about 3 months and those that develop from fall settlers live for about 8 months, though colonies kept in the laboratory have survived for up to 7 years. From ExoticsGuide.

More from the Guide:

Crabs, snails and flatworms have been observed feeding on Botryllus schlosseri. Botryllus schlosseri can overgrow other attached, filter-feeding organisms and compete with them for food.

I'll have to keep an eye on them. But to keep them well-behaved, there are plenty of snails, two hungry crabs, and a pair of flatworms. That I know about. All part of the web.

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