Polyps A-Plenty

I spent quite a while this weekend talking to Fritz, Will, and Candy about my tiny corals and, more generally, the complicated symbiotic relationships that allow lots of sea creatures to be mostly solar-powered rather than gut powered.

So, mostly for Fritz’s sake, here are some photos of my tank with a bit of explanation. I’m going to try to order this from least- to most-anatomically complicated. Everything featured here is a cnidarian (what was called ‘coelenterate’ when I was in high school) and, in particular, they are cnidarians in their polyp phase (as opposed to ‘medusa’ phases which swim around like a jellyfish) and all have symbiotic Zooxanthellae (dinoflagellates) and live under very intense LED lighting.

For starters, a rose anemone, Entacmaea quadricolor. It’s about the size of a softball now and, depending on how it feels about its locale, will either divide into a cluster of similarly-sized anemones or will just keep growing until it’s as big as a dinner plate.

Anemones and corals are closely related, and ‘coral’ isn’t a very precise term. A thing that distinguishes anemones is that they tend to be solitary polyps (if mine divides it will produce a pair of clones that tolerate each other’s company but are otherwise totally distinct) and that they will pull up stakes and creep away to a different spot if they find themselves in the shade; things called ‘corals’ are usually stuck in one spot come hell or high (or low) water.

Here are a couple of ‘corals’ that look pretty much just like anemones, but are colonial:

The first was sold to me as a zoanthid and the second as a palythoa but I wouldn’t be shocked if either was in the other’s genus. The individual polyps are about the size of a dime but they grow from a single base. Of course if I cut a colony in half then the each half would continue about its business — it’s a single organism but not tightly coupled like a frog or a person. No blood, for one thing.

Palythoa are famously toxic but that poison is reserved for things that try to eat them (or in the case of some unfortunate hobbyists, cook them in a poorly-ventilated kitchen) — their actual stings aren’t very severe; they are only mildly interested in catching things to eat, being mostly taken care of by their symbiotes.

Here’s another thing that looks like a single big thing but on closer inspection is a colony of (tinier, this time) polyps:

This is a ‘green star polyp’ coral probably in the genus Briareum They only grow over existing structures like the zoanthids above, but each polyp has a little stony shell that it can retreat into when alarmed. This mass definitely acts like a single organism — if I poke a polyp at one end, all the polyps will draw back in concert.

Now, something that looks more like what most people think of as a coral:

This is a ‘kenya tree coral’ in the genus Capnella. Those trunks and branches aren’t hard like limestone but instead sort of squishy like a mushroom… if the coral gets bonked (or some other coral or algae starts to grow on it) it will hunker down and then stretch out again in the light. Up close, you can see that it has lots of little polyps on the ends of the branches.

Despite having clearly different kinds of tissues I could still chop it into bits and each bit would grow into another tree-shaped coral, just like cutting up a jade plant. They do this on purpose, too, shedding branches so that they’ll blow around in the current and take root in new spots. Capnella live in deep water so they’re more reliant on eating and less reliant on sun bathing, but in my shallow aquarium it’s fine living on a diet of light and whatever random particles happen to wash into it.

Here, at last, is a coral that actually looks like a coral and could legitimately build a reef given enough time:

This is a Euphyllia, an example of what aquarium people call a ‘large polyp stony coral’ because it grows a stony skeleton but still features prominent squishy parts. Serious reef builders get called ‘small polyp stony’ and they really do just look like rocks unless you zoom in close to see the little tentacles — they tend to be fast growers and need a lot of supplemental calcium to grow so I’ve avoided keeping any so far.

This Euphyllia is the only thing in the tank that will actively attack and kill other cnidarians just to make living space… it has a large-investment project in the works and specific architectural plan and will brook no dissent. It’s in a corner of the tank all to itself but should it start to grow in non-geologic timescales I’ll have to prune it back to keep a multi-inch buffer zone between it and any of its neighbors.

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