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I've been trying to identify this growth on my octospawn and I'm stumped. It's a semi-circular thing that's noticeably softer than the hard skeleton of the branching octospawn. I've observed it swelling and contracting over time - it gains height off the surface of the branch and then flattens out. The edges have these fine little "hairs" that remind me of the strands on my trochus snails.

I took a few pictures to help with the ID process. This first one has the fflintix9h on to show the color difference between the growth and the octospawn branch.

This next one's the best shot I got of the little "hairs" against the tank's back wall.

I made a rough drawing to illustrate how it swells up and contracts - it's pretty interesting. Upon closer inspection, the surface seems hard, but the sides must be soft. When I touched it with my coral feeder tube, it immediately went flat against the branch.

Here's another picture, trying to capture it in its "raised" state with the hairs visible. I outlined the general shape in red - it almost looks "hinged" with the left side raising up while the right side stays flat. It kind of resembles a clam or scallop.

If anyone knows what this thing is, please let me know - I'm worried it might be a pest or parasite affecting my octospawn's extension. Thanks!

Still nothing? I'm really starting to get concerned about this growth on my octospawn. It's been a few days now and I'm still stumped. If anyone has any ideas, please share - I'm worried it might be affecting my octospawn's health.

I'm familiar with the term "octocoral" but I've never come across "octospawn" before - is that a colloquialism or something specific to your region?

It's actually a colloquialism for "octopus frogspawn" - essentially, a type of frogspawn coral with some distinct characteristics, like the smaller, more numerous colored bubbles that run down the length of its tentacles, setting it apart from the standard frogspawn.

I'm gonna take a wild guess here, but could it be a barnacle?

I'm no expert, but I'm going to take a guess here - it almost seems like this growth could be a remnant or a scar from where a head was previously broken off. The way it swells and contracts, and the soft sides, kind of remind me of how a wound might heal.

I've thought about the barnacle idea, but I'm not convinced - I just don't see the characteristic hole you'd typically find on a barnacle. And as for it being a broken-off head, I'm certain that's not the case. This thing is definitely alive, and it responds to current or stimulation by rising up and down. I've tried to get a better look at it from different angles, and I think I've spotted a spine inside the shell - it's a central line with little ribs on either side.

Here's a shot of the spine I managed to capture:

It's got some characteristics of a mollusk, possibly a type of oyster or clam, but the soft sides and movement are throwing me off.

I think you've got something attached to that octospawn, for sure. It's definitely a living creature, and I'm stumped. A chiton variant could be a possibility, but I'm not confident enough to say that's what it is. If nobody here can ID it, my next step would be to reach out to an invertebrate biologist at a local university - maybe they can take a look at your pictures and give you a more informed answer.

I'm going to take a wild guess here, but could it be an abalone? I know it's a long shot, but I've seen some weird stuff attached to coral before. Maybe it's not as far-fetched as I think.