What is this little spikey red guy?

[starbuck88]

Hi Everyone,

This was hiding under a mushroom. A clown decided it was going to host the mushroom, so the mushroom moved! This was underneath it, slowly it seems to be growing judging by the new colour and spikes around the outside edges. We thought it was just some sort of old skeleton but no, it isn't!

Apologies for the poor video but this is how we 'noticed' it today, at feeding time it's doing this!

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I have no clue....

Any help appreciated

Thank you!

 

[ISpeakForTheSeas]

Looks like either a Spondylus (Thorny Oysters) sp. or a Chamidae (Jewel Box Clams) species to me.

 

[taricha]

I was thinking maybe a kind of baby urchin, until I saw a bivalve closing sort of movement in the video.