Strange spiked brown "twigs" - not even sure if they're plant or animal

[shorediver]

I found something like this on my back glass last week and threw it out without photographing it, stupidly. It was asymmetric, not obviously part of an animal.

Then today I found this deep in a cave in my rock. It's the same thing. Look at the two brown curved lines above and to the right of the shell. The leftmost line has some spines or barbs visible on it. It's not really moving, maybe a bit in the current. Doesn't seem to respond to light.

Any idea what it is and whether to do anything about it? It's fairly nightmarish looking stuff.

Thanks!

 

[cloak]

Have you tried poking it with a stick? It kind of looks like the shell of a bivalve IMO. Half of it at least.

 

[shorediver]

Hmm thanks cloak, I'll try antagonizing the scary spiky thing and see if it responds.

What makes you think it's a bivalve shell? Curious to see what you mean. It's just two long, thin structures. The photo is bad, but if you're seeing a solid thing, it's not there: the stuff between the two "legs" is just the rock behind it.

 

[tehmadreefer]

Looks like a broken shell in the rock...

 

[shorediver]

Might not be obvious - there's a lot in the photo - circled in red below

 

[tehmadreefer]

Might not be obvious - there's a lot in the photo - circled in red below

Yep still a broken shell in the rock

 

[shorediver]

Nah no way. I fished it out (it didn't move or wriggle at all, is dead):

 

[shorediver]

Some guesses:

• I have a bit of a GHA problem on my back wall, some really long hair there. Maybe this is a thick branch of that which broke off?
• It's a larval cocoon from some kind of worm?
• Hermit crab poop?

It seems more plant-y than animal-y.