I've Never Seen Anything Like This - Any Ideas?

[SCUBA Fletch]

I was in Belize - and at the time, just snorkeling, as we were traveling out of Placencia on a catamaran. I saw this at about 20' depth, swam down and snapped a few photos. I've NEVER seen anything remotely close to it. It appears to be growing on another species. Any ideas? We've been diving for nearly 2 decades - and this is truly bizarre!

 

[SCUBA Fletch]

 

[SCUBA Fletch]

 

[diversteve]

It's an egg-case. I don't know what from but it could be a whelk case.

here's one sort of like it:

Can anyone identify this egg case? | Reef Environmental Education Foundation (REEF)

google for more examples: spiral egg case

 

[SCUBA Fletch]

Yes, that's very similar... Many thanks, Diver Steve!