Hi

It looks like the single arm of a sea star. Not sure the species. 

Maybe this larger arm has broken off the central disc and is starting to grow again. 

Hope this helps,

Todd. 

Agree with Todd. Looks like a single arm of Linckia multifora regenerating. 

Dear Todd,

Thank You very much, I agree, this might be a possibility.

Sincerely

Ralph-Walter

Dear Vladimir,

do You think it is the underside of the sea star?

I saw a lot of them but never turned them over.

Sincerely

Ralph-Walter 

Yes, I do. Retracted ambulacral podia was a first sign that we deal with sea star not with sea cucumber. Sea star's arm has an evident ambulacral groove, like we can sea on the photos, while sea cucumber's podia (except Apoda) are usually grouped in 2-5 fascias along the body.  It is also typical for sea stars that margins of the groove have spines along the ambulacrum. Unfortunately, I do not know exactly form and size of such spines for Linckia multifora. I guessed proceeding from the arm color. 

Thank You very much!!!!

I would also agree with Todd and Vladimir. I have gotten used to running into a lot of single legs here on the West Coast of Canada. It stems mainly from our issues with sea star wasting disease, although, autotomy does naturally happen in sea stars as a predator avoidance strategy.

Dear Anna,

thank You very much

Ralph,

I have listed a link to the website below that show a photograph of a Linckia multifora in the process of regenerating its central disk and remaining arms from a single arm.  The photo is of the dorsal surface so may not be much help.  It is an amazing photograph though.  Type in the link below and in the search box enter Linckia multifora.

http://eol.org/

Good Luck,

Pete

Dear Peter,

thank You very much, really interesting, I have seen many of them in Egypt and the Maledives but never realized what a wonder it is.