4. Mystery subject

If you thought the photo to be an anus, specifically, the anus of a sea cucumber, then you are correct.

As for what type of symbiosis is being displayed, did you note the presence of the small fish, most likely a blenny, residing in a pore in the skin at the 7 o'clock position? It looks as though the blenny is using the pore as a protective shelter. So, PARASITISM would be a good guess for the type of symbiosis.

The sea cucumber is an Indo-Pacific species. Apparently, such fish/sea-cucumber relationships are not found in the Caribbean region.

photograph of tiny blenny fish in pore of a sea-cucumber's skin photograph of anus of sea cucumber with fish symbiont