[caferacermike]

Are you sure it is not any sort of amphipod?

 

[caferacermike]

I'm thinking more of a tiny goby like this red head.

Or a catalina

All pictures stolen from yahoo searches by CRM. Not my pics.

 

[caferacermike]

I definitelyl saw a round amphipod. But I could not tell if there was something else as well.

 

[caferacermike]

Tkae a look at pistol shrimp/goby relationships. This sort of thing is very common in the marine world. I'd bet your's is not symbiotic so much as the shrimp like creature found a safe and secure place to live.