Symbiosis can be broadly defined as any long-term close association between organisms, as well as more narrowly referring to a mutually beneficial relationship.

I do not know of any studies testing the costs and benefits, but the overgrowth of shells by other organisms such as algae, sponges, corals, and bryozoans may be mutualistic, providing camoflage to the shell and a nice hard substrate for the epibiont.

In addition to the symbioses with zooxanthellae and with chemosynthetic bacteria, occuring in assorted mollusks, other symbiotic microbes include cellulose-digesting protists and bacteria in shipworms and light- or toxin-producing bacteria that live on certain cephalopods.

    Dr. David Campbell
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droigate Spa