Crevice dwelling and nestling bivalves
Some bivalves, especially small species, can occupy holes vacated by various boring organisms.
Some are small enough to take residence in empty barnacles where they may secrete miniature byssal threads to avoid being flushed away in the intertidal environment. Other species, such as Musculus marmoratus , live within the tissues of other organisms or are closely associated with coral or sponges. A few even construct ‘nests’ of their own, such as certain species of File Shell (Lima spp.).
Some bivalves are always found in close association with other organisms, especially echinoderms (the group including sea urchins and sea cucumbers). Some very small species, such as Montacuta substriata , are always associated with burrowing sea urchins.