Scale in the back is in millimeters.
One of yesterday's interesting finds was this live Batillaria minima with a barnacle attached to its shell. After I took this photo, I put the snail in a small dish of sea water, hoping that the barnacle would start feeding. The barnacle did open its valves but not long enough for me to take a photo. Later, I returned the inseparable pair to the sea.
I examined many of these snails yesterday and saw only one or two that had barnacles on them. These snails are not the ideal substrates for barnacles for at least 2 reasons I can think of. First, they are too small for the barnacles which, I think, can grow much larger than the one in the photo. Second, I suspect the snails' average lifespan is probably shorter than that of the barnacles, although I admit I have no idea how long barnacles live. After the snail dies, its shell will get buried in the sand. And that will be the end of the barnacle too.
Part 2 of this series features the photo of a barnacle feeding.