In this post almost 3 years ago I wrote about how I hadn't noticed 2 tiny snails while photographing a much larger spider near them until later when I was looking at the photograph on the computer monitor. It seems that when we direct our attention to a particular object our brains has a tendency to ignore the nearby objects, especially if the latter are much smaller than the former.
Here is another example of this phenomenon once again involving snails. Last night, I was looking at the pictures of the intertidal snail Batillaria minima that I took last year in Florida. I hadn't examined many of the pictures critically until then. When I first looked at the particular picture below I decided that it wasn't very descriptive of the Batillaria that was the subject and decided to delete it. Then I noticed the blurry, brown thing near the right-hand corner.
That's an Assiminea succinea, another intertidal snail that spends most of its time outside the sea. This post has better pictures of them. The funny thing is at that time I was planning to collect Assiminea at another beach where I knew they were present. When I was photographing Batillaria they were there under my nose all the while, but I wasn't seeing them.