Like pepper

 

Recently we noticed an amazing number of juvenile periwinkles (Littorina spp.) in the high intertidal zone.  It must have been a very good year for them!

These young snails are tiny.  I should have taken a picture showing an entire rock — but there were so many little black snails scattered across the boulder that it looked as if someone had shaken pepper all over the rock!  Note how the snails are concentrated along the edges of and in between the barnacles.  Do you have ideas about why that might be?

If you haven't seen periwinkle embryos before, here's a picture taken under a microscope:

 

Adult female periwinkles release transparent capsules containing fertilized eggs into the water.  The eggs and embryos develop for about a week and then emerge from the capsule as free-swimming planktonic larvae.  Eventually the swimming veligers will undergo metamorphosis and settle onto the bottom as tiny snails.