Flat Periwinkles

[Tengu]

Brown, grey, yellow, pink....

Why do they come in so many colours?

 

[daveO]

According to a book I read last week it's a camouflage response. They disguise themselves as the bladders on seaweed and depending on where they live the colour is designed to give them the best mimic. So if they are in water where fish would be looking up through the weed at them they'd be a different colour to places where predators would be looking down onto the weed. I guess the specific type of weed would be a factor too.