Dear Andy;- Spending summers at Folly Beach near Charleston, S.C., I have seen D. variabilis by the thousands on their tidal migrations. (They come in many colors and variations (thus their name) but always the same shape and about a length of 15mm). I have never noted that they are coated with algae. I have, however found halves of dead D. v with a few strings of algae attached at the ligament. I would venture to guess that their being carried up and down the beach so often would provide a sort of sand-blasting that might loosen algae from its hold. I had surmised (without proof) that the algae had only got hold dead D. v that no longer migrated. Bad guess, huh? Art