In order to withstand tidal currents, juvenile horseshoe crabs frequently burrow in the sand  Such burrowing discourages barnacles from clinging to their shells  When fully grown, however, the crabs can readily withstand tidal currents without burrowing, and thus they acquire substantial populations of barnacles
Surprisingly, in areas where tidal currents are very weak, juvenile horseshoe crabs are found not to have significant barnacle populations, even though they seldom burrow
The strength of the tidal currents in a given location varies widely over the course of a day.