When settling, the cyprid larvae of both species tend not to settle above the zone in which they can survive, but how do they photograph of of a whelk Nucella ostrina preying on barnacle spatdifferentiate between a spot on the shore that will remain moist and cool during low tide versus one that dries or heats to an extent that will kill them? Observations in San Juan Island, Washington suggest that the cyprids recognise the presence of indicator species, most likely species of diatoms or other microorganisms. Since the same factors that kill newly settled barnacles also kill potential indicator species, the absence of the latter will signify to the cyprids that the site is unfavourable. If this is true, then cues for recognition of the upper edge of the Balanus intertidal range should be more effective than cues for recognition of the lower edge of the range. This is because the predators that kill off the newly settled Balanus at the lower edge of their distribution would not usually kill off the potential indicator species. Thus, the habitat boundaries set by predators are likely to be poorly correlated with such indicator species. This is an interesting hypothesis, which would repay further investigation with other invertebrate species