On rocky shores in San Juan Island, Washington and in other similar west-coast locations, barnacles Balanus glandula and Semibalanus cariosus occupy high and low zones on the shore, respectively. The upper and lower limits of distribution of Balanus are set by tolerance to drying/extreme temperatures and by predation, respectively. The upper limit of S. cariosus is also set by mortality of young stages through desiccation or extreme temperatures. 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. Strathmann & Branscomb 1979 p. 77 In, Reproductive ecology of marine invertebrates (Stancyk, ed.) U South Caroline Press, Columbia.
NOTE the authors use settling plates set out at different intertidal levels to assess the relative importance of different factors. Their data suggest that drying and temperature are the most important, while light/shade are unimportant
A whelk Nucella ostrina appears to be preying on spat ofBalanus
glandula that have settled around the bases of several adult
Semibalanus cariosus. Note as well the presence of a few recently