Are there adequate data to assess how well theories of rarity apply to marine invertebrates?

Understanding the biology of rare species is a very important part of conse rvation biology. Most of our current understanding of rarity has, however, come from studies of terrestrial plants, birds, mammals and some insects. F reshwater and marine habitats are underrepresented in published studies of rare species or conservation biology. We therefore have little knowledge ab out how well our understanding of what makes particular species rare and ho w rare species persist applies to marine invertebrates which form a major c omponent of coastal biodiversity. In this review, I examine some theories a bout rarity with respect to intertidal and shallow subtidal invertebrates t o identify whether there are adequate data to apply these theories to marin e invertebrates and how well such theories apply. The general conclusions a re that the lack of quantitative data on abundances, ranges, habitat-requir ements, dispersal and connectedness among populations for marine invertebra tes means that their status as rare species cannot really be assessed appro priately. It is also unlikely that, without extensive sampling programmes a nd considerable expense, adequate data could be obtained for these small, c ryptic animals, which typically have very patchy, variable and unpredictabl e patterns of distribution and abundance. Intertidal and subtidal assemblag es are diverse, including species with many different life-histories from m any phyla, occupying the same suite of habitats. It is therefore suggested that future research on rare organisms in marine habitats should build upon the long and successful history of experimental marine studies to test spe cific hypotheses about processes influencing rarity in the field. Such stud ies would not only add a new dimension to our current understanding of rari ty, but would also provide badly-needed data on the status of rare marine i nvertebrates.