Biological vs. physical explanations for the non-random pattern of host occupation by a macroalga attaching to infaunal bivalve molluscs

Summary On a shallow sand flat at Princess Royal Harbour near Albany, Western Australia, the brown macrophyte Hormosira banksii attaches to shells of infaunal bivalves. Hormosira occupies shells of Katelysia rhytiphora in preference to K. scalarina. We proposed and tested four hypotheses to explain this host occupation pattern. First, by following the fate of nearly one thousand marked clams of each species, we rejected the hypothesis that K. rhytiphora exhibits greater longevity and simply possesses more frequent Hormosira because of a longer temporal integration of settlement events. Second, we rejected the hypothesis that K. rhytiphora exhibits higher densities in the top 5 mm of sediment and accumulates more Hormosira because its shell is more abundant in the depth range occupied by the attaching base of Hormosira. Third, we showed that K. rhytiphora because of its larger size is more difficult to dislodge from the sediments than K. scalarina, supporting the hypothesis that Hormosira is rare on K. scalarina because storm waves selectively dislodge and carry to the beach Hormosira attached to K. scalarina. This physical explanation for the Hormosira occupation pattern gets further support from the observation that a third infaunal bivalve, the mussel Brachidontes erosus, has a far higher frequency of Hormosira occupation than either Katelysia species, while providing a much more robust anchor because of its extensive byssal attachments to neighboring mussels. The sizes of Hormosira plants also vary consistently with this physical transport hypothesis: Hormosira is smallest on K. scalarina and largest on B. erosus. Successful colonization of initially unoccupied Katelysia during five 3–9 month periods was also more frequent on K. rhytiphora than on K. scalarina. This suggests a fourth explanation for the Hormosira distribution pattern: that spore settlement is selective for K. rhytiphora in preference to K. scalarina. Although this hypothesis requires further testing, evolution of selective spore settlement would be reasonable given the different likelihoods of subsequent host dislodgement during storm waves.