Responses of settling invertebrate larvae to bioorganic films: Effects of large-scale variation in films

Results of previous studies have shown that at two sites in-southeastern Australia (Williamstown and Mornington), some sessile invertebrate species recruit at higher densities onto surfaces that have a microbial film present, and that some species recruit in proportion to the age (0-6 days) of that film. Other sessile species, notably colonial ascidians, do not respond to the presence of these films, and some barnacles may avoid the films. We tested whether larvae can detect or respond to differences in the microbial films that develop in different geographic localities and on longer time scales. To do this, we first confirmed that laboratory-developed microbial films induced higher recruitment of benthic invertebrates at a third site, Queenscliff, in Port Phillip Bay, Victoria, Australia. Approximately two-thirds of the sessile species settling at the time of the experiment were more abundant on the filmed surfaces. We then allowed microbial films to develop at all three sites on experimental substrata that were covered with plankton mesh to exclude larvae. The sites were chosen to represent three different rates of development of microbial films. After 1 week, we removed the plankton meshes and reciprocally transplanted experimental surfaces between all three sites. At the same time, we brought a sample from each site back to the laboratory to estimate the number of recruits passing through plankton meshes. We tested two hypotheses using these data, first that settling larvae at a particular site respond more strongly to locally-developed films than to films originating elsewhere and second, that it is the density of the microbial film, rather than its origin, that determines recruitment. Williamstown appeared to have the fastest-growing, most luxurious microbial films, followed by Mornington and Queenscliff. Locally-developed films were not significantly more attractive than foreign films at Mornington or Queenscliff; none of the 15 taxa tested showed higher recruitment rates to the local treatment, and highest recruitment occurred on substrata filmed at Williamstown. At Williamstown, there were differences in recruitment rates to plates with films of different origin, but in each case, films from Williamstown received the most recruitment. These latter results could not separate our two hypotheses, but those from Mornington and Queenscliff suggest that larvae do not recognize and/or respond to films from their local area, but that more heavily-filmed surfaces may be more attractive lit settling larvae. The species showing rile strongest responses were polychaetes, encrusting bryozoans. and some solitary ascidians. We also compared the attractiveness of laboratory-developed films varying in age up to 1 month and round that some species (serpulid polychaetes and encrusting bryozoans) recruited onto these surfaces ar variable rates, while other species, notably ascidians, did not distinguish between the different substrata.