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-southeaste rn Australia (Williamstown and Mornington), some sessile invertebrate species recruit at higher densities onto surfaces that have a microbia l film present, and that some species recruit in proportion to the age (0-6 days) of that film. Other sessile species, notably colonial asci dians, do not respond to the presence of these films, and some barnacl es may avoid the films. We tested whether larvae can detect or respond to differences in the microbial films that develop in different geogr aphic localities and on longer time scales. To do this, we first confi rmed that laboratory-developed microbial films induced higher recruitm ent of benthic invertebrates at a third site, Queenscliff, in Port Phi llip 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 diff erent rates of development of microbial films. After 1 week, we remove d the plankton meshes and reciprocally transplanted experimental surfa ces between all three sites. At the same time, we brought a sample fro m 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 str ongly to locally-developed films than to films originating elsewhere a nd second, that it is the density of the microbial film, rather than i ts origin, that determines recruitment. Williamstown appeared to have the fastest-growing, most luxurious microbial films, followed by Morni ngton 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 tre atment, and highest recruitment occurred on substrata filmed at Willia mstown. At Williamstown, there were differences in recruitment rates t o 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 Queens cliff suggest that larvae do not recognize and/or respond to films fro m their local area, but that more heavily-filmed surfaces may be more attractive lit settling larvae. The species showing rile strongest res ponses were polychaetes, encrusting bryozoans. and some solitary ascid ians. We also compared the attractiveness of laboratory-developed film s varying in age up to 1 month and round that some species (serpulid p olychaetes and encrusting bryozoans) recruited onto these surfaces ar variable rates, while other species, notably ascidians, did not distin guish between the different substrata.