RESPONSES OF SETTLING INVERTEBRATE LARVAE TO BIOORGANIC FILMS - EFFECTS OF DIFFERENT TYPES OF FILMS

Larvae of benthic invertebrates are exposed to a variety of chemical cues, many of biological origin, as they near a potential settlement surface. One such cue is the bioorganic film that covers many surfaces, and some laboratory evidence suggests that larvae respond to the presence or absence of such films. Our earlier work has shown that, at two field sites in southeastern Australia, a variety of species recruit at higher densities to substrata bearing such a film, and here we begin to determine the specificity of those responses. Experimental substrata were prepared with films developing in laboratory aquaria and under field conditions for 7 days. These substrata were then exposed in the field, and analysis of 19 sessile taxa, distributed over seven phyla, showed that no species recruited at significantly different densities onto surfaces of these two origins. Field films were prepared by covering substrata with plankton meshes, to exclude larvae, and additional laboratory treatments showed that such handling does not produce different recruitment from that onto substrata that were unnetted. One further experimental treatment was plates that had been placed in sterile seawater, and recruitment onto these surfaces confirmed that both field and laboratory films did have positive (and negative) effects on recruitment. In a second experiment, we prepared laboratory microbial films of four ages: 0, 1, 3, and 6 days. These substrata were immersed in the field for 3 and 6 days in a balanced experiment. Recruitment rates varied significantly with age of film for five of 13 taxa. Two polychaete species and two arborescent bryozoans (Bugula stolonifera and Bugula neritina) showed a positive relationship, and the barnacle Balanus variegatus recruited in greater numbers onto less filmed surfaces. Seven other taxa, including one protozoan, a sponge, and five ascidian taxa, showed no relationship with age of microbial film, despite high recruitment rates of these taxa. Summed across all taxa, total recruitment increased with age of film, almost doubling from unfilmed surfaces to those that began the field period with 6-day-old films. Comparison of 3- and 6-day field immersion periods showed that mean daily recruitment of some species, including a number of those responding to the presence of films, was higher on substrata left in the field for 6-days. Some of these patterns were consistent with the hypothesis that laboratory-derived films become supplemented by further microbial organisms in the field, making them even more attractive to settling larvae. An alternative hypothesis, that higher recruitment onto plates immersed for 6 days reflects temporally patchy recruitment, with pulses of recruitment in the latter half of the experimental period, could not be rejected. The experiments show that under field conditions, invertebrate larvae settle at very different rates onto surfaces differing only in the composition of the microbial community. Such settlement choices may be important in determining recolonization sequences following small-scale disturbances.