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 c ues, many of biological origin, as they near a potential settlement su rface. One such cue is the bioorganic film that covers many surfaces, and some laboratory evidence suggests that larvae respond to the prese nce 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 substrat a 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 cover ing substrata with plankton meshes, to exclude larvae, and additional laboratory treatments showed that such handling does not produce diffe rent recruitment from that onto substrata that were unnetted. One furt her experimental treatment was plates that had been placed in sterile seawater, and recruitment onto these surfaces confirmed that both fiel d and laboratory films did have positive (and negative) effects on rec ruitment. In a second experiment, we prepared laboratory microbial fil ms 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 rate s varied significantly with age of film for five of 13 taxa. Two polyc haete species and two arborescent bryozoans (Bugula stolonifera and Bu gula neritina) showed a positive relationship, and the barnacle Balanu s variegatus recruited in greater numbers onto less filmed surfaces. S even 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 recruit ment increased with age of film, almost doubling from unfilmed surface s to those that began the field period with 6-day-old films. Compariso n of 3- and 6-day field immersion periods showed that mean daily recru itment of some species, including a number of those responding to the presence of films, was higher on substrata left in the field for 6-day s. Some of these patterns were consistent with the hypothesis that lab oratory-derived films become supplemented by further microbial organis ms in the field, making them even more attractive to settling larvae. An alternative hypothesis, that higher recruitment onto plates immerse d for 6 days reflects temporally patchy recruitment, with pulses of re cruitment in the latter half of the experimental period, could not be rejected. The experiments show that under field conditions, invertebra te 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 sm all-scale disturbances.