Larval settlement in hard substratum epifaunal assemblages: a manipulative field study of the effects of substratum filming and the presence of incumbents

Abstract

Natural larval settlement on black Perspex settlement panels was analysed for two experiments (1 and 2) conducted amongst epifaunal assemblages beneath pier pilings at Williamstown, Victoria and one experiment at Mornington, Victoria, in southeastern Australia. The overall objectives of this manipulative study were to distinguish experimentally the effects of “filming” of the substrata and the presence of previously settled incumbent invertebrates on subsequent larval settlement in the sublittoral. Filming of substrata, whilst largely excluding larval settlement, was permitted by the enclosure of settlement panels within tightly-fitting pouches of plankton gauze of 150 and 236 μm mesh size. Williamstown 1 and Mornington included only fine-mesh treatments and Williamstown 2 included both coarse and fine mesh-sizes and also an additional treatment in which panels were initially filmed intertidally and then deployed sublittorally. Each experiment was conducted over periods of ≈2 wk. For Williamstown 1 filming generally enhanced larval settlement, with the exception of various ascidian species and the erect bryozoan Bugula neritina. Inhibitory effects of incumbents were found for Bugula dentata, the colonial ascidian Trididemnum spp. and Total Settlement: this effect was possibly attributable to Didemnids (excl. Trididemnum spp.). For the repeat of the first experiment (Williamstown 2), Bugula dentata and Serpulins all showed enhanced settlement in response to films developed on panels screened with both mesh sizes. Incumbents inhibited the settlement of Didemnids (excluding Trididemnum spp.) (cf. Williamstown 1) and Encrusting Bryozoans were facilitated. There was no significant difference in sublittoral settlement during the second week of the Williamstown 2 experiment between panels upon which the initial filming was developed either intertidally or sublittorally: settling larvae apparently responded simply to whether or not a substratum was filmed and not to the source of the film, although the possibility remains of rapid modification of the intertidal film to one typical of the sublittoral. At Mornington, Arborescent Bryozoans, Tricellaria occidentaux, Slime Sponges and Spirorbins all showed enhancement of their settlement by filming of the substrata, whereas incumbents significantly inhibited settlement of Didemnids, Microporella sp., Encrusting Bryozoans and Tricellaria occidentalis. The results show clearly the importance of repeating experimental analyses such as these at a given site and of undertaking experiments at different sites. One striking outcome was the marked inhibitory effect on other settlers of Didemnids (excluding Trididemnum spp.) at Williamstown 2. Those ascidians were themselves subject to high levels of post-settlement mortality and, although the cause of their mortality remains unknown, it is possible that the bacteria and ciliates attacking the ascidians were the source of the inhibitory effects on other settlers: these micro-organisms might therefore themselves provide adaptive biological cues indicative of otherwise unfavourable substrata to settling larvae.