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Past tests of settlement by marine invertebrate larvae on different colours were not designed to distinguish between responses to substrate colour and substrate brightness. Using colour vision testing methods, the design of this study provided a true test for responses to colour by including both coloured and grayscale settlement plates. The dominant fouling taxa at the test sites (the solitary ascidian, Ciona intestinalis ; the colonial ascidians Botryllus schlosseri or Botrylloides violaceous ; and a bryozoan Bugula sp.) showed no significant differences in settlement between blue, red or green plates. In contrast, the ascidians responded to substrate brightness with significantly lower settlement on lighter plates, while the bryozoans showed no significant preference relative to substrate brightness. The methods used here are a model for future tests of responses to colour during settlement. In addition, the contrasting responses to brightness are at odds with previous laboratory experiments that showed larvae of both ascidians and bryozoans are negatively phototactic at settlement. These results suggest that other cues must supersede any phototactic responses in the bryozoans, or alternatively that the range of light intensities near darker and lighter substrata were not part of the tests in the earlier reports. [ABSTRACT FROM AUTHOR]

Abstract: Anthropogenic marine habitats, such as marinas and breakwaters, are frequently colonized by nonindigenous species (NIS). Comparative studies show that few sessile NIS are found in nearby natural habitats, but little is known about the processes affecting the spread of NIS into these habitats. In southern British Columbia, the invasive colonial ascidian Botrylloides violaceus is widespread, but is far more common on man-made structures at marinas and aquaculture facilities than adjacent natural rocky reefs. To determine whether predation by native species, one element of biotic resistance, influences the successful infiltration of B. violaceus into natural benthic habitats, we conducted a series of predator exclusion experiments. Established adult colonies and newly settled juvenile B. violaceus were transplanted onto replicate PVC panels and deployed at marina pilings and adjacent natural rocky reefs at two locations. Panels were assigned to caged, uncaged, and procedural control treatments to test whether predator exclusion changed survivorship. Both juvenile and adult ascidian survival was significantly lower in uncaged treatments, suggesting that predation can limit the abundance and distribution of this species. Results were similar between piling and rocky reef habitats regardless of life stage and our results suggest that adult and juvenile B. violaceus are vulnerable to predation in both habitats studied. If B. violaceus propagules can disperse from dock floats and infiltrate adjacent natural rock, biotic resistance, through predation by native species, may diminish the likelihood of successful colonization in these habitats. Ensuring that natural habitats and native benthic communities remain healthy and intact may reduce the likelihood that invasive ascidians will spread beyond anthropogenic introduction sites where they can dominate the fouling community. [Copyright &y& Elsevier]