A true test of colour effects on marine invertebrate larval settlement

• Experimental design: testing settlement responses to substrate colour and brightness.
• Ascidian and bryozoan species showed no settlement responses to colour.
• Solitary and colonial ascidians settled at higher density on darker substrates.
• Bryozoans showed no settlement response to substrate brightness.

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.