Further experiments on larval behaviour of the tubicolous polychaete Spirorbis inornatus L'Hardy & Quiévreux

Laboratory experiments indicate that there are many ecologically isolated populations of Spirorbisinornatus L'Hardy & Quiévreux, each tending to select the algal substratum which supported their parents. In some algae certain parts of the thallus are particularly favourable for settlement and, in some areas, this involves adaptation of the local population, e.g. to settle on fronds of Laminaria in sheltered localities. Other parts are unfavourable and these are generally the parts which are most illuminated in their natural orientation (e.g. the upper sides of Himanthalia buttons and the outer sides of Laminaria holdfast branches). These parts are active in producing antibiotics, which may thus have an antifouling role, deterring settlement of larvae whether or not those larvae come from populations adapted to settling upon other parts of the thallus.Most larvae from populations on the turf-alga Chondrus settle without any pelagic phase, whereas the majority from populations on Himanthalia and Laminaria holdfasts become pelagic. The proportions choosing holdfasts in laboratory experiments are high, however, suggesting that the need for skill in such choice, by these pelagic populations, more than balances any effect from outcrossing.