Choosing between darkness and light: The ecological genetics of photic behaviour in the planktonic larva of Spirorbis borealis

The light-dark preference of individual Spirorbis borealis larvae was measured by the relative amount of time spent in the lighted half of a container. Specimens taken from a tidal pool were 20% more photonegative than those obtained outside the pool, probably because of natural selection against being washed out of the pool at high tide. The between-population behavioural difference was maintained in the laboratory, and is genetic in origin. Applying biometrical genetic analysis to larvae grouped into half-sib families, the within-population variation was used to estimate an upper limit to the heritability of 0.36. It is concluded that the differences among populations greatly exceed heritable variation within populations. A numerical selection model based on the estimated maximum heritability suggests, however, that the populations could diverge by 20% in less than a year, in response to local differences in the selection regime.