The genetic structure of intertidal populations of two species of nudibranch molluscs with planktotrophic and pelagic lecithotrophic larval stages: are pelagic larvae "for" dispersal?

Planktonic larvae of benthic marine invertebrates may confer larval dispersal and colonization potential on the reproductive adult. Analyses of genetic heterogeneity of local populations provide an indirect means of assessing gene how and the scale of realized dispersal of larval migrants. Here we compared populations of two species of nudibranchs with long term planktotrophic (Goniodoris nodosa) and shorter term pelagic lecithotrophic (Adalaria proxima) veliger stages. Both species have annual life cycles, population turnover is absolute and free-swimming larvae hatch from benthic spawn masses. G. nodosa larvae are pelagic for up to similar to 3 months, whereas those of A. proxima can metamorphose within 1-2 d of release although, at least in the laboratory, they may delay metamorphosis for several weeks. The broad ranges of pelagic larval transport potential were estimated at 10(2)-10(3) km for C. nodosa and 10(1)-10(2) km for A. proxima: significant differentiation of populations of G. nodosa was predicted to occur on a scale of 10(3)-10(4) km, whereas for A. proxima differentiation was expected on a scale of perhaps only 10(2)-10(3) km. For both species, analyses were undertaken of allozyme variation among sites throughout the 1600 km British Isles range of A. proxima. F-statistic analyses showed no evidence of population differentiation for G. nodosa (11 populations). By contrast, comparable data for 19 populations of A. proxima showed very high levels of differentiation, but no obvious pattern of variation or significant isolation by distance. More detailed sampling of nine populations of A. proxima over a 26.18 km range in W Scotland - in a location characterised by highly dispersive tidal currents - revealed (1) similar levels of differentiation to the 1600 km range, (2) clear distance-related clinal patterns of variation, and (3) a significant inverse relationship between genetic differentiation and geographic distance. For Adalaria proxima there was evidence of significant differentiation of populations on a scale of less than or equal to 10(1) km. This was counter to our expectations of larval transport potential for A. proxima and implies that these larvae are behaviourally adapted to avoid dispersal in the water column and thereby recruit locally. Whilst the planktotrophic species, G. nodosa, conformed to our expectations, the clear implication of the data for A. proxima is that possession of a pelagic larval stage does not necessarily facilitate dispersal and colonisation potential for this sedentary invertebrate. Extrapolations and predictions of dispersal potential, and gene flow in natural populations, based on laboratory observations of the larval phase duration may be markedly error-prone. (C) 1998 Elsevier Science B.V. All rights reserved.