Larval supply and recruitment of benthic invertebrates: do larvae always disperse as much as we believe?

Abstract

The concept of larval supply, and the relative importance of pre- and post-settlement processes as structuring agents of marine benthic invertebrate assemblages and communities are discussed. Because pelagic larvae result in an essential decoupling of local reproduction from local larval recruitment, many marine populations should be considered demographically ‘open’. Accordingly, factors affecting larval input to the benthos might be especially important in structuring both populations and communities, and even supercede the importance of post-settlement processes such as competition, predation and physical disturbance. Comprehensive studies on intertidal barnacles have provided an initial focus for field studies, but perhaps the greatest challenge facing marine ecologists lies in extrapolating beyond single-species populations to assess the relative importance of pre- and post-settlement processes for whole assemblages or communities. Amongst rocky intertidal assemblages, larval supply processes do appear especially important in circumstances where recruitment levels are low: at higher recruitment levels, post-settlement processes are prominent. No comprehensive evaluation of the relative importance of pre- and post-settlement processes has been undertaken for soft sediment assemblages, but the prevailing consensus is that hydrodynamic factors, density dependent effects of food availability and post-settlement processes are of prime importance. For epifaunal ‘fouling’ assemblages, probably both pre- and post-settlement factors are very important, and illustrative data are presented to show that post-recruitment processes may mask even strong larval signals in later assemblage development. Short-term panel data showed that hard substratum epifaunal recruitment levels do not necessarily conform to simple measures of current velocity at a given site: application of a tidal flushing model did, however, assist in explaining the variation in larval recruitment rates in relation to water flux below an arbitrary critical velocity for larval settlement. An indirect means of assessing realized larval dispersal – and hence geographic scale of population ‘openness’ – is available from analyses of population genetic differentiation. Data are presented for two species of epifaunal molluscs with contrasting larval types. The planktotrophic species conformed to expectations, in showing genetic homogeneity of populations over a 1600 km range, and therefore in displaying large-scale larval dispersal. The pelagic lecithotrophic species, however, revealed population differentiation on very small scales (< 10 km), even in highly dispersive environments with strong tidal currents (up to 3.6 m s-1). The deduction is that contrary to actually facilitating dispersal, those pelagic lecithotrophic larvae are behaviourally constrained to minimize larval transport. The consequences of this to general deductions about ‘openness’ of epifaunal assemblages are discussed, in the knowledge that spatial domination there is attributable largely to three phyla which characteristically reproduce by means of short-term pelagic lecithotrophic larval stages.