Small-scale spatial genetic structure in Scopalina lophyropoda, an encrusting sponge with philopatric larval dispersal and frequent fission and fusion events

Insight from the spatial genetic structure (SGS) of a species is fundamental to understanding the patterns of effective dispersal and gene flow among its populations. Despite the ecological importance of sponges and the variety of reproduction and dispersal strategies they present, which can strongly influence SGS, there is only 1 study assessing small-scale SGS in sponges. That species had a continuous distribution and relatively wide-scale larval dispersal. Here we study the contribution of sexual and asexual reproduction, and the breeding and mating system to the SGS of a sponge species, Scopalina lophyropoda, with a patchy distribution and more limited larval dispersal. All individuals from 3 populations were mapped and genetically characterised for 7 microsatellite loci. The extent of clonality was minor (ca. 7%), possibly caused by a balance between fissions and fusions. The scarce clonality did not contribute to the SGS, which was analysed by autocorrelation statistics at both the ramet (including clones) and the genet (excluding clones) levels. The spatial autocorrelation analyses elicited a pattern of strong SGS at the small scale, confirming the predictions of philopatric larval dispersal, which fosters isolation by distance. All these patterns, however, contrast with the conspicuous lack of inbreeding detected in the populations, which is in agreement with recent data on other marine modular invertebrates and confirms that strong SGS does not necessarily imply inbreeding.