Morphological and genetic adaptation to a lagoon environment: a case studyin the bryozoan genus Alcyonidium

The Fleet (southern England) is a stable (ca. 5,000 years) coastal saline l agoon that supports a population of Alcyonidium resembling the common coast al epiphyte, Alcyonidium gelatinosum (L.). A combination of morphological, reproductive, and ecological characters was used to compare lagoonal and no n-lagoonal proximate populations. Comparisons revealed a difference in the timing of spawning, considered to be related to the temporally restricted a vailability of viable substrata within the lagoonal basin. Allochronous spa wning and spatial separation together suggest that the lagoonal taxon is re productively isolated. The two populations were further compared with seven other coastal populations of Alcyonidium using randomly amplified polymorp hic DNA (RAPD) analysis. The results confirm the individuality of the lagoo nal taxon but also a close relationship with three A. gelatinosum populatio ns. We present and consider four hypotheses that may account for the presen ce of this genetically distinct taxon: (1) diversification within the Fleet ; (2) colonisation from another lagoon; (3) a southern lagoonal species at its northern limit; and (4) introduction by shipping or other anthropogenic ally mediated dispersal mechanism. Significant diversification on the time scale involved has been demonstrated for isolated freshwater environments a nd, therefore, is feasible within a saline lagoon. Hypothesis I and, to a l esser extent, hypothesis 2 are consistent with the recognition of individua l lagoons as 'biogeographic' islands of importance for their unique or char acteristic biodiversity. The study also represents the first example of con cordant morphological, reproductive, and genetic diversification in a marin e bryozoan.