Spatial and temporal distribution of barnacle larvae in the partially mixed estuary of the Ría de Arousa (Spain)

The aim of this study was to determine the spatial and temporal distribution of the larvae of several barnacle taxa (Elminius modestus, Balanus group, Chthamalus spp., Verruca stroemia and Pollicipes pollicipes) in the partially mixed estuary of the Ría de Arousa (Galicia, NW Spain). The study was conducted during nearly three years (1999–2001) at seven larval stations distributed along the ria. The possible larval dispersal mechanisms involved are discussed using the information of the intertidal distribution in the ria of adult E. modestus, Perforatus perforatus, Balanus crenatus, Balanusimprovisus, Semibalanus balanoides, Chthamalusmontagui, Chthamalusstellatus, V. stroemia and P. pollicipes.The larvae of all barnacle species, with the exception of Balanus, showed a spatial gradient of distribution along the estuary, reflecting adult distributions in the intertidal zone. E. modestus larvae were most abundant in the inner part showing marked larval retention inside the estuary. Chthamalus larvae showed highest larval abundance in the outer part of the ria to lowest in the inner part, parallel to the spatial adult distribution along the estuary, suggesting that it completes its larval development inside the estuary due to some mechanism of larval retention. The larvae of V. stroemia and P. pollicipes entered the estuary far beyond where their adult populations were established, while Balanus larvae were widely distributed along the whole ria without any gradient.Based on the barnacle larval abundance, the sampling stations grouping divide the Ría de Arousa into two main groups along the estuarine salinity gradient: the inner one was clearly dominated by E. modestus and Balanus while the central–outer one was dominated by Balanus, Chthamalus and V. stroemia. Moreover, both areas (specially the central–outer one) could be again subdivided based on the relative presence of the barnacle species more characteristic of the extremes of the estuarine gradient (E. modestus and P. pollicipes).All the larval cirripedes in the Ría de Arousa, with the exception of V. stroemia, showed the typical seasonal abundance pattern of temperate species, generally being most common in spring and summer. Balanus and E. modestus larvae were present in the ria during the whole year, with Balanus peaking mainly in spring and summer, while E. modestus did not display a seasonal pattern that was repeated among years, normally peaking in summer, although in some years the larval abundance in autumn and/or spring was much higher. In contrast, Chthamalus had the shortest period of larval occurrence (spring and summer) with strong seasonality. Larvae of the boreal barnacle V.stroemia were collected throughout the year, with the exception of November and December, although its temporal distribution differed from the other temperate barnacles found in the ria, winter and spring being the seasons of higher abundance, instead of spring–summer.