The Effects of Wave Exposure, Tidal Height, and Crowding on Cirri and Penis Morphology of the Acorn Barnacle, Tetraclita stalactifera

Exposure to wave action can alter the morphology of intertidal barnacles. We tested several hypotheses of morphological variation in the cirri and penises of the barnacle Tetraclita stalactifera at sites in south Florida differing for three factors: wave exposure, height in the intertidal zone and level of crowding. In wave exposed sites, cirri were shorter and thicker than in protected sites. Increased thickness may be an adaptation to reduce risk of breakage in rough environments. The longer cirri of individuals from sites with low wave action may serve as an adaptation to improve food capture in lower-flow environments. We found that barnacles from higher positions in the intertidal zone had thicker cirri than those from lower positions, suggesting that they experience more risks from wave action. Barnacles at high positions may accept greater risks as a result of less time available for feeding because of reduced time submerged. The increased risks of different thresholds to wave exposure may be compensated for by the increased thickness. Penises from wave-exposed sites and from higher positions in the intertidal zone were thicker than those from calm areas or from low positions (but there were no interactions between wave-exposure and tidal height). Thicker penises are likely stronger, reducing the risk of breakage and possibly more muscular, allowing them to retain function in rough conditions. None of the morphological variables changed with crowding. Our observations of differences in cirri and penis morphology among sites of varying physical conditions suggest that these traits, observed in several other barnacle species, are adaptations shared by the species T. stalactifera, although the pattern we observed with respect to height in the intertidal was opposite of that observed by other researchers for T. japonica