Barnacle appendage plasticity: Asymmetrical response time-lags, developmental mechanics and seasonal variation

Examples of morphological plasticity now exist across a wide range of taxa and experimental systems. Much of what we know about such plasticity comes from field and laboratory manipulations assessing the magnitude of plastic responses. However, we know much less about how - and how quickly - such changes occur, nor about how changes in form track natural environmental variation. Knowing more about plastic responses under natural conditions is important to help identify developmental costs and limits that may restrict the evolution of plasticity in variable environments. In this study, I measured leg and penis form plasticity in the common Pacific acorn barnacle Balanus glandula. A ten-week transplant experiment under natural conditions in the intertidal revealed: i) barnacles took longer to change leg form than previously thought, ii) response rates depended on transplant direction, and iii) barnacles changed leg length through a combined response of growing longer leg segments and adding more leg segments. Furthermore, a two-year survey of barnacle leg and penis form in natural populations revealed that barnacle leg and penis form varied in a manner consistent with adaptation to seasonal variation in wave height. Together, these results suggest that resource acquisition under natural conditions may be an important and underappreciated limit on morphological response times in natural systems, and shed light on how such changes occur during development.

► A transplant experiment revealed longer response time‐lags than previously thought. ► Quiet-water barnacles took nearly twice as long to change leg length. ► Barnacles grew longer segments and added more segments to lengthen their legs. ► Barnacles changed appendage form seasonally consistent with adaptation to wave force.