Precisely proportioned: intertidal barnacles alter penis form to suit coastal wave action

摘要

For their size, barnacles possess the longest penis of any animal (up to eight times their body length). However,as one of few sessile animals to copulate, they face a trade-off between reaching more mates and controllingever-longer penises in turbulent flow. We observed that penises of an intertidal barnacle (Balanus glandula)from wave-exposed shores were shorter than, stouter than, and more than twice as massive for their length as,those from nearby protected bays. In addition, penis shape variation was tightly correlated with maximumvelocity of breaking waves, and, on all shores, larger barnacles had disproportionately stouter penises. Finally,field experiments confirmed that most of this variation was due to phenotypic plasticity: barnacles transplantedto a wave-exposed outer coast produced dramatically shorter and wider penises than counterparts moved to aprotected harbour. Owing to the probable trade-off between penis length and ability to function in flow, andowing to the ever-changing wave conditions on rocky shores, intertidal barnacles appear to have acquired thecapacity to change the size and shape of their penises to suit local hydrodynamic conditions. This dramaticplasticity in genital form is a valuable reminder that factors other than the usual drivers of genitaldiversification—female choice, sexual conflict and male-male competition—can influence genital form.