Ecology and Evolution of Phenotypic Plasticity in the Penis and Appendages of Barnacles

Barnacles, crustaceans of the order cirripedia, typically reproduce via pseudocopluation. Because they are sessile, their penis is long and flexible, capable of stretching to an impressive distance to reach mates. It is generally accordion-like in structure, inflated with hydrostatic pressure and controlled by longitudinal muscles. The penis is lined with setae that are likely used as chemosensory organs during the search for mates. Size, flexibility and setae patterns are all variable among species and are variable within many species in response to environmental conditions. For many barnacles, penis form is phenotypically plastic and may change form in response to waves, hydrodynamic forces or local population density. For others such variability is lacking. Species that have a distinct breeding season rapidly grow a penis in anticipation, only to cast it off with a molt shortly afterward. We will review the functional morphology of penises among the barnacles, discuss how environmental and social factors affect it and explore how this varies among different cirripede taxa. We will also consider the implications that variation in penis form has for mate competition, sex allocation, and population growth and spread.