GROWTH INCREMENTS OF THE BARNACLE BALANUS AMPHITRITE AMPHITRITE DARWIN (CIRRIPEDIA)

The microstructure and mineral growth increments of juvenile barnacles have been studied by scanning electron microscopy. Specimens were rea red in the laboratory under uniform conditions of day-length and tempe rature and were provided with food in amounts that were not growth-lim iting Growth increments were produced synchronously in the wall plates , scuta, and alae. In scuta, growth involved the deposition of many li near units, minor ridges, within a single growth increment. The size a nd number of scutal growth increments of 25-day-old barnacles raised u nder identical culture conditions showed variability. Increments of th e radius were composed of a layer of parallel needlelike crystals. By growing laterally, the crystals of the radius strengthen the junction between the wall plates. The period required to form an increment in t he scuta was 1.3-1.8 days and the number of increments increased linea rly with age. It appears that a mechanism(s) other than environmental rhythmicity is responsible for incremental skeletal growth in barnacle s that have experienced only constant culture conditions.