OPERCULAR REGULATION IN THE POLYCHAETE HYDROIDES DIANTHUS (VERRILL, 1873). I. OPERCULAR ONTOGENY, DISTRIBUTION AND FLUX

¹ Division of Biological and Medical Sciences, Brown University, Providence, Rhode Island and The Marine Biological Laboratory, Woods Hole, Massachusetts

Although any population of Hydroides dianthus maintains an equilibrium of equal proportions of right- and left-handed animals, individual members are continually modifying their condition. They may spontaneously autotomize the functional operculum, permitting the rudimentary one to enlarge. Alternatively, the rudimentary operculum may develop independently to yield an unstable situation in which the animal bears two large opercula. This may persist for several weeks; then the older of the two opercula autotomizes. Either mechanism results in a reversal of the original opercular asymmetry. The rate of variation in left- and right-handed individuals is equal, so that population balance does not shift even though its component organisms undergo continual flux.

Each animal undergoes several reversals during a normal life span, beginning with the larval stages. The rate of change declines gradually so that the older the animals, the less frequently it will reverse. Animals maintained in the ocean subject to predators show a high rate of reversal and traumatic aberrations.

In larval ontogeny the origin of an opercular cup on the second left branchial rudiment is followed by (a) loss of pinnules on the opercular stalk to form the primary operculum or (b) severance of the opercular cup from the branchia, perhaps by progressive attenuation of the terminal end of the filament. In the latter case, a primary operculum must be developed on the left. This seems to be accomplished by the formation of a left rudimentary operculum, developing later into a primary one. This operculum, no matter how it originates, may either be lost and replaced by a similar structure, on reversal may occur. These processes may be repeated until this simple operculum is permanently replaced by an adult double-cupped operculum. The time at which this occurs is not fixed, and there is a transitional period during which either type of operculum can appear. After further maturation, only the adult type of functional operculum can be formed. This coincides with the development of the ability to carry out opercular regulation.