Preliminary studies on the reproduction and population dynamics of Monodonta lineata and Gibbula umbilicalis (Mollusca, Gastropoda) on the central coast of Asturias (N. Spain)

Summary

Populations of Monodonta lineata and Gibbula umbilicalis were studied at Aramar beach (Luanco, Asturias) from October 1982 to November 1983. Both species show a sexual ratio of 1: 1 but differ in their gonadal cycle patterns. M. lineata gonadal development stages are found from November to May–June in a significant part of the population. Breeding stages occur from June–July to September, and in some individuals to November, with complete discharge of all spawn at the end of the reproductive season. Some degree of inter-annual variation is detected. Spawning stages of mature G. umbilicalis are found through the year but the higher frequencies occur in summer and autumn samples, with development phases predominating in January to May. Animals over 10 mm in shell diameter maintain their gonads close to spawning stages, completely spawned individuals being less than 10 percent. In both species variability in gonad stages for a given month reduces as size increases, indicating a certain synchrony.

The new recruits of M. lineata are detected on the bare rock by September to November and are recognizable through their first year. Due to the amplitude of the breeding period and differing growth rates size distribution within an age class is wide. Size class frequencies rapidly decrease after the second year of life. The data for G. umbilicalis are difficult to interpret given the scarce number of juveniles and size overlapping.