Reproduction, larval features and life history of <i>Philine denticulata</i> (J. Adams) (Mollusca - Tectibranchia)

Abstract

Abstract The minute and somewhat aberrant Philine denticulata, which is common in parts of the Øresund, Denmark, was studied. Its egg mass, newly hatched larvae, settling larvae and juveniles, which have hitherto been confused with those of related species, are described. The species has an annual life-cycle. Its spawns were found from May to August and the first substantial settling oflarvae was observed in July. The assessment of growth met with considerable difficulties owing to the very small increments found in the species during the first 5 months after settling and owing to the peculiar formation of the nepionic shell. To overcome these difficulties a special parameter had to be used. The absolute growth rate was found to be low in autumnwinter and high from March onwards. Growth in an exposed locality is more rapid in springsummer, but slightly slower in autumn-winter, than in a sheltered place. The population density of the new generation increases until the end of August, then decreases sharply and is very low after December. A greater population density of the adult generation results in a greater population density of the subsequent new generation, but survival rate of the young appears to be unrelated to initial settling density. High mortality is masked by repeated larval settling. An apparent. low growth rate of the species, evaluated on the basis of an average population during autumnwinter, is mainly caused by 1) a combined effect of high mortality and settling of young on the bottom until December, and 2) seemingly unfavourable changes in the environment. Seasonal variations of the vertical distribution of temperature and salinity in the Øresund were found to be rather complicated. An attempt is made to correlate these sesasonal changes of the hydrographical conditions and of the primary production with major events in the life-cycle of the species.