Reproductive success,relative abundance and population structure of two species of Nephtys in an estuarine beach

During 1978 and 1979 specimens of Nephtys hombergii in the River Tyne estuary (North East England) became gravid during the winter but did not spawn in the spring breeding season; instead their gametes were resorbed. The drastic effects of these spawning failures were apparent in the population structure in 1980, when the 1978 and 1979 year-classes were shown to be virtually absent, by the size of the individuals in the population and the numbers of growth lines in their jaws. The population structure in 1975, by contrast, indicated that recruitment during the preceding 4 to 5 yr had been good. Spawning was also effective in 1980, and an 0 group was detected in August 1980. Comparable data has been obtained for N. caeca, which occurs in the same beach. It shows that recruitment of this species during 1976–1980 was consistently good, whereas analysis of the age-class frequencies suggests that recruitment of N. caeca was poor in 1973 and 1975. The dominance relationship of the two species in the beach has been reversed during the period 1975–1980. It is suggested that the sympatric distribution of the two species is maintained in part by periodic reproductive failure. The causes of this are not yet known; possible reasons, including the extremely cold conditions experienced in 1978–1979 are discussed. The difference in frequency of the 1978, 1979 and 1980 year classes will now permit a rigorous evaluation of the relationship between the number of growth lines in the jaws and age in N. hombergii.