Variable spawning success of Nephtys hombergi (Annelida: Polychaeta) in response to environmental variation: a life history homeostasis?

The reproductive success of Nepthys caeca and Nephtys hombergi (class Polychaeta, family Nephtyidae) in an estuarine beach (R. Tyne NE England) has been followed through a twenty year period. Although the data set is not quite complete it shows that during this period one of the two species, N. caeca, has spawned completely each year whereas the other, N. hombergi, has exhibited frequent spawning failures caused either by premature oosorption or failure to spawn when gravid. The gonad index (defined as the ratio of gonad biomass to total biomass) is shown to be an age/size independant species constant with a value of 0.24 in N. caeca and 0.33 in N. hombergi. The calorific values of the gonad tissues in maturing N. caeca is 19.95 (95%cl = 0.32) and in N. hombergi 25.06 (95%cl = 0.29) and the energy requirement for maximum potential reproduction has been calculated from these relationships for each age class in the population of the two species. The energy available to these predatory polychaetes was estimated by investigation of prey density and calorific content of the most common prey species, and the prior suggestion that spawning success may relate to energy availability was tested. The ratio - energy available to energy required - was high in two good spawning years (23.7 in 1975/6, 10.5 in 1994/5) but low in a poor spawning year (2.5 in 1993/4). Relative spawning success was also found to be positively correlated with winter sea and air temperature, suggesting a possible additional link to the physical environment. The hypothesis that reproductive failure is a homeostatic mechanism involved in the trade off between reproductive effort and adult or juvenile survival is discussed in relation to these observations. (C) 1997 Elsevier Science B.V.