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 Engla nd) 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 ot her, N. hombergi, has exhibited frequent spawning failures caused eith er by premature oosorption or failure to spawn when gravid. The gonad index (defined as the ratio of gonad biomass to total biomass) is show n 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. homber gi 25.06 (95%cl = 0.29) and the energy requirement for maximum potenti al reproduction has been calculated from these relationships for each age class in the population of the two species. The energy available t o these predatory polychaetes was estimated by investigation of prey d ensity and calorific content of the most common prey species, and the prior suggestion that spawning success may relate to energy availabili ty was tested. The ratio - energy available to energy required - was h igh in two good spawning years (23.7 in 1975/6, 10.5 in 1994/5) but lo w in a poor spawning year (2.5 in 1993/4). Relative spawning success w as also found to be positively correlated with winter sea and air temp erature, suggesting a possible additional link to the physical environ ment. The hypothesis that reproductive failure is a homeostatic mechan ism 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.