Reproductive ecology of Littorina Rudis (Maton) in the estuarine Firth of Forth

Abstract

Reproductive ecology of Littorina rudis (Maton) was investigated at three localities in the Firth of Forth from virtually open sea at Aberdour to near the up-estuary limit of the species at Culross some 27 km west. Striking inter-site differences in abundance, population size-composition and minimum size at maturity were observed. Maximum brood-pouch counts (up to 612 embryos per female) occurred in late spring/early summer and minima(including several empty) occurred in late summer/early autumn. These seasonal changes in embryo numbers were more prounced at the estuarine than the marine sites. Corresponding changes occurred in the condition of ovaries and testes. Seasonal fluctuations in body protein, energy and C : N ratios were also related to reproductive condition. Numbers of embryos contained in brood-pouches on the upper shore at Aberdour were consistently 2 - 3 X those at Torrybay. Values on the lower shore at Aberdour and at Culross fell between these extremes. Embryo weight between sites varied significantly, differences in weight tending to compensate for varying numbers. Complete development of eggs ill vitro at 32 % salinity took 41 - 115 days between 5 - 15°C indicating a Q10 of 3 between 5 - 15°C. Embryos failed to develop at 20°C. Successful development was severely restricted at salinities of less than 20 - 24 % at 10°C. Newly released juveniles were less tolerant of reduced salinity than adults on the same shore. L^. rudis from the estuarine sites of Torrybay and Culross tolerated low salinities better than those at the marine Aberdour. Cyclical patterns of juvenile release were observed in a laboratory tide tank with maximum release close to successive new moons. Discrepancies in reproductive production estimates from brood-pouch loads and from juvenile release indicated that not all eggs entering the brood-pouch complete development. A tentative model to predict release from brood-pouch loads is proposed. Estimated production of juveniles at Torrybay (63.31 kJ m-2 y-1) was more than 2 X that on the upper shore at Aberdour (27.08 kJ m-2 y-1) and 16 X that on the lower (3.86 kJ m-2 y-1). At Torrybay an estimated 29% of the biomass was turned over annually as newborn compared with 14.8% and 10.3% on the upper and lower shore respectively at Aberdour.