Material and methods Directory UMM :Data Elmu:jurnal:J-a:Journal of Experimental Marine Biology and Ecology:Vol248.Issue2.May2000:

208 M .R. Cunha et al. J. Exp. Mar. Biol. Ecol. 248 2000 207 –223 salinity undergoes pronounced changes Kinne, 1970a. Amphipods are thus amongst the most successful animal groups in colonising brackish environments. Their spatial and temporal patterns of abundance are ultimately the consequences of the schedules of fecundity and survivorship that represent life history strategies. Overall the strategy adopted by an organism is a compromise allocation of energy to the various aspects of its life history, each of which contributes to total fitness Begon and Mortimer, 1986. The age of first reproduction, reproductive effort the proportion of the available resource input that is allocated to reproduction and longevity are crucial aspects of life history schedules. The egg size, brood size and number of broods per female are the main traits that determine reproductive effort in amphipods Sainte-Marie, 1991. Brood size in gammarideans is often reported as being proportional to body length of incubating females and this relationship is frequently summarised by a linear regression analysis of the raw or log-transformed data e.g. Fish, 1975; Sheader, 1978; Fish and Mills, 1979; Murdoch et al., 1986. Differences between the mean brood size of two generations or fluctuations throughout the breeding period are usually ascribed to the variation in body length of the incubating females e.g. Dauvin, 1988a,b; Beare and Moore, 1998. However, other studies have shown that this is not a rule. The scatter of values for females of the same size is often large, especially if samples over the entire breeding period are pooled, and sometimes no satisfactory explanation can be found for the variability in the number of embryos per brood, e.g. Echinogammarus obtusatus Sheader and Chia, 1970. The slope of the regression line that represents the increase in brood size with increasing body length can be used as an index of fecundity Sheader, 1978. Several authors have shown that the slope of the regression may change throughout the breeding period, which implies a temporal variation in the size-specific fecundity Sheader, 1978, 1983; Fish and Mills, 1979; Naylor et al., 1988. Environmental factors such as latitude, temperature, photoperiod, oxygen concentration and food availability may be important in determining brood size, as it has been suggested in studies on several amphipod species Kinne, 1959; Vlasblom, 1969; Fish and Preece, 1970; Nelson, 1980; Van Dolah and Bird, 1980; Sheader, 1983; Moore, 1986. In the present work, field data on brood size of Corophium multisetosum are analysed in relation to the body size of incubating females and also to the temperature and salinity throughout the breeding period. These environmental factors, currently assessed in brackish-water studies, are easy to measure and can be used as simple indices of seasonal changes. The analysis aims to produce a mathematical equation allowing the prediction of changes in fecundity according to the seasonal variation of temperature and salinity. The term ‘fecundity’ is used as a synonym for the number of embryos per brood.

2. Material and methods

2.1. Data The analysis hereafter is based on the specimens collected monthly over one year ˜ May 1988–April 1989 in Areao. This site, where C . multisetosum attains maximal M .R. Cunha et al. J. Exp. Mar. Biol. Ecol. 248 2000 207 –223 209 population densities Queiroga, 1990, is located at the upper reaches of Canal de Mira, 2 Ria de Aveiro NW Portugal. Ten corer replicates sampled area510 3 0.01 m were collected at low water of new moon spring tides and the samples were preserved in formalin. In the study area, the average depth at low water is always lower than 0.5 m and the tidal range varies between 0.2 and 1.0 m at neap and spring tides, respectively. Stratification of the water column was never observed. Water temperature and salinity were recorded at low and high water early morning and beginning of the afternoon, respectively near the bottom using a SCT meter YSI model 33. Further details on the sampling methodologies, the seasonal variation of environmental factors and the macrobenthic community in the studied site are given by Cunha and Moreira 1995. Incubating females of C . multisetosum were later separated from the remaining fauna and the developmental stage of the embryos was assessed Cunha et al., 2000a. The size of the females, expressed as head length, was measured to the nearest 1 60 mm in three replicates and the number of embryos per brood was counted only in those females with an undamaged marsupium. In order to minimise the error resulting from intramarsupial loss only females carrying embryos in an early developmental stage F females with 1 rounded embryos were considered for the statistical analysis. Further details on the abundance, biomass, production, life history and reproductive biology of C . mul- tisetosum in the site studied are given by Cunha et al. 2000a,b. 2.2. Statistical analysis The significance of the head length L , temperature T and salinity S as sources of h variation in the brood size N of C . multisetosum was first assessed by a simple e factorial ANOVA SPSS package. The relation between brood size and each factor was analysed separately and then the coefficients of the general equation were estimated using a non-linear regression model SPSS package.

3. Results