Seasonal cycles of egg production of two planktonic copepods, Centropages typicus and Temora stylifera, in the north-western Mediterranean Sea

The reproductive biology of copepods plays a key role in seasonal population oscillations and therefore determines, among other things, annual production (Kiørboe et al., 1988; Fransz et al., 1989; Ianora and Buttino, 1990; Ianora et al., 1992; Halsband and Hirche, 2001). Reproduction is affected by various environmental factors, basically by temperature and food availability (Dagg, 1978; Landry, 1978; Uye, 1981; Abou Debs and Nival, 1983; Smith and Lane, 1985; Huntley and Lopez, 1992; Hirche et al., 1997; Saiz et al., 1999). The interactions of these partly opposing factors are difficult to understand. Food can be limiting either quantitatively or qualitatively in terms of cell size, nutrient content, etc. (Runge, 1984; Beckman and Peterson, 1986; Kiørboe et al., 1988; Jónasdóttir, 1994). Moreover, food availability is difficult to define in complex natural environments (Jónasdóttir et al., 1995; Saiz et al., 1999). Temperature has direct and indirect effects on egg production. On the one hand, spawning intervals become shorter with increasing temperature (Runge, 1984), since physiological processes like maturation of oocytes are faster (Razouls, 1975; Hirche et al., 1997). On the other hand, temperature is inversely correlated with body size and thus affects the reproductive potential of females, especially in populations with several successive generations and a wide range of in situ temperatures (Landry, 1978; Ambler, 1985; Kiørboe et al., 1988; Hirche, 1992; Ban, 1994; Halsband and Hirche, 2001). While in more boreal waters a few studies on seasonal reproduction patterns of copepods are available (Kiørboe and Nielsen, 1994; Halsband and Hirche, 2001), knowledge is scarce for warm-temperate seas, which represent a large part of the world oceans. Although the Mediterranean can function as an exemplary representative of warm-temperate ecosystems [(Saiz et al., 1999) and references therein], only few studies were conducted on annual cycles of egg production (Ianora and Buttino, 1990;