Egg production, growth and development of the cyclopoid copepod Oithona similis

Egg production, growth and development rates of Oithona similis were measured in the laboratory as a function of food concentration and composition. On an optimum diet, development is isochronal and growth is near exponential. The maximum juvenile growth rate at 15°C (0.2 day") is similar to juvenile growth in calanoid copepods. The maximum weight-specific egg production rate (0.1 day"), on the other hand, is substantially less than in free-spawning calanoids, but similar to that in egg-carrying calanoids. In the Kattegat, Oithona spp. egg production is strongly limited by food during summer and controlled by temperature during winter. The seasonal signal in fecundity and population biomass is much weaker than in the co-occurring free-spawning calanoid genera, where fecundity and population biomass undergo dramatic seasonal viaration. Introduction A strong interest in marine cyclopoid copepods has developed in recent years, along with the increasing awareness of their high abundance in almost all oceanic environments and often numerical dominance within the metazoan plankton (e.g. Paffenhofer, 1983; Paffenhofer et al., 1987; Peterson et al., 1988; BottgerSchnack et al., 1989; Hay et al., 1991; Nielsen et al., 1993). However, information on the ecology of cyclopoids is still scarce and by far the majority of experimental studies have been devoted to calanoid copepods (Paffenhofer, 1993, and references therein). Among the cyclopoids, Oithonidae usually consitute a significant fraction of the copepod biomass in temperate areas, in particular during autumn and winter, when they appear able to maintain high population densities. In the North Sea, for example, Oithonidae contribute 25% (23-29%) of the total copepod biomass during October-March (Hay et al., 1991), and 38% (23-50%) in the Kattegat during late August through February (Kitfrboe and Nielsen, 1994). The oithonid Oithona similis is probably one of the most ubiquitous and abundant copepod species in neritic temperate seas. Yet, it is one of the poorest known. To our knowledge, there are only two experimental studies in the literature that provide some insight into the species. They refer to its basic feeding and reproductive biology (Eaton, 1971), and to feeding rates on flagellate diets (Drits and Semenova, 1984). Detailed autecological studies on other Oithonidae species are not less scarce. While there is some information about reproduction and development (Haq, 1965; Uchima, 1979, 1985; Lonsdale, 1981a,b; Ferrari and Ambler, 1992), egg production rates have been obtained experimentally only for Oithona plumifera (Paffenhofer, 1993). There © Oxford University Press 1329 M.Sabatini and T.Kierboe are relatively more data regarding feeding and respiration (Marshall and Orr, 1966; Lampitt and Gamble, 1982; Uchima and Hirano, 1986; Hiromi et al., 1988), but a complete lack of experimental studies on the growth of Oithonidae and only one reference for a marine cyclopoid (Paffenhofer, 1993). This study attempts to contribute to our knowledge of the growth and production of the potentially important cyclopoid copepods. We report here observations on the fecundity and growth rates of O.similis at varying food concentrations in the laboratory, and on seasonal variation in fecundity in a coastal, temperate ecosystem (Kattegat, Denmark). We compare our findings with the knowledge of other cyclopoid copepods and with information on the much better known calanoid copepods. Method Copepods were collected by a 100 p.m mesh-size WP2 net in the 0resund (Denmark) on several occasions (Table I). In the laboratory, several hundred adults and late copepodids were picked and transferred to 5 1 polypropylene beakers with 30 or 20%o salinity (S) pre-filtered (0.2 u.m) seawater, depending on in situ water salinity at the time of sampling. Animals were slowly acclimatized to ~15°C (1°C day") and subsequently used either for experiments or for culture purposes. Laboratory cultures at 20 and 30%o S were started with batches of several hundred nauplii hatched from the animals collected on 20 October 1992. The shift from collection salinity or 17%o to 30%o was made by transferring the field animals to 25%o S for 15 days before shifting them to 30%o. Both cultures were maintained through four generations. Although rather variable, a sex ratio strongly skewed towards dominance of females (10-35 females: 1 male) was found in the cultures. A high mortality was observed throughout the development of the third generation and the females of the fourth generation did not produce eggs. A mixed-flagellate diet was offered as food, following Eaton's (1971) suggestion for rearing O.similis and considering the several literature reports showing that motile flagellates support development and egg production in Oithonidae (Uchima, 1979, 1988; Lonsdale, 1981a; Lampitt and Gamble, 1982; Drits and Semenova, 1984; Uchima and Hirano, 1986; Ferrari and Ambler, Table I. Oithona similis. In situ temperature and water salinity on collection dates and types of experiment Collection date Temperature Salinity Experiment (label) ro (xo Egg production at 0.2-0.6-1.8 p.p.m.(EPl) Egg production at 0.4-1.2-3.6 p.p.m. (EP2) Development/growth (DG1) Laboratory culture [EP3-EP4; DG2-DG3] Development/growth (DG4) Length-weight relationship (carbon content) Length-weight relationship (carbon content) 1330 30 June 1992 20 October 1992 22 March 1993 09 June 1993 16.0 12.0