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We demonstrate here the existence of a range of size-independent reproductive tactics in teleostean fish involving the allocation of a size-dependent reproductive effort between fecundity and egg size. Despite considerable evidence that larger eggs and the larvae hatching from them are more likely to survive than smaller ones, we found no evidence of evolutionary trends towards greater egg sizes. Fish with pelagic eggs tend to spawn many, and therefore small, eggs, whereas demersal spawners tend to produce large, and therefore few, eggs. Maximizing egg number should increase the number of eggs hatching in suitable locations in the patchy pelagic environment and, hence, increase the reproductive success of pelagic spawners. On the other hand, the reproductive success of demersal spawners, which reduce the variance in growing conditions experienced by the off spring, should be more dependent on the survival of the individual larvae, which increases as egg size increases.

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Selection for ciliates in the presence of phytoplankton has been previously shown for some species of copepods. However, the factors determining preference for this heterotrophic prey and how crustacean zooplankton predation can affect the ciliate community are not yet fully understood. In this study, we investigated predation rates on phytoplankton and ciliates by the most abundant copepod and cladoceran species in a coastal area of the oligotrophic NW Mediterranean Sea monthly over an annual cycle. Three major results were apparent. Firstly, ciliates were important contributors to zooplankton diet, representing a median of 37 and 17% of the carbon intake, and 51 and 34% of the nitrogen intake for copepods and cladocerans, respectively. Secondly, ciliates were positively selected in most cases, this selection was species specific and apparently independent of phytoplankton concentration. And finally, in spite of the high clearance rates on ciliates, the impact of the crustacean community on the ciliate standing stock was low (median 2%), suggesting a bottom-up control of the ciliate community. KEY WORDS: Copepods · Cladocerans · Ciliates · Food web · NW MediterraneanResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 35: 65-78, 2004 al. 2002a) is commonly used to argue that ciliates may be important to zooplankton diet. Yet, relatively few studies have paid attention to the actual nutritional contribution of ciliates as carbon and nitrogen sources for zooplankton metabolism, and compared them to the one supplied by phytoplankton ingestion (e.g. Froneman et al. 1996, Batten et al. 2001. Furthermore, it is scarcely known how ciliate contribution to crustacean zooplankton diet may vary by season, in which different zooplankton species and microbial communities occur.A second aspect of interest in the trophic interactions between ciliates and zooplankton deals with feeding selection mechanisms. Copepods frequently exhibit higher clearance rates on ciliates than on phytoplankton (e.g. Gifford & Dagg 1991, Fessenden & Cowles 1994, Levinsen et al. 2000, a fact that has been attributed to positive selection patterns. What exactly determines this positive selection is not clear, and factors like food quality and encounter rates have been proposed (Jonsson & Tiselius 1990, Stoecker & Capuzzo 1990. However, the preference for a given food item could also be dependent on the availability of an alternative suitable prey, and switching responses may appear as well (Kiørboe et al. 1996, Gismervik & Andersen 1997. Unfortunately, few field studies have considered these aspects.Finally, little is known concerning the strength of the trophic control that zooplankton may exert on ciliate communities. Because of their nexus position, as both prey for zooplankton and top predators for other microbes, ciliates represent a crucial link within marine planktonic food webs. Empirical evidence up to now, however, indicates that copepods, the major component of mesozooplankton, in g...

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