This thesis represents an integrated study of the ecology and reproductive biology of Clausocalanus, a planktonic copepod genus that is widespread in oceanic and coastal regions but still poorly investigated. Principal aim of this study was characterizing the niches of eight congeneric species on the basis of investigations conducted at different space and time scales in the
Mediterranean Sea and in the Atlantic Ocean.
The combined results of field surveys and experiments showed that Clausocalanus species were characterized by different ecological and reproductive traits. Along ecological gradients of temperature and autotrophic biomass, the niches were clearly defined and separated in small (C. paululus, C. pergens, and C. furcatus) and large (C. lividus and C. mastigophorus) species, while they largely overlapped in medium-sized species (C. parapergens, C. jobei, and C. arcuicornis).
Clausocalanus presents a unique reproductive feature among planktonic copepods, since its species have two different egg-laying modes: small and medium-sized species are sac-spawners while the large species are broadcast spawners. The first data on specific sex ratio, embryo viability and hatching success were here provided for Clausocalanus wild populations. Egg production rates of broadcaster species were more than three times higher than those of sac-spanwers. Embryo viability was low in broadcast spawners and very high in sac-spawners. Despite the two groups had similar secondary production, broadcast spawners had lower recruitment, which could contribute to their lower abundance at sea. A nuclear vital probe revealed that eggs of successive clutches were fertilized even in the absence of males, suggesting that re-mating is not necessary in this genus.
The results of this thesis highlight the relevance of Clausocalanus in marine zooplankton communities and the distinctness of its species, towards a better understanding of niche differentiation among congeners in the epipelagos
