Comparative feeding patterns of early stages of mesopelagic fishes with vertical habitat partitioning

The present study analysed the trophic ecology of the early developmental stages of four species of mesopelagic fish, the myctophids Ceratoscopelus maderensis, Hygophum benoiti and Benthosema glaciale and the sternoptychid Argyropelecus hemigymnus. These species display different morphological traits and a segregated vertical distribution throughout the water column. The study was conducted off Mallorca Island (39A degrees N, 3A degrees E) in the western Mediterranean, during the summer stratification period. The results indicated that feeding patterns of myctophid larvae were strictly diurnal, while in A. hemigymnus larvae, day and night feeding occurred. In the transformation stage of C. maderensis, B. glaciale and A. hemigymnus, day and night feeding was evidenced. The feeding incidence during the larval stages was low, increasing in the transformation stages, and being particularly high for A. hemigymnus. Although an increasing tendency in size and number of ingested prey was observed, the trophic niche breadth did not indicate a trophic specialization in any of the species analysed. Gut content analysis determined that diet composition was very similar among the four species, with the different developmental stages of copepods being the dominant prey throughout the early larval development. Nevertheless, in transformation stages of C. maderensis and H. benoiti, other preys, like ostracods, become important contributors to the diet. Despite the important physical and biological structuring of the water column, no differences in feeding success were observed for larvae occurring in the layers of higher biological production.