The depth distributions and community organization of the planktonic cyclopoids (Crustacea: Copepoda) of the Cape Verde Islands Region

Abstract

The cyclopoid copepods caught in a day and a night series of horizontal hauls taken at depths between 0-1250 m in the vicinity of 18.degree. N latitude, 25.degree. W longitude in the northeastern Atlantic were examined. Forty-five species representing 4 families [Oncaeidae, Sapphirinidae, Corycaeidae, Oithonidae] were present, 22 of which were sufficiently abundant for their depth distributions and vertical migration patterns to be analyzed in detail. The remaining 23 spp. occurred in small numbers. Cyclopoids were 1.27 times more abundant in the night series than in the day series but this pattern was not uniform for all depth horizons. Patchiness is probably of major significance in the complex of factors responsible for producing the differences between day and night hauls. The sex ratios were calculated and the proportion of males was found to vary from 0.1% in members of the Oithonidae to 54% in members of the Corycaeidae. Four zones of abundance were identified: the epipelagic (0-300 m), subdivided into upper and lower layers at about 100 m, which was rich both in species and numbers; the intermediate (300-400 m) containing small numbers; the mesopelagic (400-1000 m), subdivided into shallow and deep regions at about 700 m, and the bathypelagic (1000-1250 m) characterized by very small numbers of cyclopoids. This zonation may have been partly related to the presence of several different water masses in the water column. Species diversity increased from the surface to a maximum at depths of 55-100 m, decreased to a minimum at 700-800 m and then increased slightly towards the bottom of the sampled water column. This pattern was most clearly demonstrated using the Sanders rarefaction method. Calculation of the equitability component of diversity indicated that the dominant species became proportionally more dominant in deeper samples.