Abstract

Pelagic systems are the largest on earth. Compared with the terrestrial habitat these systems, which are composed of taxa with vast ranges of distribution, do not show high species richness. In general species richness in pelagic animal groups decreases from lower to higher latitudes. Speciation processes seem to be slow in the ocean. However, the genetic diversity within a pelagic species might be very high because of the very large distributional ranges where geographic differences occur in environmental conditions. There might be a number of cryptic species as yet undetected.

There is a suggestion of a strong link between the general oceanic circulation and the productivity regime, and pelagic distributions in the different regions. The polar production cycle is characterised by a single peak in summer, the temperate cycle has a spring and autumn bloom and the (sub) tropical regime has relatively low continuous production with a slight peak in winter. The main abundance and biomass of Chaetognatha is in the upper 200 m of the water column and north of 40° latitude. Vertically and horizontally there seems to be an inverse relationship between productivity and biological diversity. The relationship might not be a direct one but might rather reflect regions with high mixing and regions with fewer perturbations.

Introduction

Pelagic ecosystems are the largest on earth. The oceans cover about 70% of the surface of the globe to an average depth of 4 km.