Abstract | Introduction | Presentation | Diversity | Summary of the species in the different oceanic zones | Cosmopolitanism and endemism | Species indicative of continental drift | Main marine currents | Species whose localization is difficult to explain | Anthropic mechanisms | Conclusion

Cosmopolitanism and endemism :

Of the 2555 recorded forms taking all orders together, the number of cosmopolitan and endemic species has been established. A total of 1151 species, i.e. 45 %, are known in only one of the 25 zones, and 69 (2.7 %) are cosmopolitan with the occasional exception of regions where they have not been identified (polar or other) and are present in at least 20 zones. One species, Oithona similis, has been reported in all zones;3 in 24 zones: Aetideus armatus, Pleuromamma xiphias, Rhincalanus nasutus; 7 in 23 zones: Euchirella rostrata, Nannocalanus minor, Oithona atlantica, Oncaea venusta, Paracalanus parvus, Pleuromamma robusta, Undeuchaeta major; and 17 in 22 zones: Calocalanus pavo, Gaetanus tenuispinus, Haloptilus acutifrons, Haloptilus longicornis, Heterorhabdus spinifrons, Heterostylites longicornis, Mecynocera clausi, Mesocalanus tenuicornis, Metridia brevicauda, Metridia lucens, Neocalanus gracilis, Oithona plumifera, Paraeuchaeta barbata, Pleuromamma gracilis, Pseudoamallothrix ovata, Scaphocalanus magnus, Undeuchaeta plumosa.

So contrary to general ideas about ancient forms, tributary to the exchange mechanisms between the various oceans, the number of cosmopolitan species seems relatively low. It is true that the distribution of many meso- and bathypelagic forms, often poorly represented in catches, is still badly documented, as is that of many podopleans because of their small size and the fact that they are not taken into consideration when samples are analysed. Hyperbenthic species are underestimated because sampling techniques are not adapted to their way of life.

The table below presents in two columns the numbers of endemic and cosmopolitan species (present in at least 20 zones) for each of the 25 zones (with the access to the lists of species with hyperlink under each number) :

It may be noted that the zone 7 includes a large number of endemic species (144). A wide area as Indian Ocean (zone 16), has a high number of endemic species (158). However, the zone 19, as wide but little explored, includes a small number of endemic species (27).

Polar areas are those where the presence of cosmopolitan species is among the least important (35 for the zone 4 and 22 for the zone 27). A small zone rather close as the Red Sea (zone 15), and recently opened to the Mediterranean through the Suez Canal, has a relatively small number of cosmopolitan species (38).

The graph opposite shows the distribution of the number of species of pelagic Copepoda with the factor N (numbers of quotations of the concerned species in the consulted publications).
850 species have been only once quoted (logically close to 1151 known species in only one of the 25 zones). And only 707 species (27.7 %) have more than 10 quotations.
Some species of the Heterorhabdus genus (belonging to the "Abyssalis" Group established by Park, 2000) show a wide distribution for only one quotation.

Below is the table of 20 most often quoted species :