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
Species indicative of continental drift :
Sewell (1956) analysed the distribution of various fresh- and brackish-water Copepoda in the light of two concepts formulated at that time about the respective dispositions of the continents and the seas.
The confirmation of Wegener's theory by later studies in plate tectonics led to a new approach to the distribution of planktonic marine Copepoda, taking into account the evolution of the continents. The difficulties remain, however, given the absence of fossils and our fragmented understanding of their phylogeny with the exception of the orders (Huys & Boxshall, 1991; Ho, 1990; 1994), of diverse superfamilies (Andronov, 1974 a; Park, 1986; Mauchline, 1988), of some families (Heptner, 1972 a) or of genera and species (Brodsky, 1972; Vaupel Klein, 1984 a).
Bradford (1979) considered possible factors in the geographical distribution of various neritic forms in temperate waters. Grindley (1984; 1990 a) analysed the distribution of Pseudodiaptomidae in the framework of historical geology. Van der Spoel & Heyman (1983, p.148) summarise the principal geomorphological events potentially responsible for the modes of distribution of various species. But these authors all emphasised the difficulties in coming to any conclusions in this domain. We give a few examples below that may perhaps fit into this schema. Certain types of distribution can only be explained in this context, as the concept of current impact is inadequate here. The case of these species is all the more interesting in that it allows the isolation of these populations to be dated, just as methods of genetic analysis are being developed. The diversification of these communities probably dates from the end of the Cretaceous (Pierrot-Bults & van der Spoel, 1979) originating in the tropical forms, in view of their mode of nutrition (Dinoflagellates, nanoplanctonic groups), then at the beginning of the Cenozoic the conquest of colder waters is probably linked to the expansion of the Diatoma (Herman, 1979).
The Tethys :
This epicontinental sea, well-formed in the Permo-Trias, allowed widespread exchange between the west Pacific and Atlanto-Mediterranean zones (Dercourt et al., 1993). The generally east-west currents were tropical in nature (Drewry et al., 1974). In the Cretaceous, communication with the east Pacific zone allowed the diffusion of tropical species (Herman, 1979).
Key species may be identified from the condition that their presence is required in zones 8, 14, 16 and unimportant in the others. One hundred and nine species meet this condition. Euterpina acutifrons, restricted to the coast, must have its origins in earlier times and is not included here.
A large proportion of the cosmopolitan species and the 24 following species may be relicts from this ancient sea: Calocalanus kristalli, C. latus, C. minor, C. neptunus, C. omaniensis, Paracartia latisetosa, Disco minutus, Lucicutia longispina, Parapontella brevicornis, Paroithona pulla, Scolecithrix fowleri, Pseudocyclops obtusatus, Scolecithrix longipes, Corycaeus (Ditrichocorycaeus) minimus, Labidocera wollastoni, Pseudochirella cryptospina, Mesorhabdus brevicaudatus, Oithona hebes, Cymbasoma longispinosum, Euaugaptilus elongatus, Oithona decipiens, Sapphirina darwini, Amallothrix gracilis, Hemirhabdus grimaldii, Oithona robusta, and ? Arietellus unisetosus , ? Spinocalanus hirtus. Seventy six species could perhaps be added to this list, but as they are also present in South Africa they may have colonised the Atlantic at the end of the Cretaceous via the Agulhas current (Scheltema, 1986; Angel, 1998; Sandy, 1991; Riccardi, 1991).
The Miocene crisis in the Mediterranean, when it was partially drained, must have caused the disappearance of a large number of Tethys species. Some of them may have been able to return later from the Atlantic via Gibraltar, others do not fulfil the condition required, for example: Pseudochirella major, Euaugaptilus digitatus, E. penicillatus, Haloptilus paralongicirrus, Pontoptilus robustus, Bathypontia elegans, B. sarsi, Foxtonia barbaula, Temorites discoveryae, Candacia ketchumi, Clausocalanus farrani, Disco inflatus, Microdisseta minuta, Neorhabdus latus, Lucicutia bella, L. intermedia, L. pallida, L. paraclausi, L. parva, L. sewelli, L. pera, Metridia discreta, Calocalanus adriaticus, Cephalophanes frigidus, Cornucalanus indicus, Onchocalanus affinis, Phyllopus impar, P. muticus, Calanopia americana, Labidocera aestiva, Pontella securifer, P. spinipes, Amallophora obtusifrons, Amallothrix falcifer, A. pseudoarcuata, Heteramalla sarsi, Lophothrix humilifrons, L. quadrispina, Racovitzanus levis, Scaphocalanus bogorovi, S. elongatus, S. longifurca, S. subbrevicornis, Scolecithricella curticauda, S. maritima, S. ctenopus, S. laminata, Scottocalanus thomasi, S. thori, Mimocalanus nudus, Monacilla tenera, Spinocalanus angusticeps, Temora turbinata, Corycaeus longistylis, Oncaea notopus, Copilia lata.
The opening up of the Atlantic :
The North-Atlantic Basin, originally fairly small (ca 160-150 Ma) and communicating with the other seas, began to change radically in the Cretaceous. The north and south Atlantic basins were first linked in the Cenomanian (94-92 Ma) and more fully in the Lutetian (46-40 Ma). Exchanges were still possible with the eastern Pacific in the Caribbean region until the Panama Isthmus was finally closed (ca 10 Ma).
Over and above the endemic forms, one hundred and twenty species are specific to this ocean including 17 that may be regarded as relicts of the primitive Atlantic: Chiridiella kuniae, C. ovata, Bathypontia minor, Candacia paenelongimana, Centropages carribeanensis, Paraeuchaeta incisa, Scottocalanus corystes, Temoropia minor, Undinella hampsoni, U. oblonga, Tortanus setacaudatus, "Xanthocalanus" paululus, Pseudolubbockia dilatata (also reported in the Gulf of Alaska), Xanthocalanus muticus (also in Chile). Three species: Scaphocalanus amplius, S. brevirostris and Scolecithrix valens that have also been reported in the western Mediterranean were perhaps introduced more recently.
Paramisophria ammophila, localised in the Bahamas and in the western Mediterranean (submarine cave in Minorca) and Exumella mediterranea found in the same habitats but only in the Balearics and near Marseille (R. Barthélémy, 1997, pers. comm .), - very similar to Exumella polyarthra - could perhaps reflect the isolation of a population when the Atlantic was opened up (relicts from the Tethys). However their maintenance in the Mediterranean poses the problem of their survival during drying-out periods of varying severity in the Miocene (Hsü, 1972; 1978) and during the ice ages.
Panama Isthmus :
The closing of the Panama Isthmus at the end of the Miocene (start of the Tortonian) must have isolated a certain number of species for the last ten million years (Stephan et al, 1990; Dercourt et al., 1993). From a study of the forms present in the Gulf of Mexico - Caribbean - Sargasso Sea area and in the eastern and north-western Pacific zones excluding all the other zones, it is clear that the following 22 species correspond to this restrictive criterion: Aetideus mexicanus, Cephalophanes tectus, Chiridiella bichela, C. brooksi, Euaugaptilus marginatus, E. mixtus, E. sublongiseta, ? Paraugaptilus buchani, Gaetanus simplex, Haloptilus longiceps, Heteroptilus attenuatus, Heterorhabdus pacificus, H. tenuis, Paraeuchaeta alaminae, ? P. confusa, P. flava, ? P. gracilicauda, ? P. vorax, Pseudolubbockia dilatata, Scaphocalanus subelongatus, Scolecithricella lobophora, Scopalatum vorax, Spinocalanus usitatus, S. pseudospinipes, Xanthocalanus muticus, Undinella frontalis, Corycaeus brehmi. It should be noted that only one of these species has been reported in South America, and all the other Pacific species are from the northern hemisphere. Gaidius inermis, not yet reported in the Caribbean but present in the Sargasso Sea (Sars, 1925; Rose, 1929) and in the Pacific (Wilson, 1950; Grice & Hulsemann, 1968; Markhaseva, 1996), may have been isolated by this event. Another feature of these species is their bathypelagic character. Pontellopsis strenua, Scolecithricella curticauda, Scolecithrix porrecta, also reported in the Indo-Pacific, seem to have experienced the same type of isolation.
The Indo-Malaysian region :
This geographic region was an important zone of communication between the tropical Pacific and the eastern Indian Ocean. It was later subjected to profound changes, during the Tertiary (Miocene et Pliocene) when Australia came closer, which must have slowed exchanges between the Pacific and the Indian Ocean. Fleminger (1986) demonstrates the role of the Pleistocene faunal barrier resulting from climatic variations during and between the ice ages. These events affected the distribution and perhaps the speciation of various species, including Pontellidae. The fact that only 49.8 % of the species are common to the Indian Ocean and the Indo-Malay Archipelago seems to confirm the role of this period.