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ON THE POPULATIONS OF BRANCHIOSTOMA LANCEOLATUM AND THEIR RELATIONS WITH THE WEST AFRICAN LANCELETS
Author(s) -
WEBB J. E.
Publication year - 1956
Publication title -
proceedings of the zoological society of london
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.915
H-Index - 96
eISSN - 1469-7998
pISSN - 0370-2774
DOI - 10.1111/j.1096-3642.1956.tb00464.x
Subject(s) - cline (biology) , population , geography , west coast , ecology , biology , demography , oceanography , geology , sociology
SUMMARY1 Samples of Branchiostoma lanceolatum from the Kattegat, North Sea, Plymouth, Polperro, Jersey, Brittany, Naples, Messina and Faro are examined and a statistical analysis of their characteristics is made. Distinct populations in the Kattegat, the Western English Channel and the central Mediterranean region are recognized and their characters described. The lancelets from the North Sea may represent a fourth population or they may indicate the presence of a series of intermediate forms linking the lancelets of the Kattegat with those of the English Channel. 2 Lancelets from Naples, Plymouth and the Kattegat are compared with the West African species B. senegalense, B. leonense, B. takoradii, B. nigeriense and B. africae.3 It is shown that there is greater uniformity among the populations of B. lanceolatum than in the West African species. The diversity of the West African forms is unexpected in view of the apparent similarity of conditions on the coast where they occur in comparison with the region occupied by B. lanceolatum. It was thought that a cline might exist in the lancelets from Rio de Oro to the Cameroons, but, when the mean values for the diagnostic characters are arranged in the geographical order in which the species occur, although trends or similarities are present in certain characters, in others no recognizable sequence exists. It appears, therefore, that factors operate along the West African coast to produce a degree of isolation of different regions sufficient to allow differentiation to take place. 4 The relationships between the European and the West African lancelets are discussed. It appears that the Guinea Coast lancelets, B. leonense, B. takoradii, B. nigeriense and B. africae form a compact group, but that B. senegalense stands somewhat apart and has affinities both with the Guinea Coast group and with the English Channel population of B. lanceolatum. It is concluded that the East Atlantic lancelets comprise three groups distributed in accordance with the main ocean currents. The populations of B. lanceolatum appear to owe their distribution to the North Atlantic and West Wind Drifts although the latter does not enter the Mediterranean. B. senegalense , on the other hand, occurs in the cold waters of the Canaries Current while the Guinea Coast lancelets are restricted to the warm Guinea Current. 5 The taxonomic rank of the East Atlantic lancelets is considered. In view of the uniformity of the samples of European lancelets it is confirmed that these should all be referred to the same species, B. lanceolatum , although it may be desirable at a later date to recognize the lancelets of the different regions as subspecies. The five lancelets from West Africa, however, have all been given specific rank, but it is probable that only B. senegalense will stand as a distinct species, the Guinea Coast forms being regarded eventually as subspecies of B. africae.