MORPHOLOGICAL VARIATION WITHIN THE EXTANT SPECIES COCCOLITHUS PELAGICUS AND RESOLUTION OF ASPECTS OF ITS BIOGEOGRAPHY
Jeremy R. Young(1), Karl-Heinz Baumann(2), Sandra Broerse(3), Mario Cachao(4), Ian Probert(5), Patrizia Ziveri(3)
1 Palaeontology Department, The Natural History Museum, Cromwell Road, London
2 Earth Science Dept., University Bremen, Bremen, Germany
3 Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
4 Centro e Departamento de Geologia, University of Lisbon, Rua Escola Politécnica, No 58, P-1294 Lisboa, Portugal
5 Laboratoire de Biologie et Biotechnologies Marines, Universite de Caen Basse Normandie, Caen, France
BackgroundCoccolithus pelagicus is the dominant sub-Arctic coccolithophore and its biogeography indicates a clear preference for water temperatures below 10¡C (McIntyre & Be 1967, Baumann unpubl). Nonetheless, anomalous occurrences in temperate latitudes, especially in coastal upwelling zones, have periodically been reported. Cachao & Moita (in press) investigated in detail such occurrences on the Portuguese shelf and demonstrated that in this environment C. pelagicus was well adapted to a quite different environment to the sub-Arctic, with characteristic temperatures of ca. 12-17C.
Morphometric study suggests that the sub-Arctic population is consistently characterised by medium sized coccoliths, 7-10microns, whereas the temperate, coastal, populations has significantly larger coccoliths, 10-16microns. Sediment trap samples from intermediate latitudes showed bimodal populations. Control samples examined by different workers using different measurement methods revealed remarkable reproducibility. These results strongly suggested major genotypic variation within the population.
New observations, culture work: Strains of the temperate population were already in culture from Plymouth (an old isolate) and the French coast (CODENET isolate) and additional ones have been isolated from the Portuguese Shelf during this year's CODENET cruises. To sample the sub-Arctic population Ian Probert (Caen) and Kerstin Muller (NHM) undertook field work in Iceland in July, and successfully isolated several strains of C. pelagicus. Preliminary culture observations indicate that the size differentiation between the sub-Arctic and temperate populations is stable. Controlled experiments on the morphometric variability and temperature tolerances of these strains are planned for early 2000, by students from U. Lisbon working at the NHM with Markus Geisen. In addition, the strains are being sampled, at U. Caen for material for molecular genetic study of differentiation within and between the populations, to be carried out by Alberto Saez (AWI).
Implications: The likely conclusion, that there are two sub-species within the N. Atlantic C. pelagicus population with discrete morphologies and ecological preferences, is in itself apparently trivial. It has, however, many interesting implications. (1) Palaeontological proxy work: Roche et al. (1975) and Pujos (1992) attempted to derive temperature transfer functions from N. Atlantic coccolith biogeography for interpretation of Quaternary climatic change. These yielded anomalous results and so were not developed further. We suspect that this may have been largely due to populations of the temperate type C. pelagicus being assigned the ecological preferences of the sub-Arctic type. More generally, there is strong signal of C. pelagicus abundance fluctuations in mid to high latitude cores and meaningful interpretation of these would be very valuable. For example C. pelagicus is virtually absent in the Southern Ocean but cores reveal the temperate form was abundant in the area until very recently, sometime within the Holocene. (2) Microevolutionary study: C. pelagicus has an exceptionally long fossil record (extending to the early Danian, ca 63Ma) and shows strong temporal size variability, even within the last 4 Ma. Geological study of the evolution of the modern sub-species is practical and in progress (U. Bremen, U. Lisbon). (3) Vicariance - C. pelagicus is also abundant in the N. Pacific, forming a biogeographically disjunct population from the N. Atlantic. Comparison of modern and Quaternary C. pelagicus between the N. Atlantic and N. Pacific should allow testing of the significance of vicariance in nannoplankton populations. Relevant studies are being developed with colleagues in Japan. (4) Comparison with other species - the basic pattern of morphometrically and ecologically discrete sub-species is broadly comparable to that emerging from the parallel studies on other CODENET species. This suggests that a general model of microevolutionary pattern and its biogeographic expression is emerging from the research.
Artcic (left) and temperate (right) Coccolithus pelagicus specimens, same magnification
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