COCCOLITHPHORID DIVERSITY IN THE GENUS POLYCRATER AND POSSIBLE RELATIONS WITH OTHER GENERA
Lluisa Cros (1), Annelies Kleijne (2) and Jeremy R. Young (3)
1 Institut de Ciències del Mar (CSIC), Passeig Joan de Borbo s/n, 08039 Barcelona, Spain
2 Faculty of Earth Sciences, Free University, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
3 Palaeontology Department, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
The genus Polycrater Manton et Oates 1980 was described from tropical waters of the Pacific (Galapagos area) and at the moment it is considered monospecific. The unique species described, Polycrater galapagensis Manton et Oates 1980, formed of aragonite, has been reported few times in the literature (Chretienot-Dinnet, 1990; Winter and Siesser, 1994; Thomsen and Buck, 1998).
Polycrater galapagensis specimens, which are morphologically identical to the P. galapagensis described by Manton et Oates, have been found repeatedly in samples from the NW Mediterranean. But, other specimens present slight to large differences from the type species of Polycrater. Thorough survey of these specimens suggests that up to seven different types of Polycrater can be distinguished in the NW Mediterranean.
Two combination coccospheres were observed from NW Mediterranean waters with coccoliths of Polycrater and of the genus Alisphaera. Two other combination coccospheres were observed involving Polycrater coccoliths and Canistrolithus coccoliths. The Polycrater specimens associated with Alisphaera and Canistrolithus were of different morphological types. The two genera, Alisphaera and Canistrolithus, are recognized as very close in the literature (Jordan and Chamberlain, 1993) and it is noteworthy that these three genera, Polycrater, Alisphaera and Canistrolithus, present common characteristics in both, coccolith morphology and coccosphere arrangement. All three genera show a longitudinal asymmetry of their coccoliths and a general coccolith arrangement based on approximately regular meridian rows around the cell, with the coccolith long axes directed equatorially, rather than longitudinally. The presence of horns, spines and extended protrusions are common in the coccoliths of Polycrater, Alisphaera and Canistrolithus. We hypothesise that these three genera may represent an unusual sub-group of coccolithophorids in which the aragonitic Polycrater coccoliths substitute for holococcoliths in the life-cycle.
REFERENCES:
Chrétienot-Dinnet, M.-J., 1990. Atlas du phytoplancton marin, v. III. ditions du Centre National de la Recherche Scientifique. Paris.
Jordan, R.W. and A.H.L. Chamberlain, 1993. Canistrolithus valliformis gen. et sp. nov. (Syracosphaeraceae, Prymnesiophyta), a comparison with the genus Alisphaera. Phycologia, v. 32 (5), 373-378.
Manton, I, and K. Oates, 1980. Polycrater galapagensis gen.et sp. nov., a putative coccolithophorid from the Galapagos Islands with an unusual aragonitic periplast. British Phycological Journal, v. 15 (1), pp. 95-103.
Thomsen, H.A. and K. Buck, 1998. Nanoflagellates of East Pacific coastal waters: morphology, taxonomy, and biogeography of weakly calcified coccolithophorids (Prymnesiophyceae). Cryptogamie, Algol., v. 19 (1-2), pp. 29-48.
Winter, A. and W.G. Siesser, 1994. Atlas of living coccolithophores. In: Coccolithophores, Ed. by Winter, A. and W.G. Siesser, p. 141, fig. 128 (micrograph by Winter and Friedinger)..
Polycrater sp. aff. galapagensis (Specimen from AlboranSea, image courtesy M. Geisen)
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