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Lluïsa Cros, Annelies Kleijne, Jeremy R. Young:
Coccolithophorid diversity in the genus Polycrater and possible relations with other genera
(Talk)

The genus Polycrater Manton & Oates, 1980 was described from tropical waters of the Pacific (Galapagos area) and at the moment it is considered monospecific. The unique species described as Polycrater galapagensis Manton & Oates, 1980, formed of aragonite, has been reported few times in the literature (Chrétienot-Dinnet, 1990; Winter & Siesser, 1994; Thomsen & Buck, 1998).

Polycrater galapagensis specimens, which are morphologically identical to the P. galapagensis described by Manton & 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. A thorough survey of these specimens suggests that upto 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 recognised as very close in the literature (Jordan & 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 subgroup 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. Éditions du Centre National de la Recherche Scientifique, Paris.

Jordan, R.W. & Chamberlain, A.H.L. 1993. Canistrolithus valliformis gen. et sp. nov. (Syracosphaeraceae, Prymnesiophyta), a comparison with the genus Alisphaera. Phycologia, 32(5): 373-378.

Manton, I. & Oates, K. 1980. Polycrater galapagensis gen. et sp. nov., a putative coccolithophorid from the Galapagos Islands with an unusual aragonitic periplast. British Phycological Journal, 15(1): 95-103.

Thomsen, H.A. & Buck, K. 1998. Nanoflagellates of East Pacific coastal waters: morphology, taxonomy, and biogeography of weakly calcified coccolithophorids (Prymnesiophyceae). Cryptogamie, Algol., 19(1-2): 29-48.

Winter, A. & Siesser, W.G. 1994. Atlas of living coccolithophores. In: A. Winter & W.G. Siesser (Eds). Coccolithophores: 141, fig.128 (micrograph by Winter & Friedinger).

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