Nanoplankton from the Galapagos Islands: two genera of spectacular coccolithophorids (OphiasterandCalciopappus), with special emphasis on unmineralized periplast components

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Abstract
On the basis of electron microscopy of dry whole mounts of wild material set upin situmainly in the Galapagos Islands but with two introductory specimens from South Africa, the presence of unmineralized periplast components has been demonstrated in two genera of fully calcified coccolithophorids (OphiasterandCalciopappus) and also in a broken cell, otherwise attributable toChrysochromulinaaff.fragilisLeadbeater. The last possesses many small elliptical plate scales with characteristic surface markings, together with fewer but larger sheet scales, each membranous, flexible, and almost without patterning except at the edge which carries a narrow zone of sparse radial striations. Both types of scale recur in the two coccolithophorid genera, the small elliptical plates as an underlayer beneath the coccoliths and the peripherally streaked membranes individually attached to the proximal surfaces of coccoliths as an integral part of their structure. Though present, these are more difficult to detect inCalciopappusthan inOphiasterin which they have been clearly demonstrated in specimens from both South Africa and the Galapagos Islands. In addition, some types ofOphiasterhave also been shown to possess completely patternless membranes, detectable only by their indirect effects, occupying the apparently vacant plate centres of coccoliths in special positions. Other aspects of coccolith substructure are discussed with special reference to recurring difficulties regarding speciation in the two genera. Revised generic descriptions are provided but specific descriptions are limited toOphiaster. These include revision of the two existing taxa (especially necessary for ‘O. formosusGran’) and the erection of three additional new taxa (O. reductussp.nov.,O. minimussp.nov. andO. formosusvar.inversusvar.nov.). The final discussion summarizes and comments on present knowledge ofChrysochromulina fragilissens. lat. in relation to several genera of coccolithophorids including, but not limited to,OphiasterandCalciopappus.

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