Family Alisphaeraceae Young Kleijne & Cros, in Young et al. (2003)
Remarks: The Alisphaeraceae are a diverse family of extant coccolithophores, of rather low abundance and producing very small coccoliths. Their fossil record is virtually undocumented but occasional specimens can be found in well preserved sediments. As explained below the coccolith structure does not fit easily in any of the main coccolith groups, and the alternate life-cycle phase produces quadrate aragonitic cup-shaped nannoliths instead of holococoliths.
Description: Dominant stage of life-cycle typically: motile, bearing heterococcoliths arranged in meridional rows. Coccoliths asymmetrical with edge directed toward flagellar opening extended into a flange or protrusion. Entire coccolith formed of rim units; proto-coccolith ring locus within tube; V-units form upper tube and distal flange; R-units form two-layered lower tube and proximal structures, sometimes with distal extension. Alternate life-cycle phase typically: motile, bearing quadrate, aragonitic, nannoliths. Nannoliths with conical upper part and cruciform base.
Type genus: Alisphaera Heimdal 1973.
Taxa included: The genera Alisphaera, and Canistrolithus are combined in this family on the grounds of very similar, and distinctive coccolith structure. Polycrater is included following observation of combination coccospheres (Cros & Fortuño 2002). This is an aragonitic nannolith (Manton & Oates 1980, Young et al. 1999) and strongly supports the family-level separation of this group.
Life-cycles and culture studies: None of these species have been cultured yet. Observations of combination coccospheres of Alisphaera with Polycrater and of Canistrolithus with Polycrater suggest that these are alternate phases (Cros et al. 2000a, Cros & Fortuño 2002). By analogy with other taxa, we predict that the heterococcolith phase (Alisphaera/Canistrolithus) is diploid and that the nannolith phase (Polycrater) is haploid.
Heterococcolith structure: As discussed by Kleijne et al. (2002), the proto-coccolith ring is located approximately halfway up the tube. Our LM observations indicate that the upper unit (D-unit of Kleijne et al. 2002) is the V-unit, this forms the distal flange and upper tube. The R-unit forms the inner and outer layers of the lower tube (with slightly different imbrication directions), and in Alisphaera the proximal flange and irregular central area grill.