A heteromorphic life cycle in two coastal coccolithophorids, Hymenomonas lacuna and Hymenomonas coronata (Prymnesiophyceae)

Species of the genus Hymenomonas, with the exception of the freshwater species Hymenomonas roseola, are marine littoral coccolithophorids, characterized by coccoliths of the tremalith type. A heteromorphic life cycle involving an alternation between coccolith-bearing cells and scale-bearing cells has been established for Hymenomonas lacuna in culture. The scaly cells are covered by two types of scales, morphologically distinct from those of the coccolith-bearing stage. Both scale- and coccolith-bearing stages may be motile or nonmotile. The number of chromosomes in coccolith-bearing cells is, however, about twice that in scaly cells as indicated by nuclear staining. Both life-cycle generations were also observed in Hymenomonas coronata, and this type of heteromorphic life cycle could be general for all heterococcolithophorids with tremaliths.