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

Abstract

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. Key words: coccolithophorids, Hymenomonas, life cycle, Prymnesiophyceae.