Morphological development of resting cysts in cultures of the marine dinoflagellateLingulodinium polyedrum(=L. Machaerophorum)

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Abstract
The previously undescribed morphological development of resting cysts of the living marine dinoflagellate Lingulodinium ("Gonyaulax") polyedrum has been observed in laboratory cultures. Referred to as L. machaerophorum in cyst‐based taxonomy, this species has a fossil record extending back at least to the Early Eocene. In our cultures, planozygotes preparing to encyst showed a distinctive interstice in the peripheral cytoplasm and often displayed a characteristic swimming behavior. The transition from motile planozygote to morphologically mature hypnozygote (resting cyst) took approximately 10–20 minutes. Encystment began with several events occurring simultaneously: (1) the cell stopped swimming and came to rest at the bottom of the observation chamber, (2) flagella were expelled from their respective thecal grooves, (3) localized swelling of a membrane external to the theca formed bubble‐like protrusions on the surface of the cell, (4) the theca began to dissociate along one or more plate sutures, and (5) a single layer of globules appeared in the interstice between the theca and cytoplasm. External protrusions then enlarged and merged to liberate a continuous membrane which surrounded the entire cell. Subsequent expansion of this membrane gave the encysting cell the appearance of an inflating balloon. In most cases, the outer membrane remained partially attached to the theca so that expansion caused thecal sections to pull away from the underlying globules and cytoplasm. As the outer membrane enlarged, globules on the surface of the cytoplasmic mass grew radially outward (i.e., centrifugally) beneath the dissociating theca to form processes. Morphological development of the resting cyst ended when the expanding membrane ruptured. The maximum lateral dimension attained by this membrane was about twice the diameter of the internal body of the cyst. In these cases, cysts developed the distinctive processes characteristic of Lingulodinium. Premature rupture of the balloon‐like membrane, however, resulted in processes showing considerable variation in size and morphology. Based on the variability of process morphology observed in laboratory cultures of L. polyedrum, three morphotypes currently designated as separate species of the genus Lingulodinium are here synonymized with L. machaerophorum.