New observations onPeridinium faeroensePaulsen (1905), and classification of small orthoperidinioid dinoflagellates

Abstract

New observations on Peridinium faeroense include details of sulcal and cingular tabulation, thecal ultrastructure, and cysts, recently suggested to be taxonomically important criteria. The organism was studied both in preserved plankton from Norwegian fjords and in cultures started with cysts from Oslofjord sediment. Two similar species were compared: Scrippsiella trochoidea (Stein) A. R. Loeblich III (cultured with cysts from Oslofjord) and Peridinium loeblichii (Cox et Arnott) comb. nov. (culture LB 1595, Indiana University Collection). P. faeroense has four sulcal plates, five cingular plates, and distinctive trichocyst pores surrounded by concentric ridges. It produces acid-resistant, organic walled cysts which are probably hypnozygotes formed in a sexual life cycle involving fusion of gametes. It has been recorded from fjords or embayments in north temperate regions of eastern and western N. Atlantic, and in N.E. Pacific Ocean. Motile cells commonly occur in the spring plankton, and cysts provide a benthic resting stage. Small orthoperidinioid dinoflagellates of the general size and shape of P. faeroense in Norwegian coastal plankton were found to include P. faeroense, S. trochoidea, and an undescribed species of Ensiculifera. Documenting the differences between these similar species is particularly important for correct identification of reference cultures. Results of this study provided relevant information for palynology. P. faeroense cysts are identical to Tertiary fossil acritarchs described under the invalid name Micrhystridium bifurcatum by G. L. Williams (unpublished Ph.D. thesis, University of Sheffield) and thus are the first documented acritarchous dinoflagellate cysts. P. faeroense is also the first documented example of fossil dinoflagellate cysts probably formed from fusion of gametes and functioning as hypnozygotes.