Introduction
Protoperidinium Bergh (1881) is one of the largest and most abundant genera of thecate dinoflagellates inhabiting marine and brackish waters across the globe (Balech 1988). Despite its long and extensive history of investigation, there has always been a great deal of taxonomic uncertainty surrounding the genus (Gribble and Anderson 2006). Balech (1974) reestablished the genus Protoperidinium , transferring 231 marine Peridinium species Ehrenberg (1830) toProtoperidinium . His demarcation of the two genera was largely based on the following characteristics: Protoperidinium species have 4C or (in terms of the interpretation) 3C and 1 transitional plate (t) configuration, with their distribution covering marine habitats, whereas Peridinium members have 5C and occur in freshwater habitats. Furthermore, Balech (1974) introduced the division of the genus Protoperidinium into three subgenera, in accordance with the number of anterior intercalary and precingular plates:Minusculum (6 precingular and 3 intercalary plates),Archaeperidinium (7 precingular, 2 intercalary plates) andProtoperidinium (7 precingular, 3 intercalary plates). Later, Faust (2006) erected the subgenus Testeria Faust forProtoperidinium species with seven precingular plates, apical plate 1’ disconnected from the tip of the apical horn, one anterior intercalary plate, and no apical pore complex. In contrast, Abé (1981) subdivided the genus Peridinium into three subgenera (Protoperidinium , Mesoperidinium , andVeroperidinium ) based on the shape of the posterior sulcal plates. He simultaneously proposed the subdivision of the subgenera into groups and sections.
Formerly, Jörgensen (1912) divided the genus Peridinium into two subgenera – Orthoperidinium (ortho–type) andMetaperidinium (meta and para–types). The author further subdivided Orthoperidinium into three sections (Tabulata ,Conica , and Oceanica ) based on the pattern of the 2a plate (quadra, penta, and hexa) and the characteristics of antapical horns. The classification system of Jörgensen was generally accepted by the later authors (Lebour 1925, Paulsen 1931_1930, Schiller 1937) with some amendments. For the present, the section Oceanica Jörgensen is placed in the subgenus Protoperidinium (Gran) Balech (1974).
Members of the section Oceanica are characterized by well-developed apical and antapical horns and ortho–quadra plate arrangement. However, some species were reported as penta or hexa–type (Taylor 1976, Throndsen et al. 2007, Phan-Tan et al. 2017). In addition, the cingulum is often markedly oblique relative to the equatorial plane of the cell (Phan-Tan et al. 2017). Even though a few studies pertaining to the section Oceanica had been carried out (such as Paulsen 1908, Lebour 1925, Schiller 1937, Taylor 1976, Abé 1981), most of the published illustrations did not reveal clear features of the anterior intercalary plates (Phan-Tan 2020). In these publications, observations were mainly based on cell outline and size, as well as on the morphology of the apical and antapical horns. The mentioned limitations cause notable confusion when it comes to the identification of Oceanicaspecies. Phan-Tan et al. (2017) presented detailed descriptions and photomicrographs of seven species (ten intraspecific taxa) belonging to the section Oceanic a, including a new species and two new combinations, from Vietnamese waters. The micromorphology of the studied specimens, together with the shape and configuration of intercalary plates, was thoroughly documented and illustrated. The paper was the first in the series aimed at presenting the true diversity ofProtoperidinium species found in Vietnamese waters. In 2020,Protoperidinium carum (Abé) Balech was first recorded in Southeast Asia and the waters of Viet Nam (Phan Tan Luom 2020). The species was re-described in detail in terms of its cell morphology and thecal plate pattern.
The present paper describes the new species, Protoperidinium vietnamicum sp. nov., from Vietnamese coastal waters.Protoperidinium curvicorne (Böhm) comb. nov. is proposed as a new combination. Detailed descriptions, accompanied by line drawing, light, and scanning electron microscopy micrographs are provided for both species.