160 J. Phycol. 36, 160–171 (2000) PYRAMIMONAS TYCHOTRETA, SP. NOV. (PRASINOPHYCEAE), A NEW MARINE SPECIES FROM ANTARCTICA: LIGHT AND ELECTRON MICROSCOPY OF THE MOTILE STAGE AND NOTES ON GROWTH RATES 1 Niels Daugbjerg 2 Botanical Institute / Phycology, University of Copenhagen, Øster Farimagsgade 2D, DK-1353 Copenhagen K, Denmark An undescribed marine prasinophyte, Pyramimo- nas tychotreta, sp. nov., was isolated from a water sam- ple collected near the ice edge in the Weddell Sea (Antarctica) and is characterized by means of light and electron microscopy. This is the second de- scribed Antarctic species in the genus and it pos- sesses a cell ultrastructure typical for members of the subgenus Vestigifera McFadden. The quadriflagel- lated cells measure 8–12  m in length and 6–7  m in width and are equipped with seven types of organic scales that cover the flagella and cell body. The scale floor of the box scales is ornamented by quadrants of parallel striations running perpendicular to one another. The scale floor is further characterized by a number of randomly positioned perforations. The wall of the box scales may be solid or possess up to five perforations. The base of the crown scales is square with rounded corners. It is formed of two crossed ribs, the extremities of which are intercon- nected by a peripheral rib. Four upright arms, at- tached to the peripheral rib in positions slightly off- set from its junction with the cross ribs, join up with the distal extremity of a central upright strut. Each arm possesses two spines. The limuloid scales are cross-striped by 10–12 ribs. Some details of the flagel- lar apparatus are briefly reported. Pyramimonas tycho- treta is compared with other species of the genus. Ex- periments were conducted to study the response of growth rate to variations in temperature and salinity in the clonal culture. The best growth rate (0.45 divi- sions  24 h - 1 ) was found at 4.6 ° C; growth ceased at temperatures in excess of 12 ° C. Growth in salinities ranging from 15 to 35 psu was similar, but was arrested at 10 psu. These studies suggest that P. tychotreta as a cold stenotherm and euryhaline taxon. New observa- tions are presented on the geographic distribution of previously described species of Pyramimonas Schmarda from the Northern Foxe Basin, Canada. Key index words: growth rates; marine nanoflagel- lates; Prasinophyceae; Pyramimonas tychotreta ; ultra- structure; Weddell Sea Abbreviations: A, B, C, D: chloroplast lobes A, B, C and D; e: eyespots; g: Golgi body; l: lipid droplet; m: mitochondrion; mb: microbody; N: nucleus; p: pyre- noid; pb: proximal band; pf: prominent proximal fi- ber; r: rhizoplast; r 1 , r 2 , r 3 , and r 4 : microtubular flagel- lar roots 1, 2, 3, and 4; s: starch sheath; 1–4: flagella or basal bodies During the past 20 years, a substantial amount of ultrastructural information has been collected on the genus Pyramimonas. This has expanded our apprecia- tion of the morphological diversity and distribution records have generated insight into the ecology and biogeography of the genus. Well-circumscribed species of Pyramimonas, with the exception of the type spe- cies (P. tetrarhyncus Schmarda), are marine (Schmarda 1850, Hori et al. 1995). In most seas more than one species is encountered in surface samples. Surveys using electron microscopy for identification pur- poses give the impression that species diversity is higher in temperate marine waters, but this may change when sampling intensifies in arctic and ant- arctic waters. Some Pyramimonas species are cosmopol- itan (e.g. P. disomata McFadden, Hill et Wetherbee, P. grossii Parke, P. orientalis Butcher ex McFadden, Hill et Wetherbee, and P. virginica Pennick; see Mc- Fadden et al. 1986, Hori et al. 1995) and a few taxa ap- pear to have a narrow biogeographic distribution. Temperature studies on arctic and antarctic species of Pyramimonas have demonstrated that cells do not sur- vive temperatures above about 10–12° C (Daugbjerg and Moestrup 1992a, 1992b; present study). Hence, marine species living throughout the year at low tem- peratures (e.g. P. aurita Daugbjerg, P. cyrtoptera Daug- bjerg, P. cyclotreta Daugbjerg, P. dichotoma Daugbjerg, P. gelidicola McFadden, Moestrup et Wetherbee, P. igloo- likensis Daugbjerg, P. quadrifolia Daugbjerg, and P. ty- chotreta Daugbjerg) are likely to have a circumpolar distribution. On the other hand, the cosmopolitan taxon P. orientalis also occurs in arctic waters (Thrond- sen 1970, Thomsen 1982, Daugbjerg and Moestrup 1993). The arctic population of P. orientalis may repre- sent an ecotype adapted to cold environments. A few studies have addressed the diversity of Pyrami- monas species from the Arctic, whereas only a single taxon (P. gelidicola) has been described from antarctic waters (McFadden et al. 1982). The present study re- ports on the motile stage of a new species, P. tychotreta, isolated into clonal culture from a water sample col- lected near the ice edge in the northern part of the Weddell Sea, Antarctica. The description is based on a light and electron microscopic investigation of cell morphology and the architecture of the body and 1 Received 17 August 1999. Accepted 22 October 1999. 2 Author for reprint requests; e-mail nielsd@bot.ku.dk.