Thalassiosira punctigera (Castracane) Hasle
This species was originally described as Ethmodiscus punctiger from Tokyo Bay during the Challenger Expedition. The epithet refers to the “dotted” appearance of the processes on the valve face. Several taxonomic synonyms are listed in Hasle (1983).
In girdle view, the cells are convex: flat in the center and gradually sloping toward the mantle (Fig. 1, girdle view, light microscope, brightfield). In valve view, the areolae are arranged in a fasciculate pattern with the radial rows of each fascicle parallel to the median radial row (Fig. 2, TEM carbon replica; Fig. 4, light microscope, phase contrast). There is a single central fultoportula and a single marginal ring of fultoportulae (Fig. 3, light microscope, phase contrast). Each valve has a single rimoportula, with a long external tube, and is located slightly inside the ring of marginal fultoportulae (Fig. 4; Fig. 5, TEM, arrows). Many valves have an irregularly spaced ring of long, tubular occluded processes, highly variable in number, centripetal to the single ring of fultoportulae (Fig.1, 2, 3). The marginal rim of the valve is distinctly ribbed (Fig. 2-5) with 10-16 ribs in 10µm. When living, cells are solitary or joined in chains by a single chitin thread; the distance between adjacent cells is normally greater than the diameter. The largest cells are usually solitary. Cells contain numerous, small irregularly discoid chloroplasts (Fig. 6, line drawing modified from Gran & Angst 1931).
Sexual reproduction in this species has been described by Chepurnov et al. (2006); T. punctigera may also be attacked by parasitic fungi (Hoppenrath et al. 2009). It may be a recent introduction to the U.S. East coast and the IRL. Although it was described from Puget Sound by Gran (Gran & Angst 1931), he failed to find it in the Bay of Fundy and Gulf of Maine four years later (Gran & Braarud 1935). More recently, Herzig & Fryxell (1986) found it to be “rare” in continental slope waters. That record and Hasle & Syvertsen’s (1997) characterization of T. punctigera as “warm water region to temperate” suggest that it might be a summer form along the U.S. east coast. However, it was found (PE Hargraves, personal observation) most abundant on Georges Bank in February, with a water temperature of 2-4 °C. It was not seen in Narragansett Bay before 1980 (PE Hargraves, personal observation), and has not been recorded from the IRL previously, though the ribbed margin and occluded processes make it easily identifiable. Its presence in the IRL is sporadic in time and place, but its morphological variability may have led to its misidentification. It supposedly was introduced on the European coast around 1979 (Throndsen et al. 2007; Hoppenrath et al. 2009), though Gomez & Souissi (2010) argue that it is not invasive.
Valve diameters from literature records are 46-186µm. Areolae on the valve face number 12-19 in 10µm, and the same on the mantle. Fultoportulae in the single marginal ring are 4-7 in 10µm. The size range in the IRL is about 45-110µm with 16-20 areolae in 10µm. A relationship between valve diameter and the presence of occluded processes, (with the largest cells lacking them) was postulated by Hasle (1983) and Fryxell (1978; as T. angstii). Absence of occluded processes in cells with a diameter >100µm is fairly common (the diameter of the cell in Fig. 4 is 103µm), but there may also be a relationship between lack of occluded processes and limiting silicate levels, which has never been examined.
Cherpurnov VA, Mann DG, von Dassow P, Aarmbrust EV, Sabbe K, Dasseveille R, Vyverman W. 2006. Oogamous reproduction with two step auxosporulation in the centric diatom Thalassiosira punctigera (Bacillariophyta). J Phycol 42:845-858.
Fryxell GA. 1978. The diatom genus Thalassiosira: T. licea sp. nov. and T. angstii (Gran) Makarova, species with occluded processes. Bot Mar 21:131-141.
Gomez F, Souissi S. 2010. The diatoms Odontella sinensis, Coscinodiscus wailesii, and Thalassiosira punctigera in the European Atlantic: recent introductions or overlooked in the past. Fresenius Environ Bull 19: 1424-1433.
Gran HH, Angst EC. 1931. Plankton diatoms of Puget Sound. Publications of the Puget Sound Biological Station 7: 417-516.
Gran HH, Braarud T. 1935. A quantitative study of the phytoplankton in the Bay of Fundy and the Gulf of Maine. J Biol Bd Canada 1:279-467.
Hasle GR, Syvertsen EE. 1997. Marine Diatoms. In: Tomas CR (Ed.) Identifying Marine Diatoms and Dinoflagellates. Academic Press, San Diego. pp. 5-385.
Herzig WN, Fryxell GA. 1986. The diatom genus Thalassiosira Cleve in the Gulf Stream warm core rings: taxonomy, with T. intrannula and T. lineoides spp. nov. Bot Mar 29:11-25.
Hoppenrath M, Elbrächter M, Drebes G. 2009. Marine Phytoplankton. Kleine Senckenberg-Reihe 49:1-264.
Hasle GR. 1983. Thalassiosira punctigera (Castr.) comb. nov., a widely distributed marine planktonic diatom. Norw J Bot 3:593-608.
Throndsen J, Hasle GR, Tangen K. 2007. Phytoplankton of Norwegian Coastal Waters. Almater Forlag AS, Oslo. 343 pp.