la. Many chloroplasts per cell, diameter usually greater than 20 fjum
S. tropicum Cleve lb. One or two chloroplasts per cell, diameter smaller 2
2a. External tubes of marginal strutted processes threadlike
S. menzelii Guillard, Carpenter, & Reimann
2b. External tubes of marginal strutted processes more robust 3
3 a. External tubes of marginal strutted processes not split longitudinally, usually extremely short S. subsalsum4 (A. Cleve) Bethge
3b. External tubes of marginal strutted processes trough shaped with a wide longitudinal slit facing the valve margin . . S. costatum4 (Greville) Cleve
* Basionyms: Melosira subsalsa A. Cleve and Melosira costata Greville, respectively.
S. costatum—cosmopolitan (absent from the high Arctic and Antarctic).
S. tropicum—warm water region.
S. subsalsutn—brackish water.
S. menzelii—probably warm water region. How to identify: Due to the unique appearance of the external tubes of the marginal strutted processes and of the colonies Skeletonema spp. will scarcely be confused with any other diatoms as long as typical chains or sibling valves are observed. However, a slight similarity between Stephanopyxis turris and large, coarsely silicified specimens of Skeletonema costatum and S. tropicum does exist. The linking structures of S. costatum, especially of cultured specimens, may be extremely short. In such cases, single cells may easily be misidentified as small Thalassiosira spp. In the same way, single valves, especially terminal valves with a central labiate process, may easily be confused with Thalassiosira spp. even when seen with EM. Whereas S. costatum and S. tropicum are differentiated on characters seen with LM, and the poorly known S. menzellii is distinguished with LM by the generally weak silicification and the thin connecting structures, SEM may be necessary to recognize the tubular connecting structures of S. subsalsum. Skeletonema costatum as presented here includes S. pseudocostatum Medlin in Medlin et al. (1991). With LM S. pseudocostatum has the appearance of an extremely small S. costatum (diameter 2-4 /xm). Skeletonema costatum forms inseparable colonies, i.e., the external tubes of the marginal strutted processes of sibling valves are permanently attached also when organic material is removed by acid cleaning. Skeletonema pseudocostatum occurs in separable colonies, i.e., permanent attachments are absent. As a consequence of this difference 5. pseudocostatum may in general appear in shorter chains than S. costatum. As shown by Medlin et al. (1991, Figs. 1-4), the distinction between the two species, i.e., the presence or absence of permanent attachment, is discernible with LM. Remarks: The length of the linking structures (external tubes of strutted processes) of S. subsalsum varies with salinity (Hasle & Evensen, 1975; Paasche et al., 1975). The presence of resting spores/resting cells in Skeletonema costatum is still disputed.
Genus Thalassiosira Cleve 1873 emend. Hasle 1973 Type: Thalassiosira nordenskioeldii Cleve.
Thalassiosira, with its more than 100 species, is probably the marine planktonic genus most thoroughly examined by modern methods. Regional investigations, some of them including descriptions of new species, have been published in a high number since the first transmission electron microscopy (TEM) examinations started to appear in the 1950s (Helmcke & Krieger, 1953, 1954) and the first examinations taking with SEM in the 1960s (Hasle, 1968a). Attempts have been made to impose structure on this genus which seems to increase in number of species parallel to the number of localities investigated. Location of labiate process linked with the presence or absence of external process tubes seemed for some time to be a promising distinctive character (Hasle, 1968a), and may still be, although several exceptions do exist. Makar-ova (1988) grouped 53 Thalassiosira species into the sections Tangentales, Fasciculigera, Thalassiosira, and Inconspicuae with keys to species for each section. Rivera (1981), Johansen & Fryxell (1985), Fryxell & Johansen (1990), and Hasle & Syvertsen (1990a) also constructed keys to species, none of them starting with the same morphological characters.
Chains or cells embedded in mucilage.
Cells in chains connected by organic thread(s) extruded from strutted process(es).
Cells usually discoid.
Valve wall with loculate areolae in various patterns or with faint radial ribs.
Characters (LM) showing differences between species: Girdle view—water mounts
Curvature/undulation of valve face.
Shape and height of valve mantle.
Connecting thread(s)—Length, thickness (indicating number of central strutted processes).
Threads extruded from the margin of valve face and/or mantle (indicating location and number of marginal rings of processes). Length (and shape) of external part of processes. The presence or absence of occluded processes. Valve view—mostly cleaned, mounted valves
Length and location (external/internal) of process tubes. Number and arrangement of strutted processes in or near valve center (see Central processes, Remarks, pg. 32). The presence or absence of strutted processes on the rest of the valve face.
Number of marginal rings of strutted processes. Distance between marginal strutted processes (number in 10 /i,m). Distance of marginal strutted processes from margin (number of areolae).
Number and location of labiate process(es).
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