Chaetoceros lauderi

Cells are joined together in colonies of medium length (4-10 cells). Frustules are lightly silicified and separate upon drying. In girdle view, cells appear to be rectangular, with “sharp corners” and narrow linear apertures. The pervalvar axis is longer than the apical axis, in most specimens. The suture between the valve mantle and the cingulum is indistinct.

Setae are long (> 150 microns) and oriented nearly perpendicular to the chain axis. Terminal setae are similar to intercalary setae in thickness and length. The terminal setae emerge parallel to the apical axis and then bend towards the chain end but remain divergent over their entire length. Intercalary setae are fused at the plane of the valve margin.

The valve face is flat. Valves are nearly circular in outline. Chloroplasts are numerous, small, and confined within the main body of the cell (no chloroplasts are present within the setae). Resting spores are formed within an elongated mother-cell. The primary valve of each resting spore is elevated from the valvar plane, hemispherical and sometimes “doorknob” shaped, and has long, sparse spine(s). The secondary valve of each resting spore is less elevated and spines may be present or absent on the valve face. Secondary valves also have a curtain of variously-developed capillate spines extending from the advalvar margin.

In SEM, valves were observed in valve view to have partially collapsed, shallow mantles. Setae are circular in cross section with spirally arranged rows of closely spaced poroids and conical spines. The resting spore morphology is variable, with primary valves varying from hemispherical to "doorknob" in shape. Spines are sparse relative to the original description (Ralfs ex Lauder 1864), but fall within the range of variation observed in the species (Paul Hargraves and Jan Rines, personal email communications - Feb. 2021). Primary valves of resting spores with radial arrangement of closely spaced granulae and sparse relatively long spines.

The genus Chaetoceros is species-rich, and for most species there is often not a single diagnostic character to distinguish taxa. Rather, a unique combination of character states distinguish taxa. Determining these character states will help the practitioner to identify Chaetoceros specimens to one of 15 monophyletic Sections (De Luca et al. 2019) or to undescribed taxa.

For C. lauderi, it is important to verify the presence of multiple small plastids confined to main body of the cell, rather than multiple plastids positioned within the main body and setae (Section Chaetoceros) or a few larger plastids within the main cell body (most other Sections). Therefore, live material or preserved cells are necessary. The presence of these character states indicates a much smaller subset of Chaetoceros species (belonging to 3 Sections). Most Chaetoceros species have one or two larger plastids, or smaller plastids found throughout the cell, including the hollow setae.

Observation of narrow apertures between cells in girdle view and nearly circular valve outlines in valve view is indicative of members of the Section Cylindrica, consisting of two species (C. lauderi and C. teres).

Distinguishing these two taxa requires examination of the primary valves of the resting spores. The primary valve face of C. lauderi is spinose, while that of C. teres lacks spines. Ultrastructure of the setae is also taxonomically informative (see above). The setae of C. teres are generally shorter (about one half the length) and finer. Finally, definitive determination between the two species requires the examination of resting spores.