Guide to Plumosigma

Species in the genus Plumosigma occur epizoically on cetaceans in temperate to polar marine waters.

Cells are attached adnately to the skin of the animal in circular patches. Cells are solitary, isovalvar, and lightly silicified. There are no reports of the living cells in the literature. According to AlgaeBase, there are 2 valid species (8 May 2025).

The valve outline is broadly sigmoid, with subacute apices. In LM, only the valve outline, raphe sternum, central nodule, and soleae (described below) at the valve apices are discernible. Externally, the valves are flat, appearing slightly concave due to their weak silicification that may cause them to collapse, bend, or become undulate. Striae are uniseriate, radiate, and slightly arcuate, although they are not resolvable in LM. Areolae are roundish to squarish, becoming slightly transapically elongated and rectangular near the raphe and strongly transapically elongated toward the valve margin. Internally, the areolae are occluded by hymenes. The valve mantle is very shallow with a smooth margin, apparently furnished with a single row of areolae (see Nagasawa et al. 1990, Pl. 1 Fig 5). The axial area is very narrow, or absent. The central area is small, irregular, and asymmetric. The raphe branches are straight for most of their length but are strongly deflected in opposite directions at a short distance from the apices. There are unornamented, elongated areas between the distal raphe ends and the areolae at the valve apices. The proximal raphe ends are straight, slightly expanded, and positioned close to one another.

Internally, the raphe sternum is thickened and raised above the flat surface of the valve interior. The internal distal raphe ends form asymmetric helictoglossae whose shape corresponds to the distal raphe ends on the external side of the valve. The proximal raphe ends are straight, slightly expanded, positioned close to one another, and separated by a small central nodule formed within the elevated raphe sternum. Both the shape of the areola openings and that of the central area are the same as on the external side of the valve. Soleae (singular = solea) are present at both apices and are so far only known in Plumosigma. A solea is a horseshoe-shaped siliceous structure that surrounds the helictoglossae with the two heels and open end of the horseshoe oriented toward the central nodule and the toe of the horseshoe located at the apex. The position of the flattened area within the solea corresponds to the deflected distal raphe ends on the external side of the valve. The function and morphogenesis of the soleae are unknown.

Up to four open girdle bands have been observed. The girdle bands have one row of circular perforations and are thickened at the closed end of the band.

Nagasawa et al. (1990, Pl. 2, Fig. 3) reported anomalous "double valves" of Plumosigma specimens that have since been observed in other specimens (Frankovich et al. in review). The double valves seem to be composed of two valves with a fully developed raphe system and areolae laying directly over each other that are connected at the valve margins and possibly the central nodule. The morphogenesis of these anomalous valves remains unknown.

The two known species of Plumosigma (P. rimosum Nemoto and P. hustedtii Nemoto) could conceivably be confused with small sigmoid species of Pleurosigma and Gyrosigma. However, the latter two genera possess a loculate valve structure, accessory ribs framing the central nodule internally, and lack soleae.

Few records of Plumosigma exist. Plumosigma has been observed on sperm and fin whales (Nemoto 1956, Nagasawa et al. 1990, Frankovich et al., in review) and a pygmy sperm whale (specimen depicted in light micrograph on this page) in the Northern Pacific Ocean and Canadian Arctic.