Pterothamnion plumula (J.Ellis) Nägeli (A)

This species forms delicate pinkish tufts, 0.1-6 cm in height (Image A). Thalli consist of ecorticate uniseriate filaments (200-260 µm at their widest) typically with paired opposite determinate branches from each indeterminate axis cell (Image B). Alternating indeterminate branches are produced every 3-6 axial cells and grow almost as long as their bearing axes to give an overall subdichotomous appearance to thalli (Image B). The indeterminate branches replace determinate branches where formed, while the latter are simple or produce adaxial branchlets (Image C). Gland cells start out lens shaped (Image C), but can become enlarged and oval to spherical (Image D). At times determinate branches are unilateral rather than paired (Image E) while in other locations, notably on cells which bear indeterminate branches, addition determinate branches can be produced to yield whorls of three (Image F). Lower on the axes a loose investment of descending rhizoids can be produced (Image G), which along with the rhizoidal filaments of the prostrate portion of the axes serve to anchor the thallus (Image H). Tetrasporangia, 30-38 µm wide by 39-50 µm long, have a decussate division pattern and are typically on a pedicel (single cell stalk) rather than being sessile, these typically borne adaxially on the branches (Image I). Spermatangia form in dense clusters along the upper branches (Image J).

Reportedly common south of Cape Cod, but extending northward in warm bays and estuaries, usually in the subtidal (to 12 m). We have a single, less than stunning (Image K), verified record from Rhode Island, which is a genetic and anatomical (Image L) match to a number of collections that we have from Norway. However, there are differences in COI-5P compared to data in GenBank for a specimen from France closer to the type locality and taxonomic study is needed. Although there are other species of Pterothamnion in BC, Pterothmanion plumula has not been encountered in the Canadian flora to my knowledge and is included on this site to assist users of the NW Atlantic algal guides and keys. Whereas more northerly morphologies of Scagelia pylaisaei (Montagne) M.J.Wynne could be confused with Pterthamnion plumula, in New England where these two species overlap, the former is typically densely whorled with 2-4 determinate branches per node while the lateral more typically produces opposite determinate branches. Furthermore, gland cells in Scagelia pylaisaei (Montagne) M.J.Wynne tend to remain lens shaped while those in Pterothamnion plumula can become enlarged and oval to spherical in shape (Image D). Careful observation will prevent confusion with species such as Antithamnion cruciatum (C.Agardh) NägeliAntithamnion hubbsii E.Y.Dawson, and Antithamnionella floccosa  (O.F.Müller) Whittick.

Image A. Pressed specimen from the subtidal (14 m) epiphytic on a kelp blade (Stora Lauvøyna (at intersection with Fv219), Norway; GWS040839; line = 2 cm).

Image B. Ecorticate axes with opposite paired determinate branches, as well as alternating indeterminate branches (arrows), which grow to the length of the main axes (GWS040826; rehydrated from press).

Image C. Indeterminate branch (double arrow) replaces a determinate branch in an opposite pair. Smaller gland cells (arrow) have a lens shape (GWS040826; rehydrated from press).

Image D. Gland cells (arrow) can become oval to spherical in shape (GWS040826; rehydrated from press).

Image E. At times determinate branches are unilateral rather than paired (arrows) (GWS040826; rehydrated from press).

Image F. Node with an indeterminate branch, its opposite determinate branch, and an additional (arrow) determinate branch, which is typically stunted (GWS040826; rehydrated from press).

Image G. Low on the erect axis a loose investment of descending rhizoids is produced (GWS040839; rehydrated from press).

Image H. Prostrate basal region of the main axes producing an extensive network of anchoring rhizoids (GWS040839; rehydrated from press).

Image I. Tetrasporangia are decussate and on a stalk cell (GWS040826; rehydrated from press).

Image J. Spermatangia in dense clusters along the upper branches, predominantly but not exclusively adaxial (subtidal (8m) on red algae, Fort Wetherill, RI; GWS005753A; not genetically verified).

Image J. Pressed specimen from subtidal (6 m) on Bryopsis (Fort Wetherill, RI; GWS005755; line = 2 cm).

Image K. Close up of our RI specimen revealing anatomical features consistent with this species despite the limited stature (GWS005755; rehydrated from press).