This species forms yellowish to orangish red tufts, 2-4 cm high, and lacks a percurrent main axis (Image A). In the upper thallus branching is alternate and distichous (some spiralling) and appears pseudodichotomous when viewed at lower magnification because many branches develop to nearly the same length as their bearing axis (Image B), which imparts a corymbose appearance at the tips (Image C). Cells here are typically 23-31 µm wide by 125-140 µm long. Mid thallus the axes are largely ecorticate, the cells varying from 40-70 µm wide by 240-510 µm long, and the branching takes on a distinctly spiral pattern (Image D). Lower mid axes cells, up to 90 µm wide by 1000 µm long, rarely develop rhizoidal filaments (Image E), and even at the base the central cells have a thin and loose covering of rhizoids (Image F). In some specimens the tips extend as hairs (Image G). Observations of reproductive structures are limited for this genetic group with only one specimen producing paired periaxial cells (Image H) from which a carpogonial branch will develop (Image I) and eventually auxiliary cells.
We have genetically verified records for this species from ME (n = 1) and Tasmania (n = 3), low intertidal pools to subtidal (4 m) on rock. Confusion with Callithamnion corymbosum (Smith) Lyngbye is highly likely although it is more richly corticated. Owing to the false corymbose tips in some collections of Pleonosporium sp. 1GWS confusion is also possible, but this species typically lacks adherent rhizoidal cortication and is not subdichotomously branched.
Taxonomic work is ongoing, but this species appears to be the same as that reported as Callithamnion byssoides Arnott ex Harvey from New England in Taylor (1962). It may also account for reports of Callithamnion pseudobyssoides Crouan & Crouan from southern Australia in Womersley (1998) and for the wider NW Atlantic flora in Schneider & Searles (1991). All of the previous records are now attributed to Aglaothamnion pseudobyssoides (P.Crouan & H.Crouan) Halos, which seems an unlikely element of our flora based on preliminary molecular data in GenBank. My valued colleague John Huisman has provided information consistent with this species being the Western Australian Callithamnion debile Harvey. It is certainly a good morphological match; a definitive identification awaits sequence from topotype material.
Image A. Specimen growing on rock in the subtidal (4 m) at Rockland, ME (GWS024032).
Image B. Alternate branching has a pseudodichotomous appearance at lower magnification near the tips (GWS024032; rehydrated from press; aniline blue stained).
Image C. Close up of the corymbose tips similar to those in the related species Callithamnion corymbosum (Smith) Lyngbye (low intertidal pool on rock, Snug Park, Tas, Australia; GWS015141; rehydrated from silica vial; aniline blue stained).
Image D. Mid thallus cells are relatively long and the spiral branching pattern (arrows) is evident (low intertidal pool on rock, Snug Park, Tas, Australia; GWS015138; rehydrated from silica vial).
Image E. A single cryptic rhizoid (arrows) extends along a lower mid axial cell (GWS015138; rehydrated from silica vial).
Image F. Axis at the base of the thallus with loose and sparse cortication (GWS024032; rehydrated from press; aniline blue stained).
Image G. Hairs at the tips in a specimen from a low intertidal pool on rock (Snug Park, Tas, Australia; GWS015119; rehydrated from silica vial; slight aniline blue).
Image H. Paired periaxial cells from which a carpogonial branch and auxiliary cells will eventually develop (Snug Park, Tas, Australia; GWS015138; rehydrated from silica vial).
Image I. Developing carpogonial branch with trichogyne (arrow) (GWS015138; rehydrated from silica vial).