Pink to purplish red tufts 2-6 cm tall with what appear to be corymbose tips (Image A). However, this corymbose appearance is an artifact of multiple tips being ‘glued’ together by diatoms and various other epiphytes (Image B). When carefully scraped clean the tips reveal a weak sigmoidal pattern with branches slightly overtopping the main axis (Image C). Cells here are 16-20 µm wide by 18-25 µm tall and the branching is regularly alternate in a single plane (Image C). Axes throughout thalli are uniseriate and essentially ecorticate. Upper mid thallus the relatively regularly alternating branches are evident and cells are 35-50 µm wide by 112-154 µm tall (Image D). Lower on the thallus axial cells are 125-220 µm wide by 550-860 µm tall and are held together by a tangle of free rhizoid-like filaments (Image E). The latter are relatively common descending rhizoids, which are typically produced from first cells of branches and can be simple (Images F & G) or branched (Image H) and variously adherent to (Image F) or free of (e.g. Image G) the major axes.
We have two genetic hits on this species, one each from dives in MA and RI. This species is allied to Lophothamnion hirtum (Hooker & Harvey) Womersley in our preliminary phylogenetic work, but likely accounts for some (all?) records of Pleonosporium borreri (Smith) Nägeli in our flora (a sequence in GenBank attributed to this species is sister to our Lophothamnion linage of 5-6 species; further Taylor (1962, p. 302) noted that specimens in the NW Atlantic were “much more slender than that from Europe”). However, other species in our collections at UNB attributable to Pleonosporium do not join this lineage. As with most things in the Cermiales, considerable taxonomic work remains.
Owing to the false corymbose tips in some collections of Lophothamnion sp. 1GWS confusion with Callithamnion corymbosum (Smith) Lyngbye is possible, but that species is different in many regards notably its dense adherent rhizoidal cortication on the lower thallus. Likewise confusion with Callithamnion debile Harvey is possible, which is less corticated than Callithamnion corymbosum (Smith) Lyngbye, but these species are both truly corymbose and have a more characteristically subdichotomous branching habit.
Image A. Press of a specimen with corymbose tips manifesting as tiny red dots near the apices (arrows) appearing much like Callithamnion corymbosum (Smith) Lyngbye (subtidal (6 m) on red algae, Fort Wetherill, RI; GWS005733).
Image B. Tip appearing corymbose owing to the filaments being glued together (GWS005733; rehydrated from press).
Image C. Clearer look at a tip with the epiphytes scraped away (GWS005733; rehydrated from press; aniline blue stained).
Image D. Branching is regularly alternate throughout the mid and upper thallus (GWS005733; rehydrated from press).
Image E. Rhizoid-like branches lower on the thalli entangle erect axes into loose strands (GWS005733; rehydrated from press).
Image F. Simple descending rhizoid extending (arrow) down the axis (GWS005733; rehydrated from press).
Image G. Simple descending rhizoid borne on the basal cell of a lateral branch (GWS005733; rehydrated from press).
Image H. Branched descending rhizoid (broken and thus truncated) (GWS005733; rehydrated from press).
Seaweed of Canada 