This species forms pink to yellowish red tufts, 2-8 cm high (Image A), which can have moderately distinct (Image B) to indistinct (Image C) main axes, but lacking a percurrent main axis. The appearance of a central axis lower on thalli can be enhanced in some individuals by multiple axes beging tangled together by an abundance of short moderately branched branches (Image D). Lower axes are covered with abundant rhizoidal filaments to form a thin but distinct cortical covering (Image E), while axes mid to upper thallus are ecorticate (Image F). Low on the thallus branching is loosely spiralled (Image G). In the upper thallus branching appears irregularly alternate (some spiralling) to pseudodichotomous when viewed at at higher (Image F) versus lower (Image H) magnification, respectively, because many branches develop to nearly the same length as their bearing axis. This development extends to the tips, which have a corymbose appearance (Image I). Carposporophytes are naked, produced in pairs and composed of 1-4 gonimolobes (Image J) while spermatangia are former in hemispherical clusters that can merge to form elongate structures (Image K).
We have genetically verified records for this species from CN to NL, subtidal (1-12 m) on algae and various hard substrata. This species tends to be in sheltered waters that warm in the summer (e.g. Northumberland Strait). Confusion with the more robust and pyramidal Callithamnion tetragonum (Withering) S.F.Gray is unlikely. However, we have collected a specimen of a second species from ME, Callithamnion sp. 1GWS, that looks much like Callithamnion corymbosum except it has extremely sparse cortication, which we have also collected Tasmania. Taxonomic work is ongoing. Owing to the false corymbose tips in some collections of Lophothamnion sp. 1GWS confusion is also possible, but this species typically lacks adherent rhizoidal cortication and is not subdichotomously branched. There is a slight chance of confusion with Seirospora interrupta (Smith) F.Schmitz, but it has irregularly (some cells lack laterals) radial branching near the tips, which are not as strongly corymbose. In Callithamnion corymbosum branching is regularly (every cell bears a lateral) alternate (transitioning to spiral) near the tips with the laterals typically as long as the main axis resulting in corymbose tips. Further, rare trichotomous branching is observed in Seirospora interrupta (Smith) F.Schmitz, but I have yet to observe this feature in Callithamnion corymbosum. Finally, Callithamnion corymbosum does not produce spores in chains as is characteristic in Seirospora interrupta (Smith) F.Schmitz.
Image A. Specimen from the drift in Rustico, PE (GWS041337).
Image B. Pressed specimen with a relatively distinct central axis low on the thallus (subtidal (2 m) on algae, Pointe du Chêne, Northumberland Strait, NB; GWS039094).
Image C. Pressed specimen lacking a distinct central axis (Gadd Point, Bonne Bay, NL; GWS008810; rehydrated from press).
Image D. Short, highly branched branchlets low on the thallus tangle major axes together into thicker strands (subtidal (1.5 m) breakwater, Port Hood Cape Breton, NS; GWS001781; rehydrated from press, aniline stained).
Image E. Lower axial cells covered with a veil of rhizoidal filaments to form a cortical covering (GWS008810; rehydrated from press).
Image F. Upper axes are ecorticate and appear alternately to spirally branched (GWS039094; rehydrated from press).
Image G. Branching lower on the thallus more typically spiralled (arrows) (GWS008810; rehydrated from press).
Image H. Some branches grow to almost the same length as their bearing branch giving an overall pseudodichomous appearance to the branching pattern (GWS039094; rehydrated from press).
Image I. Tips are corymbose (GWS008810; rehydrated from press).
Image J. Carposporophytes, naked, paired (the one on the right is intact and more developed) and enclosed in a gelatinous sheath (GWS001781; rehydrated from press, aniline stained).
Image K. Spermatangial clusters developing on vegetative cells (GWS001781; rehydrated from press, aniline stained).