This species forms erect thalli, 4-18 cm tall, that are straw-yellowish to dark red in color and that can appear subdichotomous (Image A) to irregularly alternate(radial) in branching habit (Image B). The primary branches occur every 8-15 segments. Short adventitious branches are typically formed and can be common on some specimens (Image B). Attached by a dense investment of descending rhizoids at the base (Image C), these multicellular and branched (Image D). The tips are forcipate and strongly inrolled (less so on tetrasporophytes) forming a distinctive feature for many species in this genus (Image E). Acute cells are produced at the tips on the adaxial surface of some branches (Image F). This species is fully corticated with the exception of very narrow internodal regions only near the tips (Image E). Cortication is thin near the tips and the underlying axial cells can be observed in optical section (Image G). Near the tips the segments range from 0.8-1.2 diameters long, approaching ~2 diameters long mid to lower thallus. Gland cells absent. Cross section at the nodes reveals 7-9 periaxial cells (Image H). Cystocarps are visible to the unaided eye (Image I), 300-600 µm in dimensions, and consist of 1-3 gonimolobes and 1-5 associated involucral branchlets (Image J). Tetrasporangial production is concentrated in whorls at the nodes with the sporangia covered by cortication (Image K). Tetrasporangia were irregularly cruciate (Image L) to decussate (Image M) and 52-70 µm wide by 68-86 µm tall.
This species is common south of Cape Cod growing from low intertidal pools to subtidal (12 m) on a variety of hard substrata, as well as other algae. Likely introduced to our flora (Bruce & Saunders 2015), we also have records from the British Overseas Territory of Tristan da Cunha (Saunders et al. 2019) in addition to numerous specimens from Europe. Not a component of the Canadian flora, this species is included here to assist with the identification of species in the flora covered by the new NEAS guide. South of Cape Cod this species could be confused with Ceramium virgatum Roth and Ceramium sp. 2virgatum, but in these species there are 6(7) periaxial cells in cross section, the segments reach ~1.6 diameters in length mid to lower thallus, and the mature gonimolobes are smaller (<300 µm).
Image A. Largely subdichotomously branched specimen from the subtidal (9 m) growing on algae,
Fort Wetherill, RI (GWS030047).
Image B. More irregularly alternate/radially branched specimen, which also has abundant adventitious branches (subtidal (9 m) growing on algae, Fort Wetherill, RI; GWS030048).
Image C. Dense mass of basal rhizoids, which anchor the erect axes to the substratum (low intertidal on rock, Litlebukken Island, Bergen, Norway; GWS038057; rehydrated from press).
Image D. Close up of branched anchoring rhizoids (GWS038057; rehydrated from press; aniline blue stained).
Image E. Strongly inrolled forcipate tips; branches corticated to the tips except for narrow (arrows) internodal rings near the apices (GWS038057; rehydrated from press; aniline blue stained).
Image F. Acute cells (arrows) formed in series on the adaxial surface (subtidal (3 m) on algae, Fort Wetherill, RI; GWS005743; rehydrated from press).
Image G. Cortication thin revealing the underlying axial cells in the upper thallus (GWS038057; rehydrated from press; aniline blue stained).
Image H. Section revealing periaxial cells at a node (GWS005735; rehydrated from press; aniline blue stained).
Image I. Cystocarpic specimen from a low intertidal pool growing on Ulva. Cystocarps (arrows) are visible to the unaided eye (Hazard Ave., Narragansett, RI; GWS030088).
Image J. Cystocarp with two gonimolobes and two associated involucral branchlets (GWS005743; rehydrated from press).
Image K. Tetrasporangia concentrated in whorls at the nodes and largely covered by surface cortical cells (subtidal (4 m) on algae, Fort Wetherill, RI; GWS005734; rehydrated from press; aniline blue stained).
Image L. Tetrasporangium with cruciate division (GWS005734; rehydrated from press).
Image M. Tetrasporangium with decussate division (GWS005734; rehydrated from press).