This species forms red tufts, 1-3 cm tall, of prostrate and erect filaments. The tips reveal rounded to acute apical cells and a pattern of paired opposite laterals produced from the central filament in a distichous (single plane) pattern (Image A). Mid thallus the distichous paired opposite laterals are pinnate with the basal cell markedly smaller than the adjacent cells, the axial cells here 38-50 µm wide by 95-120 µm long (Image B). Lower on the axes multicellular branched descending rhizoids can form from the basal cell of the opposite laterals (Image C).
According to Sears (2002) this was first reported in CN in the 1980’s and subsequently in RI and MA as an introduced species only known in its vegetative state. Collected in a variety of habitats from open coast to estuaries growing on stone or shell in the low intertidal to subtidal, we have no genetically verified collections. A single small collection was recently provided by my valued colleague Craig Schneider and is the source of the images here, as well as hopefully sequence data when time permits (clean sequence may require specific primers as it was a small amount of material intertwined with a Cermiuam sp. and Spermothamnion repens (Dillwyn) Magnus). More specimens would be appreciated and are necessary to better understand the range of this species in our flora as well as to confirm the identification. Not known in the Canadian flora, this species is included in these page to assist colleagues in the NW Atlantic in using identification guides for our wider flora.
This species looks superficially like its congener Antithamnion cruciatum (C.Agardh) Nägeli, but its opposite laterals develop in a decussate (alternating 90o) rather than distichous pattern (Image B). Antithamnion cf. sparsum Tokida differs in a variety of features notably in the second branching of its determinate laterals. There is a chance of confusion with Pterothamnion plumula (J.Ellis) Nägeli or Scagelia pylaisaei (Montagne) M.J.Wynne, but in these species the basal cell of the whorl branches is not noticeably reduced in size. There are similarities to Antithamnionella floccosa (O.F.Müller) Whittick but it lacks gland cells, which are reported for Antithamnion hubbsii (none were observed in the small amount of material at hand), and the determinate laterals tend to be simple rather than typically pinnate.
Image A. Close up of the tips revealing the rounded to acute terminal cells and the paired opposite development (epiphytic on Ahnfeltia plicata (Hudson) Fries, Napatree Pt., Watch Hill, RI; GWS044487; rehydrated from press).
Image B. Paired opposite laterals are more typically pinnate mid thallus with the basal cell (arrows) smaller than adjacent cells (GWS044487; rehydrated from press).
Image C. Multicellular branched descending rhizoid (arrow) formed from the basal cell of a lateral (GWS044487; rehydrated from press).