Callithamnion tetragonum (Withering) S.F. Gray (A)

This species stands out in the NW Atlantic flora for its strongly percurrent main axes and numerous short and acropetally developing lateral branches imparting the look of a Christmas tree (Image A). Regrettably our collections were heavily infested with a variety of biota hindering microscopic observations. Branches mid to lower thallus are clearly arranged in a spiralling pattern (Image B), while the main axes are covered in a thick covering of, at least on the surface, narrow rhizoids (Image C). At the tips, apical cells are slightly acute to conical in shape (Image D). Carposporophytes are produced in pairs (Image E) with the gonimolobes cordate (heart shaped; Image F) to more irregular in shape (Image G). Spermatangia formed in clusters along the lateral branch cells (Image H).

Our specimens are restricted to three genetically verified collections from Rhode Island, growing on algae in low intertidal pools. Although reported for the Canadian flora, we have not encountered this species in our surveys to date. It is included here to assist users with local identification guides for the NW Atlantic. It looks like a smaller version of the NE Pacific Callithamnion pikeanum Harvey, but is easily distinguished from Callithamnion corybosum (Smith) Lyngbye (and an allied sister species we have yet to identify) with which it shares its range.

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Image A. Specimen from a low intertidal pool growing on algae (Hazard Ave., Narragansett, RI; GWS030058).

Image B. Branches (arrows) along the main axis spiral in habit (GWS030058; rehydrated from silica vial).

Image C. Narrow rhizoids produce a cortical cover over the main axial cells (GWS030058; rehydrated from silica vial).

Image D. Branch tips are acute to conical in shape (low intertidal pool on algae, Hazard Ave., Narragansett, RI; GWS030059; rehydrated from silica vial).

Image E. Paired developing gonimoblasts (subtidal (6 m) on rock surfaces, off end of breakwater NOAH Bldg, Garbage Beach Breakwater, Woods Hole, MA; GWS001882; rehydrated from press).

Image F. Carposporophyte with a cordate gonimolobe (GWS001882; rehydrated from press).

Image G. One of a pair of carposporophytes in focus with a slightly irregularly cordate gonimolobe (GWS030058; rehydrated from silica vial).

Image H. Spermatangial clusters (GWS001882; rehydrated from press).