This species has rose red colour and is up to 8(12) cm in height with a thick and obvious central axis, at least in the lower to mid thallus (Image A), owing to dense cortication of the mid to lower axes, which are up to 500 µm wide (Image B). Cortication is sparse in the mid thallus and the rhizoids, which can branch, develop from the basal cell of a lateral filament, cells here being 142-170 µm wide by 820-875 µm long (Image C). Cells in the upper thallus are 20-25 µm wide by 51-136 µm tall reducing in size toward the tips, 8-14 µm wide by 36-43 µm tall (Image D). Here the uniseriate habit is obvious, as is the typically spiral branching that characterizes this species (Images C & D). Seirosporangia(?) are single or in short chains of 2-4 sporangia, commonly formed together in clusters (Image E).
We lack genetically verified records from the NW Atlantic, the observations here based on a specimen from Norway. It is also unlikely that this species is in the Canadian flora, but it is included here to assist in identification using guides for the NW Atlantic flora. Note it was temporarily found in St. Margarets Bay, NS, in the mid 1980’s and I completed culture investigations of that briefly introduced population for my BSc(H) (my first seminar, NEAS 25; great society!). With climate change it and many other southerly species are likely to pop up more commonly in the warm bays of NS and into the Northumberland Strait). Reportedly rare and disjunct in distribution, especially north of Cape Cod, this summer annual is an epiphyte or on shells or stones within the shallow subtidal to 8 m (Sears 2002). There is a slight chance of confusion with Callithamnion corymbosum (Smith) Lyngbye, but it has regularly (every cell bears a lateral) alternate (transitioning to spiral) branching near the tips with the branches typically as long as the main axis resulting in corymbose tips. Branching in Seirospora interrupta is irregularly (some cells lack laterals) radial near the tips, which are not as strongly corymbose. Further, rare trichotomous branching is observed in Seirospora interrupta (Smith) F.Schmitz while I have yet to observe this feature for Callithamnion corymbosum (Smith) Lyngbye. The last mentioned also does not produce spores in chains as observed for Seirospora interrupta (Image E).
Image A. Pressed voucher from the subtidal (14 m) on shells, with thick main axes (Stora Lauvøyna (at intersection with Fv219), Norway; GWS040845).
Image B. Cortication is dense near the base (GWS040845; rehydrated from press; aniline blue stained).
Image C. Cortication is sparse upper mid thallus, a rhizoid is borne on the basal cell of a lateral filament (GWS040845; rehydrated from press; aniline blue stained).
Image D. Cells upper thallus to the tips (GWS040845; rehydrated from press; aniline blue stained).
Image E. Seirosporangia (GWS040845; rehydrated from press; aniline blue stained).