Plants feathery in appearance, the thalli delicate and flaccid and collapsing when removed from water (Image A). This species has pinnate branching to give distinctive feathery thalli that typically range from 5-10 cm in height (Image B). Branches are opposite with leading and lagging branches that alternate in series of 2-3 along the axes (Image C). Cortical cells are sparse near branch apices and monosiphonous filaments of 3-7 cells are common (Image D). In subtidal specimens the apical cells can manifest as elongate multicellular rhizoids, which are quite distinct (Image E). Reproduction in our waters is strictly by parasporangia (Image F).
To the inexperienced collector confusion with Ptilota spp. (e.g. Ptilota serrata Kützing) is a possibility, but those species are more rigid and thalli don’t collapse when removed from water, and typically the monosiphonous portions are confined to the terminal 1-2 cells (but see Ptilota gunneri P.C. Silva, Maggs & L.M. Irvine, which is nonetheless distinct in having proximal and well as distal monosiphonous regions in ultimate branches). A critical distinction is the typically regular alternation of the leading and lagging branches in Ptilota spp. versus the alternating series of 2-3 in Plumaria plumosa (Image C).
This species typically forms dense matts under a variety of fucoids in the mid to low intertidal zones (Image A), but we have collections to 17 m subtidal, which can become more Ptilota like in their appearance (Image G). Ceramium deslongchampsii Chauvin ex Duby similarly forms turfs under fucoids in the same range as Plumaria plumosa, but these species are easily distinguished in the field with careful observation. We have genetically verified collections from RI to NL.
Image A. Typical mid intertidal turfy matt formed under Ascophyllum nodosum (Linnaeus) Le Jolis at East Quoddy Lighthouse, NB (EGWS000106).
Image B. Pressed specimen revealing the overall thallus structure and pinnate branching (Wallace Cove Lighthouse, NB; PHYC004).
Image C. Leading branches (LEAD) alternate with their opposing lagging branches in series of 2-3 (Cable Head, PE; GWS041368).
Image D. Sparse cortication and monosiphonpous tips in ultimate branches (GWS041368).
Image E. Tips converted to elongate multicellular rhizoids in subtidal (14 m) specimen (GWS041368).
Image F. Parasporangium (Brier Island Western Lighthouse, NS; GWS009779).
Image G. Coarse subtidal (8 m) specimen appearing much like a Ptilota spp. (Paddys Head, NS; GWS011842).