Dilsea socialis (Postels & Ruprecht) Perestenko (Ar/A)

This species ranges from red to yellowish-green (near the blade tips) in colour and has a rather distinctive look in the NW Atlantic flora with its typically simple (can be cleft or have eroded tips) pyriform blades (Image A). Multiple blades, each reaching up to 25 cm and typically taller than wide, arise from the crustose holdfast forming small clumps. In transverse section the medulla is filamentous, these 5-10 µm wide and occasionally traversing the medulla from cortex to cortex, surrounded by an inner cortex of rounded cells reducing in size to the compact outer cortex (Image B). Apparent immature tetrasporangia (see Bird & McLachlan 1992), ovate in shape and 20-27 µm wide and 30-40 µm tall, are common in the cortex (Image B).

Unless you are in (sub)Arctic waters, or relatively deep in the Maritime provinces, you are unlikely to encounter this species (please let me know if you do). In that habitat, however, D. socialis could be confused with young and or simple individuals of Palmaria palmata (Linnaeus) F.Weber & D.Mohr. That species is distinct in having a pseudoparenchymatous rather than filamentous medulla. Confusion with Turnerella pennyi (Harvey) F.Schmitz, which has a filamentous medulla, is rare as this species has round to reniform blades that are typically broader than tall, and distinctive refractive ‘gland’ cells in the cortex.

This is a cold water species found subtidally (2-25 m) with genetically verified records from St. Lawrence I., Nome and the Boulder Patch, AK, throughout the Canadian Arctic including Churchill, MB, and extending to waters off PE and NS as far south as St. Margaret’s Bay (e.g. GWS036913).


GWS001850.jpgImage A. Pressed voucher of a subtidal (6 m) specimen from the northern tip of Baffin Island, NU (GWS001850).

GWS005344-0001.jpgImage B. Transverse section (aniline stained; low quality – poor rehydration of dried specimen) mid thallus displaying the filamentous medulla (one filament near the right of the image traverses from cortex to cortex) surrounded by the rounded cells of the cortex among which are numerous putative ovate tetrasporgangial initials (subtidal, 6 m on rock, Churchill, MB; GWS005344).