Acrochaetium secundatum (Lyngbye) Nägeli (BC/A)

One of the more common members of this genus in the NW Atlantic flora, there are two distinct morphologies associated with this species – one smaller with curved axes and secund branching (Image A), the second more robust with straighter axes (Clayden & Saunders (2014). This species grows on a wide variety substrata, but is easiest to find by examining filamentous algae or small bryozoans under the microscope (Featured image above; on invertebrate). Erect filaments are <1mm tall with cells that are typically 1-3X taller than wide, characterized by a single stellate plastid with a central pyrenoid (Image A), as are cells of the monostromatic basal system from which the erect filaments develop (Image B). Monosporangia are the only known form of reproduction in the NW Atlantic flora, these typically secund along the erect filaments (Images C & D), but they can also become clustered (Clayden & Saunders (2014).

This species could be confused with Acrochetium luxurians (J.Agardh ex Kützing) Nägeli  in the NW Atlantic, however, this species typically grows as a ‘luxuriant’ carpet on seagrass in sheltered habitats with the erect filaments obtaining greater heights of ~2-3 mm.

Introduced populations of this species in California have a propensity to grow on sea otters (Bentall et al. 2016). Our genetically verified records are currently limited to BC, NB and NS, but this easily overlooked species is certainly more broadly distributed in our flora especially north into NL in the NW Atlantic (Mathieson & Dawes 2017).

GWS044246B-0063.jpgImage A. Erect filament, with weakly secund branching and a central pyrenoid in each cell, arising from a monostromatic base (on mid intertidal Vertebrata lanosa (Linnaeus) T.A.Christensen, Tiner Pt., Bay of Fundy, NB; GWS044246B).

GWS044247A-0069.jpgImage B. Single erect filament borne on a monostromatic disc (white arrows) (on low intertidal Cladophora rupestris (Linnaeus) Kützing, Tiner Pt., Bay of Fundy, NB; GWS044247A).

GWS044246B-0065.jpgImage C. Typically stalked monosporangia (arrows) produced in a secund series along an erect axis (GWS044246B).

EGWS001199Image D. Another look at the stellate plastids each with a central pyrenoid and terminal monosporangia (upper intertidal in freshwater seepage on green alga, Quaco Head Lighthouse, NB; EGWS01199).