Mastocarpus stellatus (Stackhouse) Guiry (A)

This species has an alternation of heteromorphic generations with crustose sporophyte (Image A) and erect gametophyte (Image B) stages. The former varies in colour from yellowish-green through brownish-red and forms extensive (up to 40 cm), thick (400 µm to 2 mm) and leathery crusts on rock in the mid to lower intertidal (Image A). In section a prostrate hypothallus formed of compact filaments subtends the thick perithallus, which is composed of sparsely branched and loosely adherent filaments (Image C). Perithallial cells are 5-6.5 µm wide by 6-15 µm tall becoming progressively smaller toward the surface with tetrasporangial initials developing directly from intercalary cells in the filaments (Image D). Gametophytes grow in clumps or become turf-like in habit, the upright fronds clearly dichotomously branched and typically forming surface papillae (Image B). Their colour is less variable than the tetrasporophyte phase being typically reddish-brown (Image B). In transverse section blades have a tangled mesh-like medulla of thick walled filaments (5-8 µm wide), which transitions into a relatively thick cortex of 7-10 cell layers (Image E).

There has been some question as to whether or not NW Atlantic populations are asexual and consequently uncertainty about the two phases described above belonging to a single species. Although life history work remains, COI-5P and rbcL-3P both indicate that these morphologically diverse entities are assignable to a single genetic group in what is presumably an alternation of heteromorphic generations.

Chondrus crispus Stackhouse can easily be confused with some gametophytic specimens of Mastocarpus stellatus, but individuals of the latter species typically have distinct papillae formed on their blade surfaces (Image B) and thicker margins resulting in a stronger u-shaped profile to the frond (Image F). Confusion can also manifest between M. stellatus and Fredericqia deveauniensis Maggs, L. Le Gall, Mineur, Provan & G.W.Saunders, but fronds for this latter species lack both the u-shaped profile and the papillae, and the construction is pseudoparenchymatous rather than filamentous (Image E).

We have genetically verified records of this species on rock from CN to the island of NL, typically mid through low intertidal with the crustose sporophyte extending into the shallow subtidal (3.5 m). We also have records from Europe where two genetic groups occur for this morphospecies (Le Gall & Saunders 2010a).

OLYMPUS DIGITAL CAMERAImage A. Tetrasporophyte (Petrocelis phase) stage growing lower mid intertidal on rock (Wallace Cove Lighthouse, NB; EGWS000339).

GWS010926.jpgImage B. Gametophyte stage with numerous papillae on the frond surface from low intertidal rock (Lepreau, NB; GWS010926).

GWS032398003 copy.jpgImage C. Section of the crustose sporophyte stage showing a compact hypothallus bearing the thick relatively loose straight filaments of the perithallus (lower mid intertidal on rock under rockweed, Letete, NB: GWS032398).

 

Image D. Close up of the sporophyte filaments with individual intercalary cells initiating tetrasporangial development (arrows) (low intertidal on rock, Wallace Cove Lighthouse, NB; EGWS000824).

EGWS000472-0001 copy.jpgImage E. Transverse section of the gametophytic stage showing the tangled mesh-like habit of the medulla and thick cortex of 7-10 cell layers (low intertidal on rock, Point Pleasant Park, NS; EGWS000472).

Untitled-1.jpgImage F. Side by side comparison of thickening at the margins and the consequent curling of the blade to yield a u-shaped profile in juvenile M. stellatus (right, lacks the diagnostic papillae; EGWS000472) versus Chondrus crispus Stackhouse (left; EGWS000473).