Gracilaria tikvahiae McLachlan (A)

Gracilaria tikvahiae expresses considerable morphological variation across its range from irregularly branched terete (Image A) to flattened (Image B) thalli. Consequently it can be difficult to identify, especially the morphs with terete axes when compared to Agarophyton vermiculophyllum (Ohmi) Gurgel, J.N.Norris & Fredericq. Whereas axes in Agarophyton vermiculophyllum (Ohmi) Gurgel, J.N.Norris & Fredericq are terete throughout, axes in G. tikvahiae typically have flattened regions even in largely terete individuals, notably at the branching forks (Image C). Transverse sections in G. tikvahiae reveal medullary cells that are irregularly isodiametric to oval in outline, 100-120 by 120-460 µm in dimensions (Image D), grading to a 1-2 celled inner cortex and a distinct 1-2 celled outer cortex of cuboidal to rounded cells 8-14 µm in size (Image E). In contrast, Agarophyton vermiculophyllum (Ohmi) Gurgel, J.N.Norris & Fredericq transverse sections reveal smaller medullary cells with thick and lamellate cell walls, as well as typically more cortical layers (2-3) of elongate rather than isodiametric cells.

We have genetically verified records from RI to PE from the low intertidal to subtidal (8m), typically on cobble in sheltered areas, but with some individuals growing unattached in muddy lagoon habitats. A specimen received from TX and field identified as Gracilaria debilis (Forsskål) Børgesen was a genetic match to G. tikvahiae. To date genetically verified records for Agarophyton vermiculophyllum (Ohmi) Gurgel, J.N.Norris & Fredericq are sporadic north of Cape Cod, MA (Mathieson & Dawes 2017), which can further assist in identification. When in doubt, or in particular if reporting range extensions for Agarophyton vermiculophyllum (Ohmi) Gurgel, J.N.Norris & Fredericq molecular data should be generated (Saunders 2009). This species can be highly variable in morphology, some of which is recorded in the BOLD taxonomy browser.

GWS011645.jpgImage A. Individual with irregular branching and largely terete axes from North Rustico, PE, subtidal (8 m) on cobble (GWS011645).

GWS014717Image B. Individual with irregular branching and largely flattened axes from found in the drift at Plymouth, MA (GWS014717).

PB300002.JPGImage C. A largely terete intertidal specimen displaying flattening at branch forks (arrows) (Northumberland St., NB; GWS007993). Scale = mm ruler.

GWS014717-0001.jpgImage D. Transverse section displaying the large medullary cells are rapid transition to the thin cortex (GWS014717).

GWS014717-0002.jpgImage E. Transverse section closeup on the thin1-2 layered inner and 1-2 layered outer cortex, the latter with typically isodiametric cells (GWS014717).