Titanoderma macrocarpum (Rosanoff) Y.M. Chamberlain (A)

This species forms pinkish crusts on the host alga (Image A) and commonly bears numerous uniporate conceptacles (Image B). In section crusts display the typically palisade basal layer of upward curving cells, these bearing filaments of one (typically elongate) to three cells topped by an epithallial cell (Image C). Secondary pit connections occur between cells of adjacent filaments (Image D).

We only have four genetically verified collections in this genetic group, all subtidal (5-8 m) on Chondrus crispus Stackhouse (plus one specimen on Phyllophora pseudoceranoides (S.G.Gmelin) Newroth & A.R.A.Taylor ex P.S.Dixon & L.M.Irvine awaiting genetic verification), thus far limited to semi-exposed habitats from north of Cape Cod, MA, to southern ME. Interestingly, we also have a genetic record from Norway on a kelp stipe, a host we have not seen here in the NW Atlantic, but more collections are needed.

Currently considered a var. of Titanoderma pustulatum (J.V. Lamouroux) Nägeli (Irvine  & Chamberlain 1994; Algaebase), Titanoderma macrocarpum as presented here is genetically distinct from our local seagrass-inhabiting species assigned to Titanoderma pustulatum (J.V. Lamouroux) Nägeli. The names applied to the two species of Titanoderma uncovered in the NW Atlantic flora, Titanoderma pustulatum (J.V. Lamouroux) Nägeli and Titanoderma macrocarpum, are throwbacks to Taylor (1962) pending further study. This genetic group does not conform to the var. macrocarpum as outlined in Irvine  & Chamberlain (1994) – the taxonomy of Titanoderma needs a thorough reevaluation. Further, considerably more data are needed to understand fully host and geographical range for the species of this genus in our flora.

GWS014736

Image A. Specimen subtidal (5 m) on Chondrus crispus Stackhouse ( Plymouth, MA; GWS014736).

GWS014736.jpgImage B. Closeup of crust displaying uniporate conceptacles. (GWS014736). Scale = mm ruler.

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Image C. Transverse vegetative section (aniline stained; GWS014736).

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Image D. Closeup of secondary pit connections (aniline stained; GWS014736).