Ulothrix flacca (Dillwyn) Thuret (Ar/A)

Green filaments commonly on fucoids (Featured Image above) or contributing to green turfs on rock in the intertidal (Image A) and possibly mats in estuarine or salt marsh localities (no genetically verified records to date). Filaments 1-10 cm long, soft and flaccid, can be straight resulting in tufted mats or tangled turfs, or curved and producing rope-like strands (Image B). The uniseriate filaments are attached by a slightly modified basal cell (Image C), which can be augmented by descending intramatrical (growing within the filament wall and cuticle) rhizoids (Image D). Filaments are gregarious and arise from a compact aggregation of these basal cells and rhizoids (Image E). Cells near the base can be squat to rectangular, 9-11 µm wide by 7.5-16 µm tall with the single parietal band-shaped chloroplast occupying ~70-80 % of the cell periphery and containing a single obvious pyrenoid (Image F). Cells mid filament become disc shaped, 12.5-14 µm wide by 6-8 µm tall, and again contain a single parietal chloroplast occupying ~80-90 % of the cell periphery with 1(2) obvious pyrenoid(s) (Image G). Upper filaments are 22-25 µm wide, the cells are typically more consistently disc-like, 17.5-21.5 µm wide by 5-8 µm tall (3-4:1 width to height ratio) with the chloroplast covering most of the cell periphery and containing 1-2 pyrenoids (Image H). Branching can occur in some filaments being observed as short side branches at 90 degree angles to the main filament (Image I) to more substantial development (Images J & K). At times filaments tend to adhere into a false bi(multi?)-seriate development of sorts (Image L), which may be facilitated by the typically rough and heavily epiphytized filament walls common in this species (e.g. Image H). Reproductive structures tend to be squat, but typically more elongate than adjacent vegetative cells, and tend to bulge slightly with relatively large zoids produced (Image M). Interestingly, in situ germination appears to occur from some of the reproductive structures (Image N), which we have not noted previously in our green filamentous taxa. In one collection a presumptive ‘Codiolum stage’ (zygote; Image O) was observed suggesting a sexual pattern in this population.

Our genetically verified records extend from RI in the south to Churchill, MB, in the arctic. Populations form turfs or twisted tufts/strands on all species of Fucus, as well as on various solid substrata. They range from sheltered to exposed sites, mid to upper intertidal, at times in pools and in areas of freshwater outflow. This species can be confused with and indeed grow mixed with Urospora speciosa (Carmichael) Leblond ex Hamel, but the latter has wider filaments, tends to be twisted only in reproductive regions of filaments, with the reproductive structures being slightly narrower (strongly disc-shaped) than the adjacent vegetative cells. Ulothrix flacca has narrower filaments with both the vegetative and reproductive regions having the potential to be twisted, and the reproductive structures, at least in the upper filaments, are more squat than disc-like when compared to adjacent vegetative cells. Urospora neglecta (Kornmann) Lokhorst & Trask can live in similar habitats and look very much like Ulothrix flacca when it forms turfs on rock, but the cells contain multiple pyrenoids. Ulothrix implexa (Kützing) Kützing is most likely to be confused with Ulothirx flacca, but the former is more typically estuarine in habit, has smoother filament walls lacking copious epiphytes and other adherent material, with the upper cells tending to be slightly less disc shaped.

It is common for what otherwise looks like a unialgal carpet on rock, or mat of green filaments in an estuary, to be a chaotic mix of green species including, among others, various Blidingia spp., Percursaria percursa (C.Agardh) Rosenvinge, Rhizoclonium spp., Rosenvingiella polyrhiza (Rosenvinge) P.C.Silva, Urospora spp. and Ulothrix spp. Identification should be done carefully, as should attempting to match DNA sequences to individual species. Owing to the wonders of PCR, a clean sequence could be from any one of the species present in a collection and not necessarily the dominant one on which the microscopical identification was based. I believe that the images here all derive from the same genetic group, but it has yet to be established that the correct binomial is applied here. This is my best guess after consulting the pertinent literature (mostly Scagel (1966), Burrows (1991), Sears (2002), Brodie et al. (2007), Mathieson & Dawes (2017) and Gabrielson & Lindstrom (2018)). Considerable work remains for this genus in our flora.

Image A. Collection from mid intertidal in a damp depression at an exposed site. Ulothrix flacca dominated this skein, but Rhizoclonium spp. and Urospora penicilliformis (Roth) Areschoug, among other species, were also present (Wallace Cove, NB; GWS044490).

Image B. Curved filaments uniting into rope-like strands (low intertidal on Fucus distichus Linnaeus, Lepreau – seagrass flats near bridge along highway, NB; GWS037929; rehydrated from press).

Image C. Filament with a rhizoidal basal cell for attachment (upper intertidal turf on rock, Riviere du Loup, QC; GWS008051; rehydrated from press).

Image D. Intramatrical rhizoids contributing to the attachment of the uniseriate filaments (upper intertidal turf on rock, Lepreau exposed biodiversity site, Bay of Fundy, NB; GWS003899; rehydrated from press).

Image E. Aggregation of basal cells and rhizoids give the impression of a parenchymatous crust (mid intertidal on Fucus vesiculosus Linnaeus, Wallace Cove Lighthouse (sheltered side), NB; GWS044499).

Image F. Cells near the base of the thallus (GWS044490).

Image G. Cells mid thallus (GWS044499).

Image H. Cells near the tops of the filaments (GWS044490).

Image I. Short branches formed at 90 degrees to the bearing filament (GWS008051; rehydrated from press).

Image J. Example of more robust branching observed in some filaments (mid upper intertidal on Fucus spiralis Linnaeus, Governor Sprague Bridge 17, Narragansett, RI; GWS006044; rehydrated from press).

Image K. Example of more robust branching observed in some filaments (GWS008051; rehydrated from press).

Image K. Filaments uniting together into a false biseriate filament (GWS008051; rehydrated from press).

Image M. Intercalary reproductive structures develop mid to upper filament (GWS003899; rehydrated from press).

Image N. Putative in situ germination from mature zoidangia in the filaments (GWS003899; rehydrated from press).

Image O. Presumptive ‘Codiolum stage’ indicating a possible sexual life history for this species (GWS003899; rehydrated from press).