Urospora speciosa (Carmichael) Leblond ex Hamel (BC?/Ar?/A)

Dark to bright green unbranched uniseriate filaments typically forming localized carpets on rock and other hard substrata (Image A). Filaments range from 2-12 cm long and are relatively soft and delicate in appearance (Image B). Near the base the filaments are composed of cells that are squat to rectangular, 14-17 µm wide by 15-20 µm tall, each with a parietal band-shaped chloroplast covering 0.5-0.7 % of the cell periphery and containing 1(2) pyrenoids (Image C). The basal cell of each filament is rhizoidal in habit (Image D) and the adjacent 2-3 cells of the filament can produce descending extramatrical (free of the main filament) and or intramatrical (produced within the “matrix” or cell walls and surrounding cuticle) rhizoids (Image E). Lower mid thallus the cells are 32-34 µm wide by 12-17 µm tall, being typically squat to more disc-like when compared to cells near the base (Image F). Cells upper mid, 45-48 µm wide by 11-15 µm tall, and near the top of the filaments, 52-56 µm by 8-11 µm, become disc shaped and have a parietal chloroplast that covers the cell periphery and contains 1-3 pyrenoids (Image G). Upper cells become reproductive, presumably converting to gametangial production, at which point the filaments themselves become strongly curved and up to <68 µm wide (Images H & I). We have not encountered the zygote, “Codiolum gregarium” stage, of this species in our biodiversity surveys of the Canadian flora to date, which may be a sampling artifact or owing to an asexual life history in the area from which our collections were made.

Our genetically verified collections are few (n = 8; augmented by three strictly morphological identifications) for this species and confined to the NW Atlantic. The presence of this genetic group for both the Arctic and BC await molecular confirmation. Clearly significantly more genetically verified collections are necessary to understand the full ecological and biogeographical range of this species. Our few collections are from late winter to early spring, growing on rock in the upper and mid intertidal, as well as near freshwater outflows in estuarine environments where they contribute to significant unattached(?) mats. In preliminary phylogenetic analyses with both rbcL-3P and tufA, Urospora speciosa (typically assigned to Ulothrix as Ulothrix speciosa (Carmichael) Kützing) joins Urospora penicilliformis (Roth) Areschoug (the type of the genus) to the exclusion of the other species in our flora assigned to Urospora and Ulothrix. Taxonomic work is necessary.

This species can be confused with and indeed grow mixed with Ulothrix flacca (Dillwyn) Thuret, which 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 less disc-like when compared to adjacent vegetative cells. These features contrast those in Urospora speciosa for which the twisting of filaments is typically confined to reproductive regions characterized by zoidangia that are strongly discoid in outline similar to adjacent vegetative cells.

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, 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. Further study will determine if the correct binomial is applied to the morphospecies as outlined here, but this appears to be one of the more distinct species as outlined in the published 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. Filaments growing on a piece of cobble in an area of seepage, mid intertidal at Wallace Cove Lighthouse (sheltered side), Bay of Fundy, NB (GWS044497).

Image B. Mix of vegetative (straight) and reproductive (curved) filaments viewed under low power (GWS044497).

Image C. Cells lower on filaments with band-shaped chloroplasts containing 1(2) pyrenoids (GWS044497).

Image D. View of vegetative cells and rhizoidal development near the base of the filaments (upper intertidal on rock, Blacks Harbour, NB; GWS005941; rehydrated from press).

Image E. Basal cells become elongate and rhizoidal in habit (double arrow) with the adjacent few cells forming extramatrical (free of the matrix; arrows) and intramatircal (developed within the matrix) (low left and centre in this image) rhizoids (GWS005941; rehydrated from press).

Image F. Filament lower mid thallus (GWS044497).

Image G. Cells upper mid (arrow) and near the tops of the filaments become strongly discoid in habit (GWS044497).

Image H. Upper cells converting to presumptive gametangial production (GWS044497).

Image I. Reproductive structures are clearly disc shaped and the filaments become curved in habit (GWS005941; rehydrated from press).