Dichotomaria marginata Lamarck 1816
Creators: name: Schneider, Craig W.
name: Popolizio, Thea R.
name: Spagnuolo, Dylan S.
name: Lane, Christopher E.
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Plantae
Rhodophyta
Florideophyceae
Nemaliales
Galaxauraceae
Dichotomaria
Dichotomaria marginata

description: Dichotomaria marginata (J. Ellis et Solander) Lamarck 1816, p. 146 (Figures 3–6)
Basionym
Corallina marginata J. Ellis et Solander 1786, p. 115, pl. 22 figure 6 (Figure 3).
Description
Rosy-brown plants to 10 cm tall and 13 cm across, composed of dichotomously branched flattened axes with thickened margins, these showing faint annulations; axes mottled in appearance when dried; internodes 4.7–13.7 mm long and 1.6–2.6 mm wide; axes branching dichotomously at angles of 72–89°; cortex of sporophytes composed of two layers of cells, outer cortical assimilatory cells subspherical to ovoid, 20–40 µm long and (21–) 33–49 µm diameter, and an inner layer of flared stalk cells each bearing 1–2 assimilatory cells, 15–36 µm long and 5–10 µm diameter (at narrowest point); stalk cells borne typically in pairs though occasionally singly on the outer surface of the inner cortex; stalk cells most commonly unicellular, rarely two-celled; inner cortex consisting of typically one, occasionally two, layers of transversely ovoid to rectangular cells, 50–115 µm diameter and 25–68 µm high; subcortical cells subspherical at margins; medullary filaments extending from subcortical cells, 7–13 µm diameter; gametangia and tetrasporangia not seen in our sequenced specimens.
Type locality
“… on the shore of one of the Bahama islands”, West Indies, western Atlantic Ocean (Ellis and Solander 1786).
Distribution
Formerly reported as pantropical, genetically verified specimens are known from Guadeloupe, Puerto Rico and St. Croix, USVI (West Indies).
Selected collections
St. Croix, Virgin Is. – T. R. Popolizio (TRP)/ C. E. Lane (CEL)/ E.D. Salomaki (EDS) 13-24-1 [STX 032] (Figure 4), TRP / CEL / EDS 13-25-9 [STX 054] (see Table 1 for collection details).
Remarks
As discussed above, two St. Croix collections genetically matched other sequenced specimens from Guadeloupe and Puerto Rico, as well as the description of Dichotomaria marginata provided by Papenfuss et al. (1982). This has allowed us to identify STX 032 and STX 054 (Figures 1 and 2) as true representatives of D. marginata in the West Indies, a species with a type locality in the Bahamas (Silva et al. 1996). All of the genetic collections made in St. Croix identified as D. marginata have the anatomy of sporophytes defined for the species, unfortunately none bearing tetrasporangia. Thus, we cannot provide tetrasporangial size and shape nor the anatomy or reproduction in gametophytes of these genetically significant D. marginata vouchers. Although there are reports about these features in earlier literature (e.g. BØrgesen 1916, Howe 1918b, Taylor 1960, Papenfuss et al. 1982), without a molecular tie to this species where crypsis is now reported, we are hesitant to link that information here.
Similar to what Huisman et al. (2004) found for the species, tetrasporic St. Croix representatives of D. marginata have “the presence of a distinctly thickened margin (particularly noticeable when dried)” and a “cortex of the tetrasporophyte bearing paired subspherical cells (at least some with a small terminal spine)” (Figure 5). As was pointed out in the Ellis and Solander (1786) protologue, the flattened axes of D. marginata are more pronounced when removed from the water, the first collections being washed up and dried on a Bahamian beach. Our specimens have three, or occasionally four, layers of cells including stalk and outer assimilatory cells. The assimilatory cells of St. Croix D. marginata are subspherical to ovoid in cross-section (Figure 5), and appear circular to ellipsoidal in surface view (Figure 6). Some, but not a majority, of the assimilatory cells are apiculate, these cells usually paired on stalk cells that are mostly paired on outer subcortical cells (Figure 5). The subcortical cells are transversely ovoid to rectangular, and occasionally subspherical at the margins (Figure 5).
Howe (1918b) was the first to consider Galaxaura occidentalis BØrgesen (type locality= Virgin Islands) as the gametophyte generation of G. marginata, and BØrgesen (1920) and subsequent workers (Taylor 1960, Papenfuss et al. 1982) followed his supposition. Howe (1918a) mentioned gametophytic specimens in his account of G. marginata in the Bermuda flora, but it remains unclear whether he was referring to actual Bermuda gametophytes or rather to the gametophytes he was working on at the same time from the West Indies (Howe 1918b). We have been able to observe the gametophytic holotype of G. occidentalis [in C], most reasonably the alternate generation of the sporangial D. marginata, but this requires genetic confirmation. It would be helpful to find sporophytes and gametophytes of D. marginata in a single Bahamian or West Indian population to confirm this supposition, both generations matching genetic sequences already linked to this species from the West Indies.
A second genetic species, Dichotomaria sp. 1 CRB, was collected in St. Croix and St. Kitts (Figures 1 and 2, Tables 1 and 2), and the St. Croix isolate represents the gametophytic stage of an undescribed species in the D. marginata complex. The St. Croix specimen has anatomical similarities with the type of G. occidentalis; unfortunately, it is genetically distinct from local and regional D. marginata. We only have two representatives of this species from the West Indies, thus until additional specimens are obtained and sequenced, Dichotomaria sp. 1 CRB will remain undescribed. Whether its gametophytic stage represents what was described as G. occidentalis requires further investigation. Nevertheless, its sequence demonstrates that there are at least two species in the D. marginata complex in the West Indies.
description: Published as part of Schneider, Craig W., Popolizio, Thea R., Spagnuolo, Dylan S. & Lane, Christopher E., 2016, Notes on the marine algae of the Bermudas. 15. Dichotomaria huismanii (Galaxauraceae, Rhodophyta), a new species in the D. marginata complex from the western Atlantic, pp. 13-29 in Botanica Marina (Warsaw, Poland) (Warsaw, Poland) 59 (1) on pages 17-23, DOI: 10.1515/bot-2015-0068, http://zenodo.org/record/10999925