Phytotaxa 338 (1): 090–098 http://www.mapress.com/j/pt/ Copyright © 2018 Magnolia Press Article PHYTOTAXA ISSN 1179-3155 (print edition) ISSN 1179-3163 (online edition) 90 Accepted by Jeffery Hughey: 5 Jan. 2018; published: 9 Feb. 2018 https://doi.org/10.11646/phytotaxa.338.1.7 A new species of Digenea (Rhodomelaceae, Ceramiales) based upon a molecular assessment and morphological observations of plants historically known as D. simplex in Bermuda CRAIG W. SCHNEIDER 1* , BILAL F. HAMZEH 1 , CHRISTOPHER E. LANE 2 & GARY W. SAUNDERS 3 1 Department of Biology, Trinity College, Hartford, CT 06106, USA 2 Department of Biological Sciences, University of Rhode Island, Kingston, RI 02881, USA 3 Centre for Environmental and Molecular Algal Research, Department of Biology, University of New Brunswick, Fredericton, NB E3B 5A3, Canada * Corresponding author: E-mail: cschneid@trincoll.edu Abstract Using mitochondrial COI-5P as a barcode marker, the red algal species previously identified as Digenea simplex in Bermuda is shown to be distinct from this species found in the Adriatic (type locality) and Mediterranean Seas, as well as other tropi- cal locations worldwide. This finding led to a comparative study of the morphology of Bermuda specimens and D. simplex from the type locality, as well as other congeners. Our data show the Bermuda specimens to be morphologically, as well as genetically, distinct from D. simplex necessitating the description of D. arenahauriens sp. nov. for plants found in the islands and a single collection from the Caribbean Sea. It represents the first species partitioned from the “pantropical” D. simplex and we present genetic evidence of additional undescribed isolates in the species complex requiring further study. Key words: Bermuda, COI-5P, Digenea arenahauriens sp. nov., D. simplex, rbcL, Rhodomelaceae, Rhodophyta, western Atlantic Ocean Abbreviations: COI-5P, 5’ region of the cytochrome oxidase subunit 1 gene; rbcL, ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit gene Introduction When C. Agardh (1822) described the genus Digenea, it included only D. simplex (Wulfen) C. Agardh from the Adriatic and Mediterranean Seas. An early addition to the genus, D. dichotoma Audouin (1828), was later shown by Silva & Lipkin (2003) to be Galaxaura rugosa (J. Ellis et Solander) J.V. Lamouroux. Hering (in Schimper 1837) moved Fucus lycopodium Turner nom. illeg. to Digenea, at the time resurrecting the epithet of this Red Sea species in a different genus. Digenea lycopodium (C. Agardh) Hering is presently considered a synonym of D. simplex (Guiry & Guiry 2017). Two other early taxa were included, D. wulfenii Kützing nom. illeg. (1843) and D. implexa Zanardini nom. inval. (Frauenfeld 1854) [see Guiry & Guiry (2017) for nomenclatural notes]. Kützing (1865) described D. vieillardii Kützing from New Caledonia, a species now also considered synonymous with D. simplex. Since these 19 th century names, only two taxa have been added to Digenea, D. subarticulata Simons (1970) and D. cymatophila (R.E. Norris) Díaz-Tapia et Maggs (Díaz-Tapia et al. 2017), thus prior to this paper there were only three current species in the genus (Guiry & Guiry 2017). Over the past two centuries, Digenea simplex has been collected widely and shown to have a pantropical distribution (Lüning 1990), with incursions into warm temperate waters (Guiry & Guiry 2017). It was first reported for Bermuda by Kemp (1857), quickly followed by additional 19 th century reports (Rein 1873, Hemsley 1884, Murray 1888). In 1913, specimens from Harris’ Bay were distributed as no. 1939 in the Bermuda fascicles of the exsiccata, Phycotheca Boreali-Americana (P.B.-A., Collins et al. 1913), followed by accounts of its distribution in Collins & Hervey’s (1917) Algae of Bermuda. During our concerted efforts to barcode all of the benthic marine algae from the islands beginning in 2010, several isolates of D. simplex were sequenced and shown to be distinct from specimens collected from near the type locality in the Mediterranean Sea.