Rhodachlya madagascarensis gen. et sp. nov.: a distinct acrochaetioid represents a new order and family (Rhodachlyales ord. nov., Rhodachlyaceae fam. nov.) of the Florideophyceae (Rhodophyta) JOHN A. WEST 1 *, JOSEPH L. SCOTT 2 ,KATHRYN A. WEST 1 ,ULF KARSTEN 3 ,SUSAN L. CLAYDEN 4 AND GARY W. SAUNDERS 4 1 School of Botany, University of MelbourneParkville, VIC 3010, Australia 2 Department of Biology, College of William and MaryWilliamsburg, VA 23187, USA 3 Institute of Biological Sciences - Applied Ecology, University of RostockAlbert-Einstein-Strasse 3, D-18057 Rostock, Germany 4 Department of Biology, University of New BrunswickFredericton, NB E3B 6E1, Canada J.A. WEST, J.L. SCOTT, K.A. WEST, U. KARSTEN, S.L. CLAYDEN AND G.W. SAUNDERS. 2008. Rhodachlya madagascarensis gen. et sp. nov.: a distinct acrochaetioid represents a new order and family (Rhodachlyales ord. nov., Rhodachlyaceae fam. nov.) of the Florideophyceae (Rhodophyta). Phycologia 47: 203–212. DOI: 10.2216/07-72.1 A filamentous ‘acrochaetioid’ red algal epiphyte on Posidonia from Madagascar was isolated into culture. It reproduces solely by monosporangia and the spherical spores have gliding motility that differs from amoeboid spore motility seen with time-lapse video microscopy in most Acrochaetiales and Colaconematales. Electron microscopy reveals that a peripheral encircling thylakoid is absent in the chloroplast, Golgi bodies are associated with mitochondria, and pit plugs have platelike outer and inner caps and lack a cap membrane. Low-molecular-weight carbohydrates are floridoside and trehalose. Molecular evidence (small subunit, large subunit and EF2 sequences) resolved this taxon as a completely novel lineage of the Nemaliophycidae, and it is here described as Rhodachlya madagascarensis J.A. West et al., gen. et sp. nov. In light of the unusual combination of ultrastructural features and the lack of affinity to any of the currently recognized families and orders in molecular analyses, this alga forms the basis of a new family, Rhodachlyaceae G.W. Saunders et al., fam. nov., and order, Rhodachlyales G.W. Saunders et al., ord. nov., of the subclass Nemaliophycidae. KEY WORDS: Biodiversity, Florideophyceae, Madagascar, Molecular systematics, Nemaliophycidae, Rhodachlya, Rhodophyta, Spore motility INTRODUCTION Systematics within the red algal subclass Nemaliophycidae (Saunders & Hommersand 2004) has been profoundly affected by the advent of molecular tools. Ragan et al. (1994) provided the first molecular support for a contro- versial alliance between the Acrochaetiales, Nemaliales and Palmariales, and subsequently Saunders et al. (1995) allied the Batrachospermales to this lineage and transferred a number of acrochaetioid taxa to the Palmariales. Saunders & Bailey (1997) provided supporting evidence for an alliance of the previous orders and further resolved an association of the previous to the Corallinales and Rhodogorgonales. After these initial studies, taxa of uncertain affiliation within this lineage, and in some cases other lines of the Florideophyceae, were recognized as discrete ordinal lineages in this complex largely on the basis of molecular data. The genera Balbiania and Rhododrapar- naldia, considered provisional members of the Acrochae- tiales (e.g. Vis et al. 1998), were assigned to a new order, Balbianiales (Sheath & Mu ¨ller 1999). During a systematic survey of the distantly related Ceramiales, Rhodymenio- phycidae, Choi et al. (2000) established that some marine species of the genus Ballia formed a novel lineage in the Nemaliophycidae for which they established the order Balliales. Vis et al. (1998) resolved the Thoreaceae as distinct from the Batrachospermales, in which it was positioned, and a segregate order, Thoreales, was later established (Mu ¨ ller et al. 2002). A detailed molecular survey of the Acrochaetiales resolved two divergent lineages, resulting in recognition of an additional order, Colacone- matales (Harper & Saunders 2002). Finally, Le Gall & Saunders (2007) removed the Corallinales and Rhodogor- gonales to a segregate subclass, Corallinophycidae, leaving the Nemaliophycidae with a diverse blend of orders including the Acrochaetiales, Balbianiales, Balliales, Ba- trachospermales, Colaconematales, Nemaliales, Palmar- iales and Thoreales, half of these disparate lineages assigned ordinal status on the basis largely of molecular data. Whereas many genera completely new to science have been uncovered in the major lineages of the Florideophy- ceae over the past century [including in the Nemaliophy- cidae; e.g. Psilosiphon (Entwisle 1989) and Petrohua (Vis et al. 2007)], only one completely novel ordinal-level lineage of florideophyte red algae has been identified – Rhodogor- gonales (Norris & Bucher 1989, Fredericq & Norris 1995). This taxon was unambiguously distinct using traditional taxonomic and ultrastructural criteria and molecular data were not used in the decision to erect a distinct ordinal assemblage. Although molecular data pointed to their ordinal distinctiveness, even the disclike thalli on which the recent Pihiellales was based were reported in the literature * Corresponding author (jwest@unimelb.edu.au). Phycologia (2008) Volume 47 (2), 203–212 Published 12 March 2008 203