Phycologia (1999) Volume 38 (3), 184-194 Published 16 August 1999 Two new species of Dictyota (Dictyotales, Phaeophyta) from the Indo-Malayan region O. DE CLERCK* AND E. COPPEJANS Laboratory of Botany, University of Gent, K.L. Ledeganckstraat 35, 9000 Gent, Belgium O. DE CLERCK AND E. COPPEJANS. 1999. Two new species of Dictyota (Dictyotales, Phaeophyta) fram the Indo-Malayan region. Phycologia 38: 184-194. Two new species of Dictyota (Dictyotales, Phaeophyta) are described fram the Indian Ocean, Indonesia, and Papua New Guinea. Dictyota grossedentata is a pracumbent species with large, irregularly placed marginal teeth, differing from aH other prastrate species by its stout habit and conspicuous dentation. Dictyota grossedentata differs from other species characterized by dentate margins by its prastrate habit and by the lack of a single, conspicuous stupose base. Dictyota rigida is a stiff, completely erect species with a single point of attachment. Because of its stiff texture, it resembles D. jastigiata Sonder and D. suhrii Murray but differs by its uni1ayered meduHa. Dictyota rigida also is compared with the other slender taxa. Taxonomic prablems of the genus, such as substantial morphological variation and a lack of clearly discriminating characters, are considered. INTRODUCTION Species of Dictyota are cornmon components of tropical and subtropical marine fioras, but many species lack easily defined presence-absence characters and demonstrate substantial mor- phological variability [e.g. Dictyota dichotoma (Hudson) La- mouroux]. These problems have hampered taxonomic and fio- ristic studies over the past two centuries. The lack of under- standing of the variability of vegetative characters has resulted in developmental and morphological forms being described as different species (Phillips 1992). With the work of Weber- Peukert (1985), Hornig & Schnetter (1988), and Hornig et al. (1992a, 1992b), taxonomists again showed an interest in the genus, culminating in a revision of the north Atlantic Ocean species. Phillips (1992) made a detailed study of the Austra- lian representatives of the genus Dilophus. These studies have led to a better understanding of the morphological variability of the characters within this group and their proper application in distinguishing species. Weber-Peukert (1985) emphasized the use of cell dimensions and ratios of dimensions to distin- guish certain species. All species of the genus have a single meristematic apical cell, and vegetative cells do not undergo further division once the final strap dimensions are reached in the subapical regions. Hence, elongation of the interdichoto- mies is exclusively the result of cell elongation. For this rea- son, cell sizes could represent a good taxonomic character in mature parts of the thallus. Phillips (1992) proved the value of the number of sporangial stalk cells, the diameter of the divided sporangium, and the size of the spores as taxonomic characters. The number of sporangial stalk cells was espe- cially useful for discriminating among notoriously difficult species [e.g. D. fastigiata Sonder and D. gunniana (J. Agardh) Hornig, Schnetter et Prud'homme van Reine]. De Clerck & Coppejans (1997a, 1997b) gave these new systematic insights a more stable nomenclatural basis after the study of several * Correspondence and requests for reprints should go to this author. E-mail: olivier.declerck@rug.ac.be 184 type collections (Herb. Agardh, LD; Herb. Kützing, L; Herb. Lamouroux, CN; Herb. Sonder, MEL). The concept of Dictyota used in this paper is that of Hornig et al. (1992a), which includes species characterized by a sin- gle lenticular, transversely orientated apical cell and a paren- chymatous thallus composed of a cortex and a medulla. The number of medullary layers shows considerable variation be- tween the species. Sorne species may consist entirely of a unilayered or multilayered medulla, whereas in other species, a multilayered medulla is restricted to the base, the stolonif- erous holdfasts, or the thallus margins. In the past, taxa with a multilayered medulla were referred to the genus Dilophus. However, numerous species were difficult to assign to either Dictyota or Dilophus because of the presence of a multilay- ered medulla in a specific part of the thallus (e.g. stoloniferous holdfasts) or sporadic duplications of medullary cells (Setchell & Gardner 1925; Dawson 1950). Eventually, both genera were merged, based on culture experiments demonstrating that the number of medullary layers can be altered in many species depending on the culture conditions (Hornig et al. 1992a). Phillips (1992) stated that Dilophus should be merged with Dictyota based on morphological and reproductive characters but preferred to retain Dilophus as a separate genus until the taxonomy of Dictyota was clarified. Silva et al. (1996) include Dilophus in Dictyota and list a total of 35 previously reported species for the Indian Ocean; many of these may be nothing more than misapplied names. Most publications on Dictyota from this region provide short, uninformative descriptions without indication of voucher specimens. The only detailed accounts for the Indian Ocean were by Jaasund (1970) who recognized nine species for Tan- zania, Womersley (1987) who listed many collections from the west coast of Australia, and Phillips (1992) on Australian species characterized by a multilayered medulla. The first au- thor of this paper has prepared a revision of the genus in the Indian Ocean based on a morphological-anatomical analysis of all representatives combined with molecular data of certain