SMITHSONIELLA GEN. NOV., A POSSIBLE EVOLUTIONARY LINK BETWEEN THE MULTICELLULAR AND SIPHONOUS HABITS IN THE ULVOPHYCEAE, CHLOROPHYTA

A low relief, green turf-forming alga of a heterotrichous habit was discovered in the coral reef microcosm, Museum of Natural History, Smithsonian Institution. Erect filaments bore lateral, specialized sporangia and together with basal filaments possessed septal plugs between adjacent cells, grossly similar to the "pit connections" of red algae. Data are presented which: 1) establish the identity of our plant with a plant recently described as Pilinia earleae Gallagher et Humm from the Florida Gulf coast; 2) support our establishment of the new genus Smithsoniella and our transfer of P. earleae to this new taxon. Additional data on pigmentation and cytology are related to the fine structure of other selected green algae to develop and test three hypotheses, viz. Smithsoniella earleae represents either: 1) a symbiotic association between a green and a red alga; 2) an alga which belongs to either the Ulotrichales, Chaetophorales or the Chroolepidales; or 3) an alga representing an evolutionary link between filamentous forms of the Ulvophyceae and members of the coenocytic siphonalean complex (e.g., Codiales or Caulerpales) of the Chlorophyta. Data refute hypotheses I and 2 but do lend support to the third hypothesis. RECENT FINE STRUCTURAL STUDIES of the green algae (Roberts et al., 1980; Sluiman et al., 1980; Stewart and Mattox, 1975, 1978; Stewart, Mattox and Floyd, 1973) have substantially modified our view of the relationships of members of the Chlorophyta from older systems based on vegetative and reproductive morphology. The origins of the coenocytic green algae have been especially difficult to discern (Stewart, Mattox and Floyd, 1 Received for publication 13 July 1981; revision accepted 3 November 1981. We thank individuals of the Smithsonian Institution who supported the concept of the coral reef microcosm developed by Dr. Walter Adey and colleagues and especially Dr. Adey for use of facilities in the Marine Systems Laboratory at the Smithsonian Institution. We also thank Dr. John Ogden and Nancy Ogden of the West Indies Laboratory, St. Croix, U.S.V.I. for providing space, facilities and field expertise during our visits to St. Croix, and Dr. Kenneth Towe for use of his electron microscopy facilities. Discussions with Drs. David Domozych, Ken Stewart, and Karl Mattox were especially helpful in developing our hypotheses and comments by Drs. Max Hommersand, Paul Silva, and Charles O'Kelly aided in taxonomic and nomenclatural treatments. We thank Dr. H. Humm for providing a culture of Smithsoniella from the Florida Gulf coast and Dr. Hannah Croasdale, Dartmouth College, for providing the Latin diagnosis of Smithsoniella. Support for travel to St. Croix by J. Sears was generously provided by Southeastern Massachusetts University and was greatly appreciated. 2 Present address: Department of Physiology, Health Center, University of Connecticut, Farmington, CT 06032. 1973) but recent studies (Roberts et al., 1980; Sluiman et al., 1980) suggest that they are related to the Ulvophyceae as proposed by Stewart and Mattox (1978). While surveying the green algae within the Smithsonian Institution's 6.5 kl coral reef microcosm (Brawley and Adey, 1981) in 1977, one of us discovered a most unusual heterotrichous green alga growing as a low relief, green turf on the walls and coral rock substratum of the microcosm. The fine, erect filaments bore specialized, lateral sporangia along their axes. These prominent reproductive structures were of special interest; among green algae, specialized sporangia are largely restricted to siphonous, coenocytic forms within the Caulerpales and Codiales or to multicellular forms of the Chaetophorales and Trentepohliales (Bourrelly, 1972). Even more unusual was the presence of "pit connections" between adjacent cells. Subsequent studies of these structures (Brawley and Sears, 1980, 1982) demonstrated that the apparent "pit connections" were true septal plugs occluding perforated end walls and consisted of a core and paired caps. Such structures have never been demonstrated within the Chlorophyta, although they are common in members of the Rhodophyta where they also consist of a perforated end wall occluded by a septal plug (Ramus, 1969). Stewart, Mattox and Floyd (1973) pointed out that "coe-