The taxonomy and morphological variation of distromatic ulvaceous algae (Chlorophyta) from the northeast Pacific
Creators: name: Tanner, Christopher Eugene
Publisher: name: University of British Columbia
Year: 2010

description: As a result of herbarium, field and culture studies, six species of Ulya Linnaeus, U. califomica Wille, U. fasciata Delile, U. fenestrata Postels et Ruprecht, U. rigida C. Agardh, U. stenophylla Setchell et Gardner and U. taeniata (Setchell) Setchell et Gardner, are recognized for the northeast Pacific (west coast of North America). The position of a seventh species, U. conglobata Kjellman, remains unclear. Six taxa previously recognized for the northeast Pacific are reduced to synonyms of other known species. Ulva angusta Setchell et Gardner and U. scagelii are reduced to synonyms of U. califomica. Ulva expansa (Setchell) Setchell et Gardner and U. fasciata f. lobata Setchell are reduced to synonyms of U. fenestrata. Ulva costata (Howe) Hollenberg and U. dactylifera Setchell et Gardner are reduced to synonyms of U. taeniata. Generic and specific taxonomic criteria for Ulva are examined and evaluated. A large degree of morphological and anatomical variation was observed in herbarium specimens and in field populations of Ulva. Some of this variation is related to differences in environmental parameters. Dentation in U. taeniata is reduced or eliminated with an increase of water temperature, while blade thickness increases with an increase of water temperature. As a result specimens from the southern part of its range are often incorrectly identified. The size, shape and thickness of thalli in populations of U. fenestrata from the west coast of Vancouver Island and from Vancouver vary significantly with vertical position in the intertidal zone and with wave exposure. Most species of Ulva show some seasonal variation in size and thickness. Morphology is generally more conservative than blade thickness or cell dimensions. Pyrenoid number is useful for separating U. califomica from other species; however, in species with more than one pyrenoid per cell, the number is not consistent and cannot be used as a taxonomic criterion. Studies of reproduction, development and interspecific hybridization potential confirmed observations made in the field. In culture U. taeniata does not produce marginal dentation above 16° C. Development and morphology in U. califomica varies with temperature. At relatively low temperatures (7°-11° C) upright filaments develop before the basal systems and result in plants similar to U. scagelii from British Columbia. At higher temperatures (13°-19° C) the basal system is initiated first and develops into an extensive basal system that produces numerous upright germlings. The resulting tufted plants are similar to U. califomica from southern California. These two species also form viable zygotes when crossed, supporting the synonymy of U. scagelii with U. califomica. As living specimens of U. fasciata were not encountered in the northeast Pacific, an isolate from Hawaii was used for the culture study of the reproduction and development of this species. This Hawaiian isolate demonstrated an unusual form of reproduction. Marginal vegetative cells round up and develop into aplanospores that divide into a floating multicellular globose stage. The cells of the globose stage eventually release biflagellated swarmers that develop into normal appearing germ-lings . A cytological study was undertaken to determine if species of Ulva from the northeast Pacific differed in their chromosome numbers. All of the species of Ulva studied have a similar haploid chromosome number of around 8 or 9. Following parthenogenetic development of gametes, haploid, diploid and dikaryotic cells are occasionally observed in the same germling. The dikary-otic cells may represent a transitional stage in the doubling of the chromosome number. In a few of these parthenogenetic germlings some of the cells become enlarged and multinucleate. These cells may have lost the capacity for cytokinesis. A diminutive distromatic ulvaceous alga was encountered in southern California. This alga resembles Ulva morphologically; however, its developmental pattern is distinctly different from Ulva and other ulvaceous algae. In Ulva the distromatic blade develops by the collapse of a monostromatic tubular germling. In this alga the distromatic blade forms by longitudinal divisions parallel to the surface of the germling. A new genus and species, Chloropelta caespitosa, are proposed and described for this alga.