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Contributions to the life history and ecology of the marine brown alga Phaeostrophion irregulare S. et G. on the Pacific coast of North America Mathieson, Arthur Curtis

Abstract

Until recently, few collections of the marine brown alga Phaeostrophion irregulare S. et G. had been made, little was known of its ecology, and virtually nothing of its life history. The main objectives of this investigation were to study the life history and major factors influencing growth and distribution of this species. Laboratory and field investigations were conducted during 1961-196i4. Life history studies were performed in the laboratory by culturing zoospores under constant environmental conditions. The growth of cultured germlings and the photosynthetic response of the laminate plants from the field were recorded under different temperatures, salinities, nutrients, and light conditions. The tolerance of laminate plants and germlings to extremes of temperature, salinity, and desiccation was also determined in the laboratory. The growth and reproduction of in situ plants at Glacier Point, British Columbia were correlated with temperature, salinity nutrients, tides, sand and various meteorological conditions at that locality. Life history studies were conducted at Glacier Point by observing the succession of germlings on denuded transects and by transplanting laboratory cultured germlings into the field. The laminate thallus of P. irregulare sometimes bears both unilocular and plurilocular sporangia at the same time. Previously, only unilocular sporangia were reported in this plant. Zoospores from the unilocular sporangia (unispores) and plurilocular sporangia (plurispores) develop identicallys, and each is capable of producing a laminate thallus directly or after a succession of filamentous and discoid plethysmothalli. The "direct-type" of development of the zoospores (unispores) from the unilocular sporangium is probably due to a suppression of meiosis in the unilocular sporangium. Morphological and cultural evidence is presented to support this hypothesis, although no cytological evidence was obtained. At Glacier Point, P. irregulare is restricted to sandy areas, and the greatest number of plants occur where large fluctuations of sand occur annually. The plants are regularly buried four to six months per year, and their growth and reproduction is limited to the period when sand is absent. Competition with other plants probably accounts for the occurrence of P. irregulare in sandy areas, since it will grow in rocky areas if other algae are eliminated. The period of maximum growth (February to April) is associated with a corresponding increase in light intensity and water temperature in this area. After April, growth in non-tide pool populations decreases much more rapidly than growth of tide pool populations, because of the increased exposure of plants to desiccation during daylight. A period of decreased growth for tide pool plants occurs in May to June; this decrease probably results from high surface water temperatures, high light intensities, or a combination of both. The morphology of the laminate plants of P. irregulare is extremely variable and the range of variability observed at Glacier Point overlaps that described for P. australe from Callifornia. P. australe is considered to be a growth form of P. irregular, and is therefore a taxonomic synonym of P. irregulare, Distributional evidence also supports this conclusion. The known range of P. irregulare extends from Point Conception, California to Khantaak Island, near Yakutat, Alaska. Temperature is considered to be the primary factor controlling its gross distribution. Nitrate and phosphate deficiency may partially restrict the distribution of P. irregulare south of Point Conceptions, California. The sporadic distribution of P. irregulare on the Pacific Coast is correlated with the presence of sand, and local conditions are most important in determining its regional distribution. Experimental studies show that P. irregulare is well adapted to a sandy habitat, and several features are discussed to explain this adaptation. The laminate plants and germlings of P. irregulare tolerate a wider range in temperature and salinity in culture than that to which they are subjected in nature. However, the laminate plants and germlings of P. irregulare are very sensitive to desiccation; under experimental conditions both tolerate less desiccation than that to which they are subjected under natural conditions.

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