Fecundity, spore recruitment and size in Gelidium sesquipedale (Gelidiales, Rhodophyta)

Abstract

Gelidium sesquipedale fecundity was quantified by counting tetrasporangial sori and cystocarps per meter squared and by estimating the number of spores contained inside them . These were obtained by regression on a size metric of reproductive structures . Tetrasporangial sori length and cystocarp thickness were the best estimators of spore number. To assess spore recruitment, 12 pottery tiles were fixed to the bottom, and the appearance of small fronds was monitored.No clear seasonal pattern of reproduction was found . Tetraspore production peaked in March 1990 with 10.4 x 10 6 spores m -2 , whereas the carpospore peak was lower, 4 .9 x 10 5 spores m -2 in July 1989 . Recruitment followed tetraspore peaks . The probability of a G. sesquipedale tetraspore making the transition to a recruit was 4 .7 x 10 -5 . Frond length was significantly related to tetrasporangial sori number, while cystocarp number was only related to frond branching order. Minimum size for reproduction was 6 .9 cm for gametophytes and 5 .4 cm for tetrasporophytes; very rarely were cystocarpic fronds smaller than 9 cm, while tetrasporic fronds were often longer than 15 cm . Cystocarpic fronds were significantly shorter and had more branches than tetrasporic fronds .

IS THERE AN ECOPHYSIOLOGICAL EXPLANATION FOR THE GAMETOPHYTE–TETRASPOROPHYTE RATIO IN GELIDIUM SESQUIPEDALE (RHODOPHYTA)?1

“…The reproductive subpopulation of G. sesquipedale of São Rafael, southern Portugal, was dominated by tetrasporic fronds, as described elsewhere for other populations of the same species (Gorostiaga 1990, Salinas 1991, Santos and Duarte 1996 or of the same genus (Montalva and Santelices 1981, Carter 1985, Akatsuka 1986, Macler and West 1987, Santelices 1990, Melo and Neushul 1993, Sosa et al 1993). The G:T ratio of São Rafael reproductive subpopulation varied between 1:7 and 1:10 in the fall, the most fertile season, when up to 61% of the fronds were reproductive.…”

“…Other studies have been reported on the species carposporophyte:tetrasporophyte ratio because inconspicuous spermatangial sori of male fronds were not reported. In a commercially harvested population of the central western coast of Portugal, Santos and Duarte (1996) observed a carposporophyte:tetrasporophyte ratio of 1:10, higher than in São Rafael, where it was up to 1:20 (because half of the gametophytes were carposporophytes). Other commercially harvested populations of G. sesquipedale in northern Spain had carposporophyte:tetrasporophyte ratios of 1:46 (Salinas 1991) and 1:9 (Gorostiaga 1990), indicating no clear effects of harvest on the phase structure of this species.…”

“…Sexual reproduction and recruitment from spores has also been observed in this species (Gorostiaga 1990, Santos andDuarte 1996). The populations of G. sesquipedale show a predominance of tetrasporophytes (Dixon 1959, Gorostiaga 1990, Salinas 1991, Santos and Duarte 1996, which is a common feature of this genus (Montalva and Santelices 1981, Carter 1985, Akatsuka 1986, Macler and West 1987, Santelices 1990, Melo and Neushul 1993, Sosa et al 1993, independent of the latitude and the environmental conditions. The deviation of the G:T ratio toward the tetrasporophytes appears to be characteristic of the evolutionary branch of Gelidiales, as opposed to the Gigartinales, whose populations are often dominated by gametophytes (Dyck et al 1985, Hannach and Santelices 1985, May 1986, Littler et al 1987, DeWreede and Green 1990.…”

“…The vegetative growth of erect fronds, both from the prostrate axis and from the re-attached frond fragments, has been suggested as the major process of population recovery from disturbances such as storms and commercial harvest (Santos 1994(Santos , 1995. Sexual reproduction and recruitment from spores has also been observed in this species (Gorostiaga 1990, Santos andDuarte 1996). The populations of G. sesquipedale show a predominance of tetrasporophytes (Dixon 1959, Gorostiaga 1990, Salinas 1991, Santos and Duarte 1996, which is a common feature of this genus (Montalva and Santelices 1981, Carter 1985, Akatsuka 1986, Macler and West 1987, Santelices 1990, Melo and Neushul 1993, Sosa et al 1993, independent of the latitude and the environmental conditions.…”

“…The relative abundance of isomorphic life-history phases is well documented in the Rhodophyta. Some studies reported the dominance of gametophytes (Dyck et al 1985, Hannach and Santelices 1985, May 1986, Littler et al 1987, others found a dominance of tetrasporophytes (Hansen and Doyle 1976, Norall et al 1981, Akatsuka 1986, DeWreede and Green 1990, Santos and Duarte 1996, while still others observed similar numbers of gametophytes and tetrasporophytes (Hay andNorris 1984, Destombe et al 1989). The deviation from a 1:1 gametophyte-tetrasporophyte ratio (G:T) in biphasic life cycles has been used in past studies to infer an evolutionary advantage of diploids.…”