Studies on the Reproductive Ecology of Sargassum Thunbergii

【摘要】：Three life-history stages including reproductive allocation (RA), gametogenesis andrecruitment influence the population structure of Sargassum thunbergii. Based on thislife-history cycle, the reproductive ecology of the species was researched. Annual reproduction initiated in mid-June and peaked in mid-July (90% fertilethalli and 75% RA mean). Both RA and percentage of fertile thalli exhibitedsignificant temporal variations during reproduction. Sterile thalli were only observedin small hierarchies at peak reproduction and mean values of RA showed asignificantly hierarchical variation, suggesting that the size of thalli played importantroles in reproduction and both reproduction and RA were size-dependent. Numerousreceptacles were produced along the lateral branches during the reproductive period.A distinct seasonal pattern was observed wherein the presence of lateral branches wasfollowed by the onset of reproduction. RA was positively correlated with the numberof lateral branches, as well as the total length of lateral branches, suggesting that theappearance of lateral branches is of importance for reproduction. In addition, massiveshedding of modules occurred after the reproductive season. Moreover, theprobability of shedding was evidently higher for fertile than for sterile modules,because all surviving thalli were sterile and short during July-August. So, a trade-offbetween reproduction and survival may exist at individual levels in S. thunbergii. Much attentions have been paid to the gamete release of fucoid algae, but theinteractions between the environmental cues and their importance order for thegamete release remain little known. In this study, a 2~(8-4) fractional factorial design(ResolutionⅣ) was used to identify the main effects and interactions ofenvironmental factors of S. thunbergii. Of the factors tested, water motion,temperature, photoperiod and salinity were found to be the most important factorsaffecting egg release. Moreover, the interactions between any two factors were allsignificant. Based on the size of effect estimated by ANOVA, the effects whichcontribute to the release of eggs, in order of importance, are: salinity, watermotion~*photoperiod (possibly temperature~* salinity), water motion~*salinity (possiblytemperature~* photoperiod), temperature, water motion~*temperature (photoperiod~* salinity), water motion and photoperiod. The optimum conditions of 25℃, 30 psusalinity, slack seawater and 12 h irradiance that achieved 100% release rate may bethe "window of opportunity" of egg release in S. thunbergii. These findings giveinsight into the egg release in S. thunbergii under laboratory conditions. The development of ovum and zygote, and the growth of juvenile sporophytewere observed on the laboratory conditions. Eggs adhered to the receptacle followingrelease from their conceptacles. The zygote detached from receptacle 20 happroximately after fertilization. After taking transverse division twice, one terminalcell developed into rhizoid and the two others developed into the germling. Thedevelopment of lateral branch started when juveniles grew up to 2mm in length. Commercial farming of S. thunbergii is being developed in China. Asynchronousdischarge of gametes and growth of epiphytic algae are the two main constraints forthe seedling production of this species. In this study, 40% and 65% reproductiveoutput for nature and farmed population were respectively recorded during peakreproduction. Furthermore, lower degree of temporal dispersion of germlingoccurrences for farmed population in related to nature ones was found, althoughgermling occurrence exhibited significant temporal variations for both populations.These results indicated that using farmed population as parental plants to collectgermlings is an effective strategy in achieving mass discharge of gametes. At the peakperiod of discharge, as more as 3.2×10~5 germlings per kg wet weight plants from thefarmed population was obtained, which was significantly more than that of natureones and could meet the demand of large scale seedling production. Anotherconstraint (i.e. growth of epiphytic algae) interfering the seedling cultivation wasattempted to eliminate by a strategy of jet-washing and high density seeding. Theattachment capacity of germlings was found to be significantly increased with theduration of attachment. A detachment of less than 10% was observed on collectorsthat were jet-washed using an intensity of 1 kg cm~(-1) by 48 h post-attachment, whichenables the jet-washing to be early conducted, thus preventing the growth of epiphyticalgae from early life stage. High density had adverse effects on length mean, sizeequality and occurrence of branches of germlings. However, 30-50 individuals cm~(-2)were thought to be usable in the seedling production of S. thunbergii, because theyunderwent less density effects, and withstood the impact of continual jet washing. Noevident epiphytic algae growing on collectors or adhering to host plants were observed in the treatments which combined jet-washing and high seeding density,suggesting that both jet-washing and high density significantly reduced epiphyticalgae. Most seedlings in a length of＞0.5 cm were achieved after one month of tankculture under less controlled conditions. The effects of environmental factors including disturbance, temperature, wavemotion, epiphytes and grazer (i.e. Chlorostoma rustica) on biomass were conducted,using step-wise regression analysis. The results showed that S. thunbergii distributedbetween the low-tidal zone and mid-tidal zone at 50～125 cm tidal level. Both biomassand mean thallus length of S. thunbergii exhibited a statistically significant temporalvariation, with a similar unimodal trajectory. The biomass and mean thallus lengthreached a highest value in July and decreased to a lowest value in September. Thegrowth pattern of S. thunbergii may be divided into four phases: inactivity period(before March), growth period (from early-April to mid-June), reproductive period(from mid-June to late-July) and senescence period (from late-July to September).Small plants were recorded throughout the year and showed a seasonal variation,indicating that the vegetative reproduction of S. thunbergii occurred year round. AfterJuly, the number of smallest plants increased rapidly due to the degeneration of largeplants, recruitment of new plants and vegetative reproduction. During therapid-growth period, the length distribution was not dominated by a few size classes,and frequencies were more or less evenly distributed in most size classes. Step-wiseregression analysis demonstrated that water temperature, wave motion anddisturbance significantly influenced the increase of biomass; however, the effect ofdesiccation was not significant. That is considered to be the results that S. thunbergiiclumped during sampling period, because fronds covered each other at low-tide andwater lose was efficiently prevented. On balance, the population investigated inpresent study is significantly different from others growing on the coast of Japan andKorea, which results from the differences of environmental factors.