Studies on the reproductive ecology of Sargassum thunbergii.pdf

1、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 sp

2、ecies 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 rep

3、roduction． 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 reprodu

4、ction 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

5、 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

6、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 surviv

7、ing 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 al

8、gae，but theinteractions between the environmental cues and their importance order for thegamete release remain little known． In this study，a 28-4 fractional factorial design(Resolution Ⅳ) was used to identify the main ef

9、fects 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 inte

10、ractions 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 (possib

11、ly temperature*salinity)，water motion*salinity (possiblytemperature*photoperiod)，temperature，water motion*temperature (photoperiod* salinity)，water motion and photoperiod． The optimum conditions of 25℃，30 psusalinity，sla

12、ck 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 conditi

13、ons． 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 fr

14、om 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

15、 when juveniles grew up to 2ram in length． Commercial farming of S． thunbergii is being developed in China． Asynchronousdischarge of gametes and growth of epiphytie algae are the two main constraints forthe seedling

16、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 germlingocc

17、urrences 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 parent

18、al plants to collectgermlings is an effective strategy in achieving mass discharge of gametes． At the peakperiod of discharge，as more as 3.2×105 germlings per kg wet weight plants from thefarmed population was obtained，w

19、hich 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 el

20、iminate 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 col

21、lectorsthat 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

22、adverse effects on length mean，sizeequality and occurrence of branches of germlings． However，30-50 individuals cm-2were thought to be usable in the seedling production of S． thunbergii，because theyunderwent less density

23、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，su

24、ggesting 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

25、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 d

26、istributedbetween 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 traje

27、ctory． 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)，

28、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 variat

29、ion，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 vegetat

30、ive 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 demo

31、nstrated 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． thunbergiicl

32、umped 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