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標題: | 台灣北部暫時性池塘大型鰓足類的分佈與孵化策略 Large branchiopod’s distribution and hatching strategies in a freshwater temporary pool in northern Taiwan |
作者: | Chun-Chieh Wang 王俊傑 |
指導教授: | 周蓮香 |
關鍵字: | 鵠沼枝額蟲,真湖蚌蟲,貓眼蚌蟲,積水相,分佈,休眠卵庫,風險分散策略, bet hedging,Branchinella kugenumaensis,distribution,egg bank,Eulimnadia braueriana,hydroregime,Lynceus biformis, |
出版年 : | 2016 |
學位: | 博士 |
摘要: | 暫時性水池的積水相變動隨機且不穩定,時常在積水與乾涸之間頻繁交替,棲息於其內的大型鰓足類已演化出特殊的生活史,產下能忍受乾旱等極端環境的休眠卵以適應暫時性水池。休眠卵非同步孵化的特性,使得部份未孵化的休眠卵會逐漸累積在底質中形成休眠卵庫,持續休眠直到下一個積水期的到來。能否維持休眠卵庫是大型鰓足類在暫時性水池存續的關鍵。在空間上,牠們必須將休眠卵產在最理想的區域─水池邊緣,以確保休眠卵能在池水水量較充沛、積水期較長時孵化,而在時間上,休眠卵除了得在一個積水期內的適當時間孵化以避免生殖失敗,也應該適當分配孵化量至不同積水期,減少無效孵化所造成的休眠卵庫損失。因此,本論文分別從空間與時間面向,瞭解鵠沼枝額蟲、真湖蚌蟲與貓眼蚌蟲等三種共域於向天池的大型鰓足類,是否達成理想的休眠卵邊緣分佈與是否表現出理想的孵化率,以探討各物種的生存策略。
論文第一章關注大型鰓足類成體的空間分佈模式。涵蓋四個積水期的逐日穿越線垂直拖網採樣結果顯示,向天池的三種大型鰓足類中,只有鵠沼枝額蟲表現出穩定聚集於水域邊緣的現象,然而其在向天池的分佈熱區卻與貓眼蚌蟲類似,都集中在中心三處最深的小凹洞區域;真湖蚌蟲的熱區則相對廣布,但是並不均勻,較偏向低植被覆蓋的步道區域。此分佈模式可能與各物種的性成熟時間和積水範圍的日漸縮減有關,雖然鵠沼枝額蟲會主動聚集在水域邊緣,但是最後卻與更晚成熟的貓眼蚌蟲同樣侷限在向天池三個水坑中心附近的區域,而較早成熟的真湖蚌蟲在棲地利用上限制較少,因而能分佈於更廣闊的範圍,然而其不均勻的空間分佈現象可能意味著此物種有特殊的微棲地偏好。 延續空間分佈的議題,論文第二章轉向休眠卵,估算大型鰓足類的休眠卵庫數量,以及釐清牠們在向天池內的分佈狀況。系統性採集向天池表層底質並經過辨識、計數休眠卵數後,發現貓眼蚌蟲是休眠卵庫數量上最優勢的物種,而鵠沼枝額蟲的休眠卵庫數量接近批次生產力,表示休眠卵庫可能會達到數量上的動態平衡,而非無限制的累積。在分佈方面,鵠沼枝額蟲的休眠卵集中在向天池的中心三處水坑,真湖蚌蟲偏向於西北側,而貓眼蚌蟲則廣泛分佈,特別是池子的南側直到更邊緣的區域。這般種間各異的模式可能是由許多不同的機制導致。 一併檢視成體與休眠卵庫的分佈,發現鵠沼枝額蟲在兩者熱區的分佈模式上互相吻合,但對於真湖蚌蟲與貓眼蚌蟲而言卻並非如此,意指鵠沼枝額蟲休眠卵庫的水平分佈主要由成體的分佈模式決定,然而真湖蚌蟲與貓眼蚌蟲的休眠卵還受到休眠卵的再散佈影響。由於較晚成熟與成熟時受限的積水範圍,鵠沼枝額蟲沒有辦法將大多數休眠卵產在理想的向天池邊緣區域,但是更晚熟的貓眼蚌蟲卻可能因為休眠卵的再散佈,而得以重新傳播休眠卵到池子邊緣。整體而言,在先天發育速率與積水相造成的空間利用限制下,大型鰓足類仍舊可能間接受益於其它機制,以達成理想的休眠卵分佈模式。 就時間面向探討,論文第三章針對鵠沼枝額蟲與真湖蚌蟲的孵化物候,藉由實驗室孵化實驗,每週重複浸泡、乾燥休眠卵,並紀錄積水期內每天的孵化數量,結果顯示這兩種的孵化時間均集中在一個積水期剛開始的二到五天內,以確保孵化的個體能盡早成熟。此外,主要的孵化量都發生在早先的二到三次積水期內,而非均勻分散至往後更多的積水期;如果每批孵化的個體有較高的機率能順利成長並生殖,孵化策略上便無需太過保守,可以趁早孵化。向天池積水相的特色─與豪大雨同步發生的積水期,使得積水時通常擁有豐富的水量且能維持足夠時間,因此降低了大型鰓足類在積水期內孵化卻未能生殖的風險。 接續孵化物候,為了探究大型鰓足類的孵化策略,論文第四章試圖釐清鵠沼枝額蟲與真湖蚌蟲是否因應在向天池面臨到的長期生殖成功率變化,而表現出理想的孵化率。分別在兩種孵化的風險情境─孵化率是以每次積水期、或是以每個年度為評估基準,驗證孵化率與生殖成功率是否一致(柯恩氏假說)。結果發現鵠沼枝額蟲的孵化率在兩種情境中都與生殖成功率有顯著差異,而真湖蚌蟲的孵化率則在後者情境中與生殖成功率吻合,意味著真湖蚌蟲的孵化策略趨於保守,以年度內的數次積水期為單位進行賭注,減少了族群生存適度在積水期間的變動幅度。鵠沼枝額蟲的孵化率較理想狀況為低,可能是缺乏理想化孵化率的選汰壓力,亦有可能肇因於先天生理限制而無法表現出理想孵化率。 綜觀這三種共域的大型鰓足類,研究較多的鵠沼枝額蟲與真湖蚌蟲可能分別依賴生命週期的不同階段以生存於向天池。鵠沼枝額蟲活動階段的特色─較大的族群量與機會性的彈性生長模式等,可能讓鵠沼枝額蟲擁有較高的生殖力,以抵銷偏低的孵化率對維持休眠卵庫的影響。其它兩種則明顯不同,真湖蚌蟲的休眠階段除了有較高的孵化率,孵化時的保守性風險分散策略可減少生殖失敗造成的休眠卵庫損失,避免族群生存適度的大幅度變動,而目前貓眼蚌蟲的生活史與孵化物候尚待孵化養殖技術突破才能進一步釐清。 Temporary pools are characterized by stochastic hydroregimes, which fluctuate frequently betweeen wet and dry periods. Large branchiopods have adapted to these conditions via specific life history traits including the production of dormant eggs which accumulate in the substrate as an egg bank, remaining viable until the next hydroperiod. Maintaining the egg bank is key to long term persistence. Spatially, eggs should be placed in optimal regions to ensure hatching when there is sufficient water to complete the life cycle. Temporally, they should hatch at the optimal point within a hydroperiod to prevent reproductive failure, and in optimal fractions across hydroperiods to reduce egg bank depletion. I studied the large branchiopods in Siangtian Pond, i.e. Branchinella kugenumaensis, Eulimnadia braueriana, and Lynceus biformis. I explored their optimal spatial and temporal egg and adult distributions, optimal hatching rates, and discuss their life strategies. The first chapter focuses on distributional patterns of free swimming adults. Daily transect line surveys with vertical townet samplings across four hydroperiods showed that only B. kugenumaensis consistently aggregated at the edge, while overall density hotspots for both B. kugenumaensis and L. biformis were concentrated centrally in the three deepest places. Even though B. kugenumaensis aggregated at the edge of the inundation area, generally late maturation limited the available region for it and L. biformis near the center. In contrast, E. braueriana hotspots were widespread but skewed towards less vegetated places. Early maturing E. braueriana suffered less restriction on habitat utilization and was able to distribute widely, although the skewing implies special microhabitat preference. Continuing in spatial aspects, the second chapter estimated egg bank sizes and clarified structure. Identifying and counting eggs in the superficial substrate of core areas revealed that L. biformis was dominant numerically. Egg bank size for B. kugenumaensis was close to cohort fecundity, meaning that egg bank would balance around a certain number rather than accumulate unlimitedly. Regarding spatial structure, B. kugenumaensis eggs were distributed in the central three deepest places, inclined to the northwest in E. braueriana, and spread widely towards the southern margin in L. biformis. Species specific patterns can be realized from multiple mechanisms. Taking the above two chapters together, egg bank spatial patterns were coincident with adult hotspots in B. kugenumaensis, but not in E. braueriana or L. biformis. This indicates that horizontal structure for B. kugenumaensis eggs was mainly determined by adult distribution, while it was also affected by factors such as egg redispersal in E. braueriana and L. biformis. The peripherally aggregated B. kugenumaensis failed to distribute eggs along the basin margin due to late maturation and accordingly restricted inundation areas near the three deepest places. However, redispersal may work and redistribute eggs to the basin edge for the late maturing L. biformis, providing opportunity in spite of developmental limitations. Regarding temporal aspects, the third chapter examined the hatching phenologies of B. kugenumaensis and E. braueriana. Laboratory experiments through repetitive inundations and dehydrations at weekly intervals demonstrated that hatching of both species concentrated in the second through fifth day of a hydroperiod, which could assure early maturation. Additionally, the majority of hatching fractions occurred in the first two to three inundations rather than spread across more inundations. Since the hydroregime of Siangtian Pond, i.e. synchronization between large scale precipitations and hydroperiods, can ensure abundant water and longer hydroperiods, the risk of abortive hatching could be low and thus large branchiopods have evolved an early hatching time. As regards hatching strategy, the fourth chapter tested if B. kugenumaensis and E. braueriana showed optimal hatching following the variation of successful reproduction (Cohen’s hypothesis) under two scenarios wherein a species hatches according to a sporadic hydroperiod or a year with several hydroperiods. I compared hatching rate with evaluated successful reproduction rates based on long term life history and climate data. The two rates were different for B. kugenumaensis in both scenarios but were identical for E. braueriana in the latter scenario, suggesting that E. braueriana bet hatching on yearly cycles. Eulimnadia braueriana adopted a more conservative hatching strategy, further lowering fitness variation. The apparently unfavorable hatching rate in B. kugenumaensis may result from weak selection for optimization or physiological limitations. As a whole, B. kugenumaensis and E. braueriana may rely on different stages of their life cycle to survive in Siangtian Pond. Characters of free swimming individuals such as abundant population sizes and flexible growth patterns could lead B. kugenumaensis to be more fecund and balance hatching performance inferiority. On the other hand, high hatching rate accompanying conservative bet hedging hatching in E. braueriana eggs help to reduce the depletion from abortive hatchings and avoid fitness extremes. Breakthroughs in triggering L. biformis to hatch under laboratory conditions will contribute to the understanding of its life history and hatching phenology in the future. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/4019 |
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