- 學位論文
綠島(台灣)星野黑皮海綿(Terpios hoshinota)之有性生殖研究及野外族群幼生排放之觀察
Sexual reproduction of the black sponge, Terpios hoshinota, and in situ observation of its larval release in Green Island, Taiwan
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摘要
在2006年台灣珊瑚總體檢時，發現了以往未曾被注意到的現象：在綠島及蘭嶼，一種黑色的海綿覆蓋了多種石珊瑚，鑑定結果發現，是一種有藍菌共生的海綿Terpios hoshinota（星野黑皮海綿），其擁有生長快速的特性，且能覆蓋跨種類的石珊瑚，嚴重降低當地石珊瑚的覆蓋率及生物多樣性。雖然早在1973年就有星野黑皮海綿在關島大爆發的報導及初步的研究，但直到現在，對於其族群量遽增的原因及其族群動態仍不了解。此篇研究針對在綠島的星野黑皮海綿族群，以標記特定群體(colony)進行密集採樣(2011年6月16日到8月17日，2-3天採樣一次)，再以組織切片染色法探討其配子發育及胚胎發育；並每2-3天觀察紀錄25個群體幼生排放的情況。 組織切片的結果顯示：(1)星野黑皮海綿的精發育時間很短，小於8天，精子的釋放有半月周期存在，時間與滿月及新月的時間相同，(2)卵(< 40 μm)及早期胚胎在各個時間點都有被觀察到，顯示卵持續被製造出來，(3)成熟胚胎(length: ~400 µm, width 285 µm)出現在滿月及新月附近，且在組織切片中發現成熟胚胎的時間點與野外觀察到的幼生排放時間點相符合；可能是受到精子排放的半月週期影響，導致胚胎成熟及排放的時間近似半月週期。 從組織切片的結果，推測星野黑皮海綿的胚胎發育過程可分為三個階段：(1)在一個由扁平細胞(pinacocytes)包圍的腔室中，由成體提供的共生藍菌及海綿細胞逐漸累積；早期胚胎並非位於此腔室中，而是在此腔室之外進行卵裂，(2)卵裂進行到某個階段後，胚胎進入扁平細胞包圍的腔室中，繼續分裂增生，(3) 最後胚胎細胞包圍在整團細胞(大部分是藍菌)的最外層，完成胚胎發育。此種胚胎發育過程相當獨特，未曾有研究指出類似的胚胎發育過程，將來可對星野黑皮海綿的胚胎做更進一步的研究，提供更多關於海綿胚胎發育的資訊。在成體海綿體內共生的藍菌也出現在胚胎的中央空腔內及幼生體內，顯示星野黑皮海綿在胚胎發育過程中獲得和親代相同的共生菌。 星野黑皮海綿的幼生排放後會附著於海綿表面，在野外可直接用肉眼觀察，經過兩個月的野外觀察(每兩到三天觀察一次)，發現在六月中、七月初、七月中及八月中有大規模的幼生釋放，此四次幼生釋放事件為整個公館港的族群同時進行，一次釋放持續約八天，中間日為釋放的高峰；每株海綿排放幼生的數量及面積差異很大，但同一個群體的幼生是同時釋放，亦即幼生排放事件在單一群體內及整個族群都是同時發生；其中七月十五日在25株海綿中有24株排放幼生(96%)。
並列摘要
Outbreaks of the black cyanobacteriosponge, Terpios hoshinota have been reported in Green Island and Orchid Island, Taiwan since 2006. Because T. hoshinota grows fast and has the ability to kill and overgrow scleractinian corals, the outbreak of T. hoshinota results in serious decrease of coral coverage and biodiversity. Although the outbreak of T. hoshinota has also been reported in Guam and Ryukyu Archipelago (Japan) since 1970s, little is known about its life history and ecology of T. hoshinota, which are essential to understand its outbreak mechanisms and population dynamics after the outbreak. Accordingly, in this study, some reproductive information of T. hoshinota (gametogenesis, embryogenesis and larval release pattern) was examined in Green Island, Taiwan. Intensive sampling and field observation of 2-3 day intervals were performed on 15 tagged colonies as well as haphazardly chosen colonies for 2 months (June 16-August 17) in 2011. Histological observations revealed some patterns in sexual reproduction of T. hoshinota: (1) spermatogenesis is very short (< 8 days); sperm was released on a semi-lunar cycle around the full moon and new moon, (2) oocytes (< 40 μm) and early embryos were always presented, and (3) mature embryos (length: ~400 µm, width 285 µm) were presented only around the full moon and new moon. The semi-lunar cycle of embryo maturation is also coincided well with larval release timing observed in the field (around the full moon and new moon). One larval release event continued for ~8 days. The semi-lunar cycle of embryo maturation and larval release may be determined by the semi-lunar cycle of sperm-release. Embryogenesis of T. hoshinota was unique and never been reported: (1) embryo at morula stage developed outside the embryo-nursing chamber, which consisted of a single layer of pinacocytes and contained numerous cyanobacteria and maternal sponge cells transferred gradually from adult, (2) and then embryo penetrated into the nursing chamber and underwent blastulation, (3) finally, embryonic cells migrated to the periphery of the nursing chamber, packed all of the cyanobacteria and maternal sponge cells inside, and formed a compact single outer layer to become a mature embryo. Symbiotic cyanobacteria were not found in oocyte, but numerously in central cavity of mature embryo and larvae, suggesting vertical transmission of the symbionts from mother sponges to larvae during embryogenesis.
參考文獻
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