以電腦斷層掃描影像測量浮游性有孔蟲之形態特徵

Abstract

Globigerinoidesella fistulosa 是一已滅絕的熱帶浮游性有孔蟲物種，大約在 330 萬到 170 萬 年前生存於全球熱帶海域。其滅絕事件與現代西太平洋暖池(WPWP)的形成和初步擴張 之發生時間重合且可能有潛在的連結。了解其古生態學有助於檢驗氣候誘導的假說，並進 一步增進我們對古氣候與生物圈間相互作用的認識。迄今為止，該物種的生態位仍存在許 多不確定性，如其生物量與光合共生藻之互動關係。在本研究中，我們採用了三維形態計 量方法嘗試重建了該物種的生態位，包括使用 μ-CT 重建進行體積和表面分析。基於前人 研究對其祖先和親緣物種 Trilobatus sacculifer plexus 之生態特性已有較多的著墨，我們將 G. fistulosa 和其親緣物種 T. sacculifer plexus 進行比較，以更好地理解前者。3D 立體型態 分析結果表明，G. fistulosa 的生物量和殼體表面積較 T. sacculifer plexus 更大，這可能歸 因於混合層較深處相對豐富的營養供應，並因此形成較強的共生光合作用。此外，表面積 與總體積(S-V)之比值提供了檢驗兩種物種在個體發育過程中的表面積變化的機會。與 T. sacculifer plexus 相比，G. fistulosa 通常在相似的體型下具有較高的 S-V 比值，且不同個 體間的 S-V 比值差異也更大。對於 G. fistulosa，較高的 S-V 比值(可能是由於較扁平、不 規則的殼體型態和指狀突起所致，此假設需要更多理論研究確認)增加了殼體表面容納更 多刺的空間。此型態特徵可能會抵消在假設上殼體較高的殼體密度和總密度所帶來的沉降 力。我們對 G. fistulosa 生態學的解釋基於先前對其殼體地球化學的研究，其表明根據 Mg/Ca 比率和 δ18O 數據，G. fistulosa 的造殼深度可能比其祖先 T. sacculifer plexus 更深。

Globigerinoidesella fistulosa is an extinct planktonic foraminifera species occurred around 3.3- 1.7 million years ago (Ma) in global tropical ocean. Its extinction event coincided and may be triggered by the formation and expansion of the modern Western Pacific Warm Pool (WPWP). Understanding its paleoecology would help testing the climate-induced hypothesis and further our knowledge to the climate-ecology interaction. So far, much uncertainty still exists about its ecological niche, such as biomass and photosymbiotic ecology. In this study, we performed 3D morphometric method to reconstruct the species’ ecological niche, including volumetric and surface analysis using μ-CT reconstructions. Based on the previous studies on the ecology of modern Trilobatus sacculifer, a comparison between G. fistulosa and its relative T. sacculifer can be made here to better understand the former. The results of 3D analysis indicate larger biomass and surface area of G. fistulosa compared to T. sacculifer, which could be attributed to relatively abundant nutrient supply and enhanced symbiotic photosynthesis in the deeper part of the water column. Moreover, the surface area to total volume (S-V) ratio provides chance to inspect the variation of surface area through ontogeny of the two species. Compared to T. sacculifer, G. fistulosa generally registers higher S-V ratio under similar body size, as well as higher variation. For G. fistulosa, the higher surface area per total volume (might resulted from the flat, irregular gross morphology and protuberance. More theoretical studies would be required to confirm the hypothesis) might bring about a larger potential for spines across the surface space. The morphological traits would likely counteract the settling force provided by the hypothesized increase in shell test and overall density. Our interpretations toward the ecology of G. fistulosa is based on the previous study of shell geochemistry, as it implies the calcification depth of G. fistulosa might been deeper in depth than its ancestor T. sacculifer based on the Mg/Ca ratio and δ18O data.