Biogeographic and morphological variation in Late Pleistocene to Holocene globorotalid foraminifera
Biogeographic and morphological variation in Late Pleistocene to Holocene globorotalid foraminifera
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晚更新世至全新世球状有孔虫的生物地理和形态变异
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发表时间：
2007
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通讯作者：
K. Brown
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作者：
K. Brown
浮游有孔虫是海洋的、分泌方解石的原生生物。他们在工业界和学术界都有悠久的研究历史。各个物种表现出独特的生物地理分布和生态耐受性。传统上，物种概念是基于有孔虫测试的总体形态。两个物种的形态越接近，它们的相关性就越近。这导致同一个物种被来自全球不同地点的几位作者命名，而且对于长寿物种，命名的时间间隔也不同。这项工作研究了晚更新世 - 全新世 menardiform globorotalids 的形态变异，并将这种形态变异与不同的生态和环境条件联系起来。为了实现这一目标，我们在有限的时间内对全球 70 个样本点进行了调查，涵盖了一系列不同的环境条件。在可能的情况下，使用了全新世的样本，在无法获得来自 Emiliani Huxleyi acme 区域的绝对年代样本的情况下，给出的上限年龄为 65 – 7 万年。形态变异分析允许鉴定综合形态素和总共六种不同的形态类型（例如 menardi 形态形态类型 α、β、χ 和 η 以及两种 tumid 形态形态类型 e 和 φ）。形态类型显示出具有不同但重叠的生物地理分布。在螺旋高度与轴直径的双变量形态空间中，方程 y = 2.07x –15 将形态素（G. menardii 形态）与形态素（G. tumida 形态）分开。在形态斜线内，方程 y = 3.2x –160 的直线将形态型 α (G. menardii menardii) 与形态型 β (G. menardii cultrata) 分开。形态型 β 被解释为 G. menardii cultrata，并且被认为在年平均海面温度超过 25°C 的环境中占主导地位。形态型 α 被解释为 G. menardii menardii，并且随着海面温度变冷而变得更加占主导地位。在样本中同时存在两种形态的区域，我们将这种情况解释为替代营养深度采用。 G. menardii cultrata 生活在较浅的深度，而 G. menardii menardii 则生活在较深的地方
水柱内。这种解释得到了对墨西哥湾和加勒比地区样品进行的稳定同位素研究的支持，其中两种形态显示出显着不同的同位素信号。 G. menardii cultrata 在形态上具有扁平光滑的测试，很少有二次结壳，而在同位素上它具有浅层栖息地和可能的共生关系。 G. menardii menardii 在形态测量上显示出测试中更大的膨胀和结壳，并且在同位素上它显示出更深和更冷的深度栖息地。两个公认的具有不同同位素特征的形态群中所有个体发育阶段的存在表明，G. menardii 可能有两个生活在加勒比海不同深度的不同亚群。
对取自单个样本的相同大小部分的形态型 α 和 β 的成年形式进行的超微结构研究表明，即使在幼年生长阶段也存在差异。前房大小和生长速度表明α形态具有r选择（快速生长，机会主义）的生活模式。而形态β是k选择（长寿、共生、专家）的生活方式。
形态型 η 被解释为 G. menardii gibberula，这是 G. menardii 组内最高的尖顶形态型，仅在样本集的南端发现。已在西太平洋的样本地点发现了标本，这扩大了其已知的生物地理范围。它还具有所有梅纳迪形式中最高的尖顶，并显示出与最冷的海面温度的相关性。
形态型 χ 仅在印度洋北部发现，被解释为 G. menardii neoflexuosa。它具有最终室的明显弯曲，但随着最终“弯曲”室的移除，形态类型落入形态类型β形态空间，其显示出相似的纹理结构。弯曲的原因尚不清楚，但由于在夏季季风期间发现弯曲的数量增加，因此有人认为这是对盐度降低和地表水浊度增加的反应。
在 morphocline 2 形态类型 e (G. tumida) 中，观察到形态相似的
但质地不同的形态型 φ (G. unculata)。 G. unculata 较小的尺寸和精致的结构表明它是居住在浅层的幼体，而 G. tumida 是居住在较深的、更健壮的成体。然而，同位素研究表明，两种形态类型的栖息地深度不同，当比较来自相同样本地点的同等大小的标本时，较重的结壳 G. tumida 显示出比更光滑、更精致的 G. unculata 更深的信号。 G. unculata 的首次出现尚不清楚，但据信发生在更新世晚期。因此，结果被解释为表明物种内的生态表型变异，而不仅仅是个体发生变异，形态类型 φ 代表浅层居住形态，形态类型 e 代表深层居住形态。本研究中使用的所有标本的二次结壳表明，结壳是有孔虫生存的一种功能，而不是其个体发育或配子发生阶段的指示。
Planktonic foraminifera are marine, calcite secreting protists. They have a long history of study in both industry and academia. Individual species show distinct biogeographical distributions and ecological tolerances. Traditionally species concepts are based on the gross morphology of the foraminiferal test. The closer the morphology of two species, the closer they are related. This has resulted in a single species being named by several authors from differing global locations and also, in long lived species, differing time intervals. This work investigates morphological variation of Late Pleistocene – Holocene menardiform globorotalids, and links this morphological variation to different ecological and environmental conditions. To achieve this 70 global sample sites are investigated covering a range of differing environmental conditions, but within constrained time limits. Where possible samples dated as Holocene have been used, where absolute dating was unavailable samples from about the Emiliani huxleyi acme zone, giving an upper age is given of 65 – 70 thousand years. Analysis of morphological variation allowed identification ofintergrading morphoclines and a total of six distinct morphotypes (e.g. the menardi-form morphotypes α, β, χ and η and the two tumid form morphotypes e and φ). The morphotypes are shown to have distinct though overlapping biogeographic distributions. In the bivariate morphospace of spiral height versus axial diameter the equation y = 2.07x –15 separates morphocline(G. menardii morphologies) from morphocline(G. tumida morphologies). Within morphoclinethe line with equation y = 3.2x –160 separates morphotypes α (G. menardii menardii) from morphotype β (G. menardii cultrata). Morphotype β is interpreted as G. menardii cultrata and is seen to dominate environments with mean annual sea surface temperatures over 25°C. Morphotype α is interpreted as G. menardii menardii and becomes more dominant as sea surface temperatures become cooler. In areas where both morphologies are present in a sample we interpreted the situations a vicariant trophic depth adoption. G. menardii cultrata lives at shallow depths, while G. menardii menardii occurs deeper
within the water column. This interpretation is supported by stable isotope studies carried out on samples from the Gulf of Mexico and Caribbean region where the two morphologies show significantly different isotopic signals. G. menardii cultrata morphologically has a flattened smooth test with little secondary encrusting, while isotopically it has a shallow depth habitat and possible symbiotic relationship. G. menardii menardii morphometrically shows greater inflation and encrusting of the test and isotopically it shows a deeper and colder depth habitat. The presence of all ontogenetic stages within the two recognized morphological groups with distinct isotopic signatures, suggests that G. menardii may have two distinct subpopulations living at different depths within the Caribbean.
Ultrastructural studies on adult forms of morphotypes α and β from the same size fractions taken from a single sample, show that differences are present even in juvenile growth stages. Prolocular size and rate of growth suggest that morphotype α has a r-selected (rapid growth, opportunistic) mode of life. While morphotype β is k-selected (longer living, symbiont bearing, specialist) mode of life.
Morphotype η is interpreted as G. menardii gibberula this is the highest spired morphotype within the G. menardii group and is found only at the southerly extent of the sample set. Specimens have been identified in sample sites from the Western Pacific, which extends its known biogeographic range. It also has the highest spire of all the menardii forms and shows a correlation to the coldest sea surface temperatures.
Morphotype χ is only found in the northern part of the Indian Ocean and is interpreted as G. menardii neoflexuosa. It has a distinct flexure of the final chamber, but with removal of the final “flexed” chamber, the morphotype falls within morphotype β morphospace, to which is shows similar textural structure. The cause of the flexing is not clear, but as it is found in increased numbers during the summer monsoon, it has been suggested that it is a response to lowered salinity and an increase in turbidity of the surface waters.
Within morphocline 2 morphotype e (G. tumida) is seen to intergrade the morphologically similar
but texturally different morphotype φ (G. ungulata). The diminutive size and delicate structure of G. ungulata is suggestive of it being the shallow dwelling juvenile form with being G. tumida the deeper dwelling more robust adult form. However, isotopic studies show differing depth habitats for the two morphotypes, with the heavier encrusted G. tumida showing a constantly deeper signal than the smoother more delicate form of G. ungulata, when comparing size equivalent specimens from the same sample sites. The first occurrence of G. ungulata is unclear but is believed to occur during the late Pleistocene. Because of this the results are interpreted as indicating ecophenotypic variation within a species, rather than just ontogenetic variation, with morphotype φ representing the shallow dwelling morphology, and morphotype e the deeper dwelling morphology. Secondary encrusting of all specimens used in this present study indicates that encrusting is a function of which the foraminifera lived an not an indication of its stage of ontogeny or gametogenesis.