Morphology of Cycladophora davisiana davisiana from the Atlantic Ocean

In recent years, temporal fluctuations in the abundance of C. d. davisiana have been used frequently as a highresolution stratigraphic and paleoenvironmental tool. The modern ecology and morphologic variation (temporal and geographic) of this radiolarian species is evaluated to ascertain its potential stratigraphic and paleoenvironmental significance.
Statistics were obtained on the width and height of all C. d. davisiana segments from Pleistocene populations of differing ages from the Northern Hemisphere (Labrador Sea and Iceland-Faeroe Ridge) and Southern Hemisphere (Namibian shelf and Meteor Rise). Results reveal that segment height variations between and within populations are more conservative than segment width. The mean sizes of the thorax and first abdominal segment have distinguishable differences between C. d. davisiana found in the North and South Atlantic. All populations have no significant difference in first abdominal segment width, however, mean heights of this segment differ greatly between populations of the North and South Atlantic. Second abdominal segment sizes show no clear population grouping. Size differences in post-cephalic segment size of these populations would appear to be related to some isolation of gene pools and possibly unknown paleoenvironmental factors. Temporal changes in the postcephalic size of C. d. davisiana may be used to: (1) identify temporally equivalent peaks in abundance of the species in a given region, (2) possibly evaluate the degree of mixing of water'masses between regions, and (3) trace the initial spread of the species from its area of origin.
Cleve's 1887 plankton samples, between Greenland and Spitzsbergen, were studied and used in conjunction with other data to make the following conclusions on the modern ecology of C. d. davisiana in the Arctic and Greenland-Norwegian Seas. (1) It is presently absent in surface water plankton samples, (2) it currently lives at depths below 500 m, where it is rare, (3) it does not live in the upper 200 m under Arctic ice but is rare at greater depths, (4) it is absent in the upper 200 m near permanent Greenland Sea ice where normal oceanic salinity prevails, and (5) it is most common in deep marginal fjord environments which may serve as a refuge for the species during interglacial periods.
In the Atlantic Ocean, the abundance of C. d. davisiana does not exceed 1% of the assemblage between the Subtropical Convergence of each hemisphere. In the Norwegian and Labrador Seas the species may occasionally be in the range of 1-5% of the modern radiolarian assemblage and never more than 5% in the southern high latitudes. Apparently only in the modern Sea of Okhotsk, does the species presently occur in high abundance. We concur with Morley and Hays (1983) that increased abundances are likely caused by the development of a strong low-salinity surface layer associated with seasonal sea ice melting and a strong temperature minimum above warmer and higher salinity intermediate waters. Similar conditions were frequent during the Pleistocene in the high latitudes and its modern scarcity outside the Sea of Okhotsk must be related to the absence of the presently unique conditions in the latter region.

近年来，C. d. davisiana（戴维斯放射虫）的丰度时间波动常被用作高分辨率地层学与古环境研究工具。本研究对该放射虫物种的现代生态学特征及（时间与空间尺度上的）形态变异展开评估，以明确其在地层学与古环境研究中的潜在应用价值。

研究采集了来自北半球（拉布拉多海、冰岛-法罗海脊）与南半球（纳米比亚陆架、梅特罗海隆）不同年代更新世（Pleistocene）种群的全部C. d. davisiana壳体节段的宽度与高度数据。结果显示，种群间与种群内部的节段高度变异相较于节段宽度更为保守。北大西洋与南大西洋海域的C. d. davisiana，其胸部（thorax）与第一腹节（abdominal segment）的平均尺寸存在显著差异。不过所有种群的第一腹节宽度均无显著差异，而北大西洋与南大西洋种群的该腹节平均高度则存在显著差异。第二腹节的尺寸则未呈现明确的种群聚类特征。这些种群的头后（post-cephalic）节段尺寸差异，似乎与基因库的一定程度隔离及潜在的未知古环境因素相关。C. d. davisiana的头后节段尺寸时间变化可用于以下三方面：(1) 识别特定区域内该物种丰度的时间等效峰值；(2) 评估不同区域间的水团（water masses）混合程度；(3) 追溯该物种从起源地的初始扩散路径。

1887年克利夫采集的格陵兰与斯匹次卑尔根之间的浮游生物样本，结合其他数据展开研究后，我们就北极及格陵兰-挪威海海域C. d. davisiana的现代生态学特征得出以下结论：(1) 当前该物种未在表层水浮游生物样本中被检出；(2) 目前仅在500米以深的海域零星分布；(3) 北极冰盖下方200米以深的上层水域无该物种分布，但在更深海域仍有零星出现；(4) 在格陵兰海常年冰盖附近、盐度接近正常海洋标准的200米以深上层水域，该物种同样未被检出；(5) 该物种在边缘深海峡湾环境中最为常见，这类环境或可作为间冰期（interglacial periods）该物种的避难所。

在大西洋各半球亚热带辐合带（Subtropical Convergence）之间的海域，C. d. davisiana的丰度均不超过群落总丰度的1%。在挪威海与拉布拉多海，该物种在现代放射虫群落中的占比偶尔可达1%~5%，而南极高纬度海域的该占比从未超过5%。显然，当前仅鄂霍次克海（Sea of Okhotsk）的C. d. davisiana丰度较高。我们同意莫利与海斯（1983）的观点，即该物种丰度升高，可能与季节性海冰消融形成的强低盐度表层水，以及暖性高盐度中层水上方显著的温度最小值相关。更新世时期，高纬度海域频繁出现类似环境条件；而当前鄂霍次克海以外海域该物种的稀缺性，必然与后者缺乏当前这类独特环境条件有关。