海岸表层沉积物应用在现生微型贝类种群丰度评估的可行性探讨——以南中国海涠洲岛小笠原腹螺研究为例

The Feasibility of Assessing the Abundance of Extant Microgastropod Fauna through Coastal Surface Sediments—A Case Study of Alvania ogasawarana (Gastropoda: Rissoidae) from Weizhou Island, South China Sea

摘要: 为探讨海岸表层近期的沉积物应用在现生微型贝类种群丰度评估的可行性，针对南中国海涠洲岛的微型腹足贝类物种小笠原腹螺Alvania ogasawarana (Pilsbry, 1904)，进行种群大小与分布调查。于2019~2020年，参照涠洲岛沿岸地形地貌特征选择8个样区，在潮间带进行等距定点砂样调查，共采集99个样点砂样，总重约225公斤，使用筛析法分析粒径组成分；且从中随机选取59个样点砂样，分离出小笠原腹螺遗壳1522个，进行样区种群大小与分布分析。结果显示：有44处样点分离出小笠原腹螺遗壳，发现率高达74.6% (44/59)；小笠原腹螺在8个样区之间种群平均个体数存在统计学上显着差异(Kruskal-Wallis Test, χ² = 47.349, p = 0.000)。其中，种群密度较高前三者样区依序为贝壳沙滩西区、东海岸沙滩区、贝壳沙滩东区，在每1000公克砂样里约含遗壳数分别为56、27、25个；而以南湾海滩东区最少，约为0个。按地理方位东部沿岸、西部沿岸、南部沿岸和北部沿岸实施小笠原腹螺丰度比较，结果表明：单一样区平均遗壳数仅东部沿岸53个(±46.72, n = 16)和北部沿岸60个(±58.11, n = 9)两者之间的丰度未达统计学显着差异(Mann-Whitney检验，Z = −0.623，P = 0.533)；而南部沿岸则与其他方位沿岸都达统计学极显着差异(p = 0.000)。沿岸砂样的组合结构数据表明：南部海岸粉砂百分比例较其他样区为多，相对于北部(47.0%)和东部海岸(42.8%)的粗砂–中粗砂组合最高；两者均可以反映出涠洲岛海域底质现况，即表现为北部海岸区潮下带珊瑚覆盖度较高、南部沙滩区砂质底质居多。推测小笠原腹螺的种群丰度在北部海岸区显着大于南部沙滩区，是与海岸环境因素有关；支持海岸表层近期的沉积物中含有来自潮下带底栖环境因海水动力带上来的生物个体，适宜评估现生的微型贝类种群丰度。

Abstract: According to the population size of Alvania ogasawarana (Pilsbry, 1904) among various geographical types on Weizhou Island, the feasibility of assessing the abundance of extant microgastropod fauna by coastal surface sediments was evaluated. There were eight study areas selected, i.e. Shell Beach west area, Shell Beach east area, East Coast beach area, Hengling beach area, Nanwan beach east area, Nanwan beach west area, Dishui Danping beach area and Beigang beach area. From spring 2019 to winter 2020, we checked about 225 kg of sediments with the sieve analysis method from 99 locations among all sample areas. Further, 1522 specimens of A. ogasawarana were also collected from 59 of all locations. The results show that the A. ogasawarana distribute widely around coast of Weizhou Island (74.6%, by 44/59), and the population size are more abundant on the eastern (53 ± 46.72, n =16) and the northern (60 ± 58.11, n = 9) study regions than southern region of island (Mann-Whitney test, p = 0.000), especially, the density of population at Shell beach west area, East Coast beach area, and Shell beach east area are 56, 27 and 25 (mean inds. of specimen/1000g sand), respectively. From the distribution of substrate texture, the particle size is larger (>0.450 mm) in the eastern (42.8%) and the northern (47.0%) study regions than in southern region. Our results suggest that the abundance of the extant A. ogasawarana is significantly different between the northern and southern parts of the island, which is related to the benthic environment along the coast and the dynamics of seawater intrusion under tidal oscillations. And the aspects of depositional assemblage of organisms such as A. ogasawarana in surface sediments, which are suitable for evaluating the richness of extant microgastropods.