Influence of asexual reproduction on Holothuria atra and Stichopus chloronotus populations on the Great Barrier Reef

Densities and individual wet weights of Holothuria atra and Stichopus chloronotus were measured on near-shore reefs of the central Great Barrier Reef, to determine the effect of asexual reproduction by transverse fission on the population structure of holothurians. Permanent stations were established at Fantome Island, Brook Island and at two sites in Little Cannon Bay, Great Palm Island.Holothurian densities were estimated every 1 to 3 months between March 1995 and August 1996 and again in April 1997 after Cyclone Justin, which occurred in March 1997. Fixed transect lines were established parallel to the shore and for each survey, a number of sub-transects, each 10 m long were deployed at random intervals, perpendicular to the transect line and all individuals within 1 m either side of each 10 m transect were counted. Only Holothuria atra was counted at Brook and Fantome Islands as Stichopus chloronotus does not occur at these sites.At Brook Island, transect lines were established at 5-15 m and 25-35 m seawards from the mean high tide level and 10 sub-transects were deployed per line. Twenty sub-transects were deployed from a single 100 m transect line at Fantome Island. At Great Palm Island, 10 sub-transects were deployed along a single transect line at one site and along two transect lines at a second site, which were set 10-20 m and 30-40 seaward of the mean high tide level. Additional 50 x 2m line transects were counted at both sites on Great Palm Island on a number of occasions.Weight frequency distributions were determined for Stichopus chloronotus from both sites at Great Palm Island and for Holothuria atra from Fantome Island and Brook Island. Random samples of each population, including intact individuals and fission products were collected and wet weights were measured on shore after allowing individuals to eject water from their respiratory trees. In addition, 151 recently divided Holothuria atra and 120 Stichopus chloronotus were weighed to determine the size range of fission products.All weights were grouped into 10 or 15 g size classes and biomass for each sampling month was calculated as the frequency of each weight class x the modal value of the class x the density. For months where size structure or density was not estimated, values for closest sampling occasion were used in calculations.A conceptual model based on these and previous findings was developed and 5 factors (mortality, habitat stability, optimum individual size, food availability and larval supply) were hypothesized as having an influence on promotion or repression of transverse fission. This study was undertaken to determine the effect of asexual reproduction by transverse fission on the population structure of holothurians.

本研究于大堡礁（Great Barrier Reef）中部近岸礁区开展糙海参（Holothuria atra）与绿刺参（Stichopus chloronotus）的种群密度及个体湿重测定，以探究横裂无性繁殖对海参种群结构的影响。
研究团队在幻岛（Fantome Island）、布鲁克岛（Brook Island）以及大棕榈岛（Great Palm Island）的小坎农湾（Little Cannon Bay）两处站点设立了固定监测样区。
于1995年3月至1996年8月期间，每1至3个月开展一次海参种群密度估算，并在1997年3月发生的贾斯汀飓风（Cyclone Justin）过后，于1997年4月补充开展了一次密度调查。研究设置了平行于岸线的固定主样带，每次调查时均沿主样带以随机间隔布设若干条长10米的副样带（垂直于主样带），并统计每条10米副样带两侧1米范围内的所有海参个体。因布鲁克岛与幻岛海域无绿刺参分布，故仅统计了该两处站点的糙海参种群密度。
布鲁克岛的主样带设置于平均高潮线向海侧5-15米与25-35米处，每条主样带布设10条副样带；幻岛仅设置1条长100米的主样带，其上布设20条副样带。
大棕榈岛的两处监测站点中，一处仅设置1条主样带并布设10条副样带；另一处设置2条主样带，分别位于平均高潮线向海侧10-20米与30-40米处。研究团队还在大棕榈岛的两处站点多次开展了额外的50×2米样带调查。
针对大棕榈岛两处站点的绿刺参，以及幻岛与布鲁克岛的糙海参，研究人员统计了其体重频率分布。
采集各种群的随机样本，包括完整个体与横裂产物，待个体将呼吸树内的水分排出后，在岸上测定其湿重。此外，还测定了151头新近完成横裂的糙海参与120头绿刺参的湿重，以明确横裂产物的体型范围。
所有体重数据按10克或15克的组距划分为不同体重等级，各采样月的种群生物量通过以下公式计算：各体重等级的个体频率 × 该等级的组中值 × 种群密度。
若某采样月未开展体型结构或种群密度调查，则以距其最近的采样数据替代用于生物量计算。
基于本次及既往研究结果，本研究构建了一个概念模型，并提出5个可能影响横裂繁殖发生（促进或抑制）的关键因子：死亡率、生境稳定性、个体最适体型、食物可获得性以及幼体补给。
本研究旨在探究横裂无性繁殖对海参种群结构的影响。