Metabarcoding of ichthyoplankton communities associated with a highly dynamic shelf region of the southwest Indian Ocean

Drifting fish eggs and larvae (ichthyoplankton) can be identified to species using DNA metabarcoding, allowing for community analyses at a high taxonomic resolution. We undertook a regional-scale study of ichthyoplankton distribution along the east coast of South Africa, focused on narrow shelf and sheltered bight habitats in the contrasting environments of the tropical Delagoa and subtropical Natal ecoregions. Zooplankton samples were collected with tow nets at discrete stations along cross-shelf transects (20–200 m depth) spaced along a latitudinal gradient that incorporates a known biogeographical boundary. Metabarcoding detected 67 fish species, of which 64 matched prior distribution records of fishes from South Africa, with the remaining three known from the western Indian Ocean. Detections included fish species classified as coastal, neritic, and oceanic, with adult habitats ranging from epi- and mesopelagic to benthopelagic and benthic. By family, Myctophidae (10 species), Carangidae, Clupeidae, Labridae (each with 4 species) and Haemulidae (3 species) were most speciose. Ichthyoplankton community composition varied significantly with latitude, distance to coast, distance to the shelf edge and transect. Small pelagic fishes had the highest frequency of occurrence: <em>Engraulis capensis, Emmelichthys nitidus </em>and <em>Benthosema pterotum</em> increased in frequency towards the north, whereas <em>Etrumeus whiteheadi </em>increased towards the south. Chub mackerel <em>Scomber japonicus</em> accounted for most variability related to distance from the coast, whilst African scad <em>Trachurus delagoa </em>showed relatively strong correlation with distance to the shelf edge. Dissimilarity between communities in the Delagoa and Natal ecoregions was 98-100%, whereas neighbouring transects located within the sheltered KwaZulu-Natal Bight had lower dissimilarity (56-86%). Onshore transport of ichthyoplankton by intrusions of south-flowing Agulhas Current water may explain the large proportion of oceanic species collected over the shelf. The value of metabarcoding as a tool to facilitate marine ecological studies at a high taxonomic resolution is discussed.

利用DNA宏条形码（DNA metabarcoding）技术可对漂流性鱼卵与仔鱼（ichthyoplankton）进行物种级鉴定，从而实现高分类学分辨率的群落分析。本研究沿南非东海岸开展了区域尺度的鱼卵仔鱼分布调查，聚焦热带德尔加戈（Delagoa）与亚热带纳塔尔（Natal）生态区的对比环境中的窄陆架及遮蔽海湾生境。研究团队采用拖网在沿纬度梯度布设的离散站位采集浮游动物样品，该梯度涵盖已知生物地理边界，站位分布于水深20至200米的跨陆架断面上。DNA宏条形码检测到67种鱼类，其中64种符合南非已有的鱼类分布记录，剩余3种则记录于西印度洋海域。本次检测到的鱼类涵盖沿岸性、近岸性与大洋性类群，成体栖息生境涵盖大洋上层、大洋中层、底栖上层至底栖环境。按科统计，物种丰富度最高的类群为灯笼鱼科（Myctophidae，10种）、鲹科（Carangidae）、鲱科（Clupeidae）、隆头鱼科（Labridae，各4种）以及石鲈科（Haemulidae，3种）。鱼卵仔鱼群落组成随纬度、离岸距离、陆架边缘距离以及断面位置呈现显著变化。小型中上层鱼类出现频率最高：南非鳀（*Engraulis capensis*）、光亮软鳍鲷（*Emmelichthys nitidus*）与翼底灯鱼（*Benthosema pterotum*）的出现频率向北递增，而怀特氏圆腹鲱（*Etrumeus whiteheadi*）的出现频率则向南升高。日本鲭（*Scomber japonicus*）是与离岸距离相关的群落变异的主要贡献物种，而德尔加多竹荚鱼（*Trachurus delagoa*）则与陆架边缘距离呈现较强相关性。德尔加戈生态区与纳塔尔生态区的鱼卵仔鱼群落相异度达98%至100%，而位于遮蔽性夸祖鲁-纳塔尔湾内的相邻断面间相异度则较低（56%至86%）。向南流动的阿古拉斯海流（Agulhas Current）侵入所驱动的鱼卵仔鱼向岸输运，或可解释陆架上采集到的大洋性物种占比偏高的现象。本文探讨了DNA宏条形码作为高分类学分辨率研究工具，在海洋生态学研究中的应用价值。