Morphological and molecular evidence for first records and range extension of the Japanese seahorse, Hippocampus mohnikei (Bleeker 1853) in a bay-estuarine system of Goa, central west coast of India

Accurate information of taxonomy and geographic range of seahorse species (genus Hippocampus) is the first step in preparing threat assessments and designing effective conservation measures. Here, we report first records and a range extension of the Japanese seahorse, Hippocampus mohnikei (Bleeker, 1853) from the Mandovi estuarine ecosystem of Goa, central west coast of India (CWCI) based on morphological and molecular analyses. The morphometric and meristic traits, particularly short snout (29–35% head length), double cheek spine, low coronet, long tail (51.2–57.9% of standard length), 11 trunk rings, 37–39 tail rings, 15–16 dorsal and 12–14 fin rays observed in four collected specimens matched with the reported key diagnostic morphological criteria of vouchered specimens of H. mohnikei. The seahorse mitochondrial cytochrome oxidase subunit I (COI) and cytochrome b (Cyt b) genes were partially sequenced for conclusive genetic identification of the species under study. Molecular analysis showed that all four individuals clustered together suggesting a monophyletic lineage. Using the maximum similarity with GenBank database, maximum likelihood network and subsequent morphological analysis, the identity of the collected seahorse species was reconfirmed as H. mohnikei. With this new report, the geographic range of H. mohnikei extended significantly to the west from its previously known range. This new sighting of H. mohnikei could indicate a long-distance dispersal facilitated by the prevailing oceanic circulation in the Indo-Pacific region or increased habitat suitability in bay-estuarine systems of Goa, CWCI. Comparison of the pair-wise genetic distances (Kimura 2-parameter) based on COI and Cyt b sequences revealed that the specimens examined in this study are genetically closer to H. mohnikei populations from Vietnam and Thailand than they are to those in Japan and China. To test the hypothesis whether H. mohnikei are vagrants or previously unreported established population, long-term inter-annual sampling and analyses are warranted.准确掌握海马属（Hippocampus）物种的分类学信息与地理分布范围，是开展濒危等级评估、制定有效保护措施的首要前提。本研究基于形态学与分子生物学分析，报道了在印度中西部海岸（CWCI）果阿邦曼多维河口生态系统中发现的日本海马（Hippocampus mohnikei，Bleeker, 1853）的首次记录及其分布范围扩张。本次采集的4份标本的形态测量与计数性状——尤其是短吻（占头长的29%~35%）、双颊棘、低矮的冠状突、长尾（占标准体长的51.2%~57.9%）、11个躯干环、37~39个尾环、15~16个背鳍鳍条与12~14个胸鳍鳍条——均符合已发表的日本海马凭证标本的关键诊断形态学标准。本研究对采集标本的线粒体细胞色素氧化酶亚基I（cytochrome oxidase subunit I, COI）与细胞色素b（cytochrome b, Cyt b）基因片段进行了测序，以实现物种的精准遗传学鉴定。分子分析结果显示，4个个体聚为一支，形成单系谱系。结合与GenBank数据库的最高序列相似度比对、最大似然系统发育网络分析以及后续形态学验证，本研究采集的海马物种最终被确认为日本海马（H. mohnikei）。本次新记录的发表，使日本海马的已知地理分布范围从原分布区向西大幅扩张。此次在印度中西部海岸果阿的海湾-河口生态系统中的新发现，可能暗示了印度-太平洋区域主流海洋环流介导的长距离扩散，或是该区域栖息地适宜性提升所致。基于COI与Cyt b序列的成对遗传距离（Kimura 2-parameter）比较结果显示，本研究的标本与越南、泰国的日本海马种群遗传相似度更高，而与日本、中国的种群亲缘关系相对较远。为验证“日本海马为迷游个体，还是当地此前未被记录的定居种群”这一假说，后续需开展长期的年度间采样与相关分析。