Morphology and molecular phylogeny of trypanorhynchid metacestodes infecting commercial fish of the Mediterranean Sea

ABSTRACT Members of the order Trypanorhyncha are cestode parasites that are frequently found infecting the muscles of several marine fish species, affecting fish health, and resulting in consumers’ rejection of fish. Fifty-two specimens of marine fish were freshly caught throughout the year 2020 from boat landing sites at the Alexandria coast along the Mediterranean Sea in Egypt, including the grey trigger fish Balistes carolinensis (F: Balistidae); the mottled grouper Mycteroperca rubra (F: Serranidae) and the common sole Solea vulgaris (F: Soleidae). Blastocysts were isolated and ruptured; the generated pleurocerci were described morphologically and morphometrically by light and scanning electron microscopy. Also, multiple-sequence alignment was performed, and a phylogenetic tree was constructed following maximum likelihood analysis of the 18s and 28s ribosomal RNA sequences of the recovered worms. Thirty fish were infected; the infection was recorded as blastocysts embedded in fish flesh. Three different parasitic species were recovered and classified morphologically as Gymnorhynchus isuri, Pseudotobothrium dipsacum and Heteronybelinia estigmena. The taxonomic position of these parasites was justified by molecular analysis of their 18s and 28s rRNAs, which revealed high percentages of homology with species recovered from the GenBank. The accession numbers ON157059, ON139663 and ON139662 were respectively assigned to the recovered parasites after their deposition in GenBank. The results obtained from the molecular analyses confirmed the morphological records of the recovered parasites. Since metacestodes are found in the musculature of infected fish specimens, it is necessary to remove these areas in the commercialization of fish.

摘要：吻腔目（Trypanorhyncha）成员为绦虫寄生虫（cestode parasites），常寄生于多种海水鱼类的肌肉组织中，可影响鱼类健康，并导致消费者拒收受感染水产品。本研究于2020年全年在埃及地中海沿岸亚历山大港的渔船卸货点采集了52尾新鲜海水鱼样本，涵盖灰鳞鲀（*Balistes carolinensis*，鳞鲀科（Balistidae））、花斑石斑鱼（*Mycteroperca rubra*，鮨科（Serranidae））及普通鳎（*Solea vulgaris*，鳎科（Soleidae））。研究人员分离并裂解了样本中的囊状体（blastocysts），释放出的原尾蚴（pleurocerci）通过光学显微镜（light microscopy）与扫描电子显微镜（scanning electron microscopy）完成形态学与形态测量学分析。同时对分离获得的虫体18S及28S核糖体RNA序列开展多重序列比对，并基于最大似然法构建系统发育树。共30尾鱼呈现感染状态，感染特征为鱼肉组织中嵌入囊状体。本次研究共分离得到3种寄生蠕虫，经形态学鉴定分别为伊氏等棘吻虫（*Gymnorhynchus isuri*）、水螅假叶槽绦虫（*Pseudotobothrium dipsacum*）及异尼贝林绦虫（*Heteronybelinia estigmena*）。通过对这3种寄生虫的18S及28S rRNA进行分子分析，验证了其分类学地位，结果显示它们与GenBank数据库中收录的同源物种具有较高的序列相似性。上述分离得到的寄生虫提交至GenBank后，分别被赋予登录号ON157059、ON139663及ON139662。分子分析结果与形态学鉴定结果相互印证，验证了形态记录的准确性。由于受感染鱼类样本的肌肉组织中存在后绦虫幼虫（metacestodes），因此在鱼类商业化加工与流通环节中，需去除受感染的肌肉区域。