The siphonostomatoid copepod Caligus undulatus Shen & Li, 1959 has been widely reported from plankton samples obtained from neritic and oceanic waters off coasts of the Indo-West Pacific and Atlantic Oceans. Until now, its fish host has remained unknown. This copepod belongs to an intriguing group of congeners that, despite being part of a chiefly parasitic group, are consistently found as zooplankters. Quite unexpectedly, in October 2019, a fish host of C. undulatus was discovered in the Seto Inland Sea, Japan-namely, the Japanese sardinella Sardinella zunasi (Bleeker, 1854). Both juvenile (chalimus) and adult individuals of this caligid were observed as parasites of the fish host. The discovery suggests that the species has an alternative life cycle as previously proposed for other purportedly 'planktonic' congeners and might frequently switch hosts during the adult stage. Thus, the C. undulatus group is newly proposed as a species group in the genus, in which five species are known as planktonic. Some hypotheses on the modified life cycle of caligids also briefly discussed.

中文翻译：

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1959年发现了“浮游生物”的硬ig（Caligs undulatus Shen＆Li）鱼的寄主（甲壳纲：Co足类：虹吸纲）。

虹吸食足动物的co足类Caligs undulatus Shen＆Li，1959年，从印度洋-西太平洋和大西洋沿岸的浅水和洋水获得的浮游生物样品中得到了广泛报道。到目前为止，它的鱼类寄主还不为人所知。该co足动物属于有趣的同类动物群，尽管它是主要寄生虫群的一部分，但始终被视为浮游动物。出乎意料的是，2019年10月，在日本濑户内海发现了一条梭状芽胞杆菌（C. undulatus）的鱼类寄主，即日本沙丁鱼沙丁鱼zunasi（Bleeker，1854）。幼年（cha属）和成年个体都被视为鱼寄主的寄生虫。该发现表明该物种具有替代生命周期，如先前针对其他据称是“浮游生物”的提议 成年动物，在成年阶段可能经常更换宿主。因此，新提出了C. undulatus组作为该属中的一个物种组，其中五个物种被称为浮游生物。还简要讨论了有关哈利吉人生命周期修改的一些假设。