A description and the molecular characterization of two new species in the Haploporidae and Haplosplanchnidae families are provided herein. Parasaccocoelium armatum n. sp. was collected from the intestine of a Mugil cephalus Linnaeus, 1758 from the Primorsky region, Russia, and Pseudohaplosplanchnus catbaensis n. g. n. sp. was collected from Moolgarda seheli (Forsskål, 1775) in the coastal waters of Cat Ba Island, Vietnam. The morphological features of P. armatum n. sp. closely resemble those of Parasaccocoelium polyovum, but these species differ from one another by hermaphroditic sac and vitellaria area length and by maximal egg size. The main difference between P. armatum n. sp. and P. polyovum is the presence of an armed hermaphroditic duct in the new species. Molecular data support the case for inclusion of the studied trematodes in P. armatum n. sp. Worms P. catbaensis n. g. n. sp. from the mullet from Vietnam are morphologically close to Haplosplanchnus (Haplosplosplanchninae). The only difference between P. catbaensis n. g. n. sp. and species of Haplosplanchnus is the presence of few (1–7) large eggs, measuring 135–142 × 92–104 μm, versus numerous small eggs with a maximal size of 75 × 50 μm. Phylogenetic analysis showed that there is a contradiction between the morphological similarity of the worms and their position in the Haplosplanchnidae system, based on the genetic data. Results of this study indicate that P. catbaensis n. g. n. sp. is genetically distant from other representatives of Haplosplanchnus, despite their morphological similarity. According to the molecular data, P. catbaensis n. g. n. sp. is close to Hymenocotta mulli Manter, 1961 (Hymenocottinae). However, these species are considerably different to each other morphologically. Molecular data argue for the possibility of establishing a new subfamily for P. catbaensis n. g. n. sp. However, considering earlier studies of Haplosplanchnidae, we support the view that creating new subfamilies within this family is unreasonable because of the lack of molecular data for most haplosplanchnid species, which are necessary to resolve the problematic systematics and phylogeny of this family.

中文翻译：

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俄罗斯和越南远东地区鲻鱼的新种 Parasaccocoelium (Haploporidae) 和新属 Pseudohaplosplanchnus (Haplosplanchnidae)：形态学和分子数据

本文提供了 Haploporidae 和 Haplosplanchnidae 家族中两个新物种的描述和分子特征。Parasaccocoelium armatumn. sp。是从一个人的肠子里收集的鲶鱼林奈，1758 年来自俄罗斯滨海边疆区，假单叶鳖ngn sp。收集自穆尔加达·塞赫里(Forsskål, 1775) 在越南吉婆岛的沿海水域。形态特征P. armatumn. sp。与那些非常相似多卵副球囊菌，但这些物种在雌雄同体囊和珍珠母区域长度以及最大卵子大小方面有所不同。之间的主要区别P. armatumn. sp。和P. polyovum是新物种中存在武装的雌雄同体导管。分子数据支持将所研究的吸虫纳入P. armatumn. sp。蠕虫P. catbaensisngn sp。来自越南的鲻鱼在形态上接近单翅目(Haplosplosplanchninae)。之间的唯一区别P. catbaensisngn sp。和种类单翅目是存在少量（1-7）个大鸡蛋，尺寸为 135-142 × 92-104 μm，而最大尺寸为 75 × 50 μm 的许多小鸡蛋。系统发育分析表明，基于遗传数据，蠕虫的形态相似性与其在Haplosplanchnidae系统中的位置之间存在矛盾。本研究结果表明P. catbaensisngn sp。与其他代表在遗传上相距甚远单翅目，尽管它们的形态相似。根据分子数据，P. catbaensisngn sp。接近膜藻曼特，1961 年（膜藻科）。然而，这些物种在形态上彼此有很大不同。分子数据证明了建立新亚科的可能性P. catbaensisngn sp。然而，考虑到早期对单足纲动物的研究，我们支持这样一种观点，即在该科中创建新的亚科是不合理的，因为缺乏大多数单双纲纲物种的分子数据，这是解决该科有问题的系统学和系统发育所必需的。