Monstrilloid copepods are unusual semi-parasitic crustaceans whose juveniles develop within benthic invertebrate hosts, whereas the non-feeding adults occur briefly in the plankton for reproduction. Because adults are rarely encountered in routine zooplankton surveys, their diversity and distribution remain poorly understood in many marine regions. Recent examinations of zooplankton samples from coastal, estuarine, and fringing-reef habitats in the northern South China Sea revealed five previously unknown species belonging to three genera: Monstrillopsis plumosa, Monstrilla pseudograndis, Cymbasoma stricturum, Cymbasoma elongatum sp. nov. (manuscript under review), and Monstrilla inflata sp. nov. (manuscript under review). These records include the first occurrence of Monstrillopsis in the China Seas and extend the known regional diversity of Monstrilla and Cymbasoma.
Species identification was based on detailed comparisons of body proportions, antennular segmentation and armature, cephalothoracic ornamentation, female fifth-leg structure, genital morphology, caudal setation, and the length and configuration of ovigerous spines. Particular emphasis was placed on Cymbasoma elongatum, collected at night from plankton over a shallow fringing reef off Wenchang, Hainan. This species is characterized by an extremely elongate body, exceptionally long ovigerous spines, short antennules, and a distinctive V-shaped pattern of cuticular ridges on the forehead. Molecular analyses based on concatenated mitochondrial COI and nuclear 28S rRNA sequences supported its distinctiveness and recovered it within a strongly supported Cymbasoma lineage.
Together, these discoveries demonstrate that the monstrilloid fauna of the northern South China Sea has been substantially underestimated. They also highlight the importance of habitat-targeted sampling, careful morphological examination, and molecular reference data for detecting rare or overlooked components of plankton diversity. Improved taxonomic resolution of monstrilloids will provide a stronger foundation for future investigations of zooplankton community composition, biogeography, and reef-associated biodiversity.
 

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