**1. Introduction**

Monstrilloida Sars, 1901 is a small group of marine copepods that comprise approximately 170 nominal species in seven genera currently considered as valid: *Monstrilla* Dana, 1849; *Cymbasoma* Thompson, 1888; *Monstrillopsis* Sars, 1921; *Maemonstrilla* Grygier & Ohtsuka, 2008; *Australomonstrillopsis* Suárez-Morales & McKinnon, 2014; *Caromiobenella* Jeon, Lee & Soh, 2018; *Spinomonstrilla* Suárez-Morales, 2019 [1–7]. This group is distinguished from other copepods by their peculiar life cycle and strange morphology. They have a protelean life-cycle consisting of an endoparasitic juvenile phase and a planktonic adult stage. The juveniles are known to infect various marine invertebrates such as polychaetes, gastropods and bivalve mollusks [8–18]. The adults, often regarded as an exclusively reproductive stage, lack all mouth parts as well as the antennae and are non-feeding.

Most of the recent work on monstrilloids has focused on taxonomy and the morphological description of adults, but studies on the parasitic life stages and host-parasite interactions have been on the increase [2,17–19]. As well, research into monstrilloid diversity has been expanding into lesser-studied regions. For example, Suárez-Morales and McKinnon [6,20] recently described 34 species of four genera from Australian waters, with a promised treatment of similar numbers of Australian *Monstrilla* still to come, and such findings imply the possible occurrence of large numbers of undescribed taxa in other such areas as well.

East Asia is clearly a region where the monstrilloid fauna has not been yet fully revealed [16,21]. Serious research on monstrilloids in Korea began less than a decade ago with the first record of *Cymbasoma striifrons* Chang, 2012. So far nine species in four genera have been reported from Korea's eastern and southern coastal waters [3,22–26]. The site of the present study, the Chuja Islands, is a group of 42 small islands, four of which are inhabited. They are located between Jeju Island and the mainland of Korea and are affected by the Tsushima Warm Current. Temperate and subtropical creatures coexist there [27,28], leading to the expectation of high biodiversity in their surrounding waters. Here we report a new type of monstrilloid with a novel and unusual morphological feature for the group, and described it as a new species.

#### **2. Materials and Methods**

A plankton sample was collected by Min Ho Seo (Marine Ecology Research Center, Korea) using a hand-made polyvinyl chloride (PVC) light trap containing an light emitting diode (LED) flashlight as a light source. The type materials of the present new species were collected from 19:00 to 7:00 on 11–12 September 2017 alongside a wharf at the type locality (Figure 1). The captured contents were filtered using a 63 μm mesh test sieve, and the retained material was immediately washed with 99.5% ethanol. The sample was initially fixed with 99.5% ethanol on-site and the fixative was exchanged for fresh 99.5% ethanol in the laboratory. Monstrilloids were sorted out from the bulk collection under a SMZ645 stereomicroscope (Nikon, Tokyo, Japan) and kept refrigerated at 4 ◦C. The osmotic shock of ethanol fixation caused the cephalothorax of many specimens to collapse, so monstrilloid specimens used for morphological analysis were exposed to 0.25% trisodium phosphate dodecahydrate solution (Na3PO4·12H2O; Daejung Chemicals and Metals, Siheung, Korea) to restore its original shape [3,12,31]. An Eclipse 80i compound microscope with a drawing tube and differential interference optics was used for preparing illustrations. The holotype specimen was dissected into five parts, and each was mounted on a slide separately with lactophenol for detailed examination. Body measurements were obtained by using AxioVision LE64 software (AxioVs40x64 v 4.9.1.0; Carl Zeiss, Oberkochen, Germany) based on the illustrations of the type material. Terminology from Grygier and Ohtsuka [2] and Jeon, et al. [32] was used to describe body segmentation and antennular setation patterns, respectively.

**Figure 1.** Maps showing the investigated area and the sampling locality. (**A**) Location of Chuja Islands (red dot), Korea; (**B**) sampling site (red dot) in Chuja Islands. The maps were prepared using QGIS [29], a free and open-source geographic information system, with the OpenStreetMap data [30].

*Diversity* **2020**, *12*, 9
