The calanoid family Candaciidae, defined by Giesbrecht in 1892 (as Candaciidae) and Giesbrecht and Schmeil in 1898 (as Candaciidae), includes two genera and 36 valid species (Razouls et al., 2017). This family was placed in the superfamily Diaptomoidea by Andronov (1974) and Boxshall and Halsey (2004). The early history of taxonomy about this family was discussed by Grice (1963) in detail. Sars (1903) found that there were two morphological types of males in the species of genus Candacia, and he suggested this genus could be divided into two genera. Grice (1963) examined the specimens deposited in the U.S. National Museum, and established a new genus, Paracandacia Grice, 1963, based on two diagnostic characteristics, which were the finger-like terminal process on the apical segment of the female fifth pereiopod and the non-chelate form of the male right fifth pereiopod with its apical plumose seta. von Vaupel Klein and Gassmann (1998) analyzed the phylogenetic relationship of 27 available species in family Candaciidae based on 60 morphological characteristics. The results showed that three species of Paracandacia were in a terminal cluster, which indicated the genus Paracandacia was holophyletic. However, 23 species of Candacia could not form a separate clade, which indicated the genus Candacia was paraphyletic, at least based on morphological data in that study. More importantly, the family Candaciidae was also not monophyletic based on this phylogenetic analysis. Therefore, the current classification of genus taxon in this family is not in accord with the phylogenetic relationship among species, and the genus Candacia should be revised, especially based on molecular data in future. In the present study, we followed the treatment in Boxshall and Halsey (2004), which regarded the family Candaciidae as a monophyly and thus combined four species of Paracandacia into Candacia.

The family Candaciidae was wildly distributed in the tropical and temperate regions of the Atlantic, Pacific and Indian Oceans (Grice, 1963; Razouls et al., 2017). In China, Zheng et al. (1984) reported thirteen species distributed in China seas and a key of these species was given. Nineteen species of this family were recorded in the Checklist of Marine Biota of China Seas (Liu, 2008). More recently, Zhang et al. (2010) summarized eighteen Candaciidae species and provided an illustrated guide. The species Candacia varicans Giesbrecht, 1892 was recorded in China seas and the female specimens were illustrated and descripted by Lian and Lin (1978), but the male of this species had not been listed or confirmed in taxonomic literatures involving China seas. There were two possible reasons: first, the number of female individuals was over one time more than that of male individuals in natural population of pelagic copepods (Kiørboe, 2006) (in the author’s personal observation (Liu Zhensheng), female specimens were over four times more than that of male individuals), thus female specimens were much easier to obtain; second, and there might be some misidentification, because of the existence of morphologically similar species (Table 1).

In the present study, we examined some specimens from the South China Sea. The diagnosis and microscope photos of female and male were given. Besides, a comparison of diagnosis characteristics about several confusing species was provided.

Samples examined in the present study were collected from the central basin of the South China Sea (10.996 5°N, 113.503°E), in the summer of 2012, by 0–200 m vertical tows using a WP2 plankton net (with a 0.25 m² mouth opening area and a mesh size of 200 μm). The collected samples were preserved immediately in 5% (v/v) buffered formalin-seawater solution. The specimens were observed and studied using a microscope (Zeiss Discovery V20) equipped with a digital camera (AxioCam ICc5). All photos were also obtained using the same microscope system. Specimens used in this study are preserved in the Second Institute of Oceanography, State Oceanic Administration. The descriptive terminology and measurements follows Zheng et al. (1984) and Boxshall and Halsey (2004).

Family Candaciidae Giesbrecht, 1893

Genus Candacia Dana, 1846

Candacia varicans Giesbrecht, 1892 (Figs 1 and 2)

Giesbrecht, 1892 (p. 424, 439, 771); Scott, 1894 (p. 62); Rose, 1933 (p. 252); Grice, 1962 (p. 234); Grice, 1963 (p. 179); Owre and Foyo, 1967 (p. 94); Zheng et al., 1984 (p. 250); Bradford-Grieve, 1999 (p. 174); Harding, 2004 (p. 25); Vives and Shmeleva, 2007 (p. 456); Liu, 2008 (p. 614); Zhang et al., 2010 (p. 158).

Type material not examined. Two adult female specimens and one adult male specimen were examined in the present study. The specimens were collected from the central basin of the South China Sea (10.996 5°N, 113.503°E), in the summer of 2012.

Prosome cylindraceous and rather narrow; forehead wide and truncate; the posterolateral corners of fifth thoracic segment prominently sharply protuberant and symmetrical, with spines; the terminal of posterolateral corners reached the middle part of genital segment. Urosome 3-segmented; genital segment symmetrically oval-shaped for dorsal view and with a prominently triangular protuberance towards ventral surface for lateral view; the terminal of this protuberance truncate; the second segment approximately square; anal segment rather short; caudal furca longer than wide slightly; caudal setae showed dark brown; ventral surface of urosome segments also showed dark brown in some specimens.

Antennule 24-segmented, reached up to the posterior of anal segment of urosome; basal seven segments of antennule robust. The first to fourth pereiopods showed dark brown; endopod of first pereiopod 2-segmented. The fifth pereiopod uniramous, 3-segmented, and with slightly asymmetrical third segments; lateral surface of terminal segment with two spines, and inner surface without spine; the terminal of the third segment with two asymmetrical strong spines; these two spines of right pereiopod stronger than those of left pereiopod.

Body length, 2.30–2.53 mm (two specimens)

New record in China seas.

Prosome also cylindraceous, but narrower than that of female; forehead wide and truncate; the posterolateral corners of the fifth thoracic segment prominently sharply protuberant and symmetrical; the terminal of posterolateral corners over the middle part of genital segment. Urosome 4-segmented; the surface of urosome segments showed dark brown; genital segment trapezoidal and symmetrical; right side of posterior edge of this segment with a small backward protuberance; the second and third segments approximately square; anal segment rather short; caudal furca longer than wide (length/width≈1.6); caudal setae showed dark brown.

Antennule reached up to the middle part of urosome. The first to fourth pereiopods showed dark brown; endopod of first pereiopod 2-segmented. The fifth pereiopod asymmetric. In left fifth pereiopod, the length of the second segment equaled to that of terminal segment; the length of the third segment 1.5 times longer than that of the second segment, and with a lateral spine; terminal segment approximately oval-shaped, with a lateral spine and two terminal spines; the inner surface of terminal segment obviously swollen. In right fifth pereiopod, terminal two segments pincer-shaped; the base of the second segment obviously robust, posterior of this segment bended outward and fan-shaped; the dorsal surface of the third segment with a squama-shaped protuberance; the terminal of this segment with a short and thick spine.

Body length, 2.15 mm (one specimen).

In China seas, this species is morphologically associated with male individuals of Candacia curta (Dana, 1849) and Candacia armata (Boeck, 1872). Several key diagnoses were provided here to distinguish them (Table 1).