Ceratodoris plebeia (Bergh, 1902)

(Figs 1E, 2G‒H, 4E‒H)

Idalia plebeia Bergh (1902): 186; plate III; Figs. 15‒19.

Okenia cf. plebeia — Hung & Huang (2021): 111, text-figs.

Material examined. MNCN 15.05 /94477, Kinmen, Taiwan, intertidal, 28 November 2019, col. by S . T. Huang, 96% EtOH, dissected (SEM: Radula, labial cuticle, penis) ; MNCN 15.05 /94478, Kinmen, Taiwan, intertidal, 28 November 2019, col. by S . T. Huang, 96% EtOH, dissected (SEM: Radula, labial cuticle, penis) ; MNCN 15.05 /94479, Kinmen, Taiwan, intertidal, 28 November 2019, col. by S . T. Huang, 96% EtOH .

External morphology and color pattern (Figs. 2C‒D). Preserved specimens 10–20 mm length. The species was described in detail by Bergh (1902) in its original description.

Foregut anatomy (Figs. 2G, 4E‒G). Buccal bulb thick and muscular (Fig. 2G). Rounded, wide, dorsal buccal pump expanding posteriorly. Radular sac located ventrally, expanding backwards. Esophagus begins from buccal bulb behind buccal pump. Salivary glands elongated and granulated, formed by several small, rounded glands. Salivary glands located at junction of esophagus with buccal bulb. Nervous system covers this junction. Esophagus continues posteriorly and inserts into digestive-hermaphroditic gland. Labial cuticle surrounds lips and expands within buccal pump. Part of labial cuticle located within buccal pump with honey-comb elements (Fig. 4E). Radular formula 25‒26 × 1.1.0.1.1. Inner lateral tooth with single large and thin cusp, and wide, rectangular base (Figs. 4F– G). Cusp large and pointed, with a masticatory margin bearing 22–25 small, pointed denticles (Fig. 4G). Denticles with rounded tips. Denticles located in middle part of masticatory margin larger than lateral ones. Outer base ends in somewhat prominent wing, with rounded external edge. Outer lateral tooth much smaller (Figs. 4F–G). Base of outer lateral tooth rectangular, top with an extension, with a rounded base from which an oval peak emerges (Fig. 4G).

Reproductive system (Figs. 2H, 4H).Reproductive system located in anterior third of body.Thin hermaphroditic duct begins at ovotestis, located inside digestive-hermaphroditic gland. Hermaphroditic duct expands into elongate, sausage-shape ampulla. Postampullary duct emerges from ampulla and divides into two, thin ducts. Short oviduct enters inside female gland mass, other connects with prostate. Prostate very big, with fluffy texture, covers almost half size of reproductive system. It continues as a very thin vas deferens. End of vas deferens widens and expands to ejaculatory duct. Penis with penial spines (Fig. 4H). Penial spines at base short and hooked, arranged in longitudinal rows. Spines become larger, straight in middle, and more spread out, slight shorter and hooked towards opening area. Vagina small, wider than vas deferens, connects with large, oval bursa copulatrix. From base of bursa copulatrix arises a very thin duct that connects with an elongated receptaculum seminis. Approximately in middle part of vagina, a very thin uterine duct arises and enters female gland mass. Uterine duct wider at part that enters female gland mass.

Distribution. Thailand (Bergh 1902; Gosliner et al. 2018), India (Gosliner et al. 2018) and Taiwan (Hung & Huang 2021; present study).

Natural history. The species lives on rocks, inhabiting shallow reefs and gravel areas. Ceratodoris plebeia feeds on bryozoan. Its egg mass is yellowish white and is a simple spiral consisting of 4-5 whorls (Hung & Huang 2021).

Remarks. Since its original description, C. plebeia has hardly been found (Gosliner et al. 2018; Hung & Huang 2021). The external morphology of this species was described in detail by Bergh (1902) and our specimens agree with it. Bergh (1902) also described and drawn the radular teeth, which are shown for first time by scanning electron microscope in the present study. Moreover, Bergh (1902) described some details of the reproductive system, mainly focused on the male part. Here we provide a complete description.