A New Species of Predatory Nudibranch (Gastropoda: Trinchesiidae) of the Scleractinian Coral Goniopora

© 2020 Academia Sinica, Taiwan Open Access A New Species of Predatory Nudibranch (Gastropoda: Trinchesiidae) of the Scleractinian Coral Goniopora Juntong Hu1, Yanjie Zhang1,2, Sam King Fung Yiu1, James Yang Xie3, and Jian-Wen Qiu1,2,* 1Department of Biology and Hong Kong Branch of the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Hong Kong Baptist University, Hong Kong, China. *Correspondence: E-mail: [email protected] (Qiu). Phone: +852-34117055. E-mail: [email protected] (Hu); [email protected] (Zhang); [email protected] (Yiu) 2HKBU Institute of Research and Continuing Education, Virtual University Park, Shenzhen, China 3Agriculture, Fisheries and Conservation Department, Hong Kong SAR Government, China. E-mail: [email protected].hk (Xie) Received 18 July 2020 / Accepted 22 October 2020 / Published 23 November 2020 Communicated by Benny K.K. Chan Members of the nudibranch genus Phestilla are common predators of scleractinian corals, but currently this genus has 10 described species only. Here we describe Phestilla goniophaga sp. nov., the first formally named predatory nudibranch species of the stony corals from the genus Goniopora. The new species can be distinguished from its congeneric species by the large number of long cerata (up to 16 rows and 23 cerata per row), and white rounded hump on the notum. The hump resembles the mouth of the coral poly, while the cerata resemble the coral tentacles. The egg masses of P. goniophaga sp. nov. are unique among Phestilla spp. egg masses in being bright orange in color, and forming a coiled ribbon. Analysis of the COI, 16S rRNA and H3 genes of P. goniophaga sp. nov. also showed that this species is distinct from other congeneric species. Key words: Coral, Coral-eating, Corallivory, Mollusca, Phestilla. BACKGROUND The genus Phestilla Bergh, 1874 is a small group of tergipedid nudibranchs with reduced cephalic tentacles and without ceratal cnidocacs (MolluscaBase 2020). With the exception that P. chaetopterana (originally Tenellia chaetopterana) lives inside the tube of the polychaete Chaetopterus sp. (Ekimova et al. 2017), all Phestilla spp. are obligate predators of scleractinian corals. There has been some controversy in the systematics of nudibranchs of the superfamily Fionoidea Nordsieck, 1972 with a total of 317 species including those classified in Phestilla Bergh, 1874 (Tergipedidae) (MolluscaBase 2020). Cella et al. (2016) merged the families Calmidae, Eubranchidae, Fionidae and Tergipedidae into a single family Fionidae based on the molecular phylogenetic result that this would make the family monophyletic. In addition, they treated Phestilla and several other related genera (Trinchesia, Catriona and Cuthona) as junior synonyms of Tenellia. Ekimova et al. (2017) and Fritts-Penniman et al. (2020) supported this classification scheme, and in the latter publication the authors pointed out that otherwise Cuthona had to be divided into many genera. But Korshunova et al. (2017) reverted the classification of these families and genera because they thought Cella et al. (2016) did not fully consider the morphological and ontological characteristics when making the decision. Three recent studies (Wang et al. 2019; Hu et al. 2020; Mehrotra et al. 2020) all considered it more appropriate to retain Phestilla because this genus is monophyletic (except P. sibogae), and all the species except P. chaetopterana are obligate corallivores. Due to the inclusion of P. chaetopterana and P. viei in Phestilla, Mehrotra et al. (2020) provided an updated diagnosis of the genus with respect to the morphology of the oral tentacles, oral Citation: Hu J, Zhang Y, Yiu SKF, Xie JY, Qiu JW. 2020. A new species of predatory nudibranch (Gastropoda: Trinchesiidae) of the scleractinian coral Goniopora. Zool Stud 59:62. doi:10.6620/ZS.2020.59-62. Zoological Studies 59:62 (2020) doi:10.6620/ZS.2020.59-62 1 © 2020 Academia Sinica, Taiwan veil, presence of a swollen hump on notum, shape and arrangement of cerata, masticatory processes, radular teeth and penile stylet. Previous studies have found that corallivorous Phestilla spp. are associated with only few genera/ species of scleractinican corals. Specifically, P. lugubris, P. minor, P. panamica, and P. poritophages are predators of Porties (Harris 1975; Rudman 1979 1981 1982; Gosliner et al. 2018). Phestilla melanobrachia is a predator of dendrophylliid corals Tubastraea spp. and Dendrophyllia spp., and the its color changes according the color of the prey (Harris 1968 1975; Salvini-Plawen 1972). Phestilla subodiosa feeds on Montipora spp., common corals in the aquarium trade (Wang et al. 2020). Phestilla viei (Mehrotra et al. 2020) and P. fuscostriata (Hu et al. 2020) feeds on Pavona explanulata and Pavona decussata, respectively. As for P. sibogae, it was treated as a junior synonym of Phestilla lugubris by Rudman (1981). However, Gosliner et al. (2018) consider that they are different species and P. sibogae feeds on hydroids of the genus Sertularella. There have been several records of nudibranchs feeding on Goniopora corals in Indo-Pacific locations (Gosliner et al. 2018) including Singapore (Robertson 1970), Papua New Guinea (Gosliner 1992; Gosliner et al. 1996), and Guam and Palau (Ritson-Williams et al. 2003; Faucci et al. 2007). Nevertheless, no formal description of these nudibranchs is available. RitsonWilliams et al. (2003 2007 2009) reported Phestilla sp. 2 as a specialist predator of the scleractinian genus Goniopora (Poritidae) and studied its dietary breadth and larval development in Guam. They found that, when Porities cylindrica, Goniopora fruticosa, G. minor, G. lobata and G. eclipsensis were offered to the nudribranch, it showed higher preference for G. fruticosa, followed by G. minor and G. lobata (Ritson-Williams et al. 2003). Its larval development took 5 days after which the larvae would undergo metamorphosis in response to the cues from its favorite coral G. fruticosa (Ritson-Williams et al. 2007). Moreover, water-soluble cues from G. fruticosa caused a higher percentage of competent larvae of Phestilla sp. 2 to undergo metamorphosis than those cues from Porites annae, P. Cylindrica, Psammocora contigua, G. fruticosa, G. tenuidens, G. eclipsensis, G. minor, and G. lobata (Ritson-Williams et al. 2009). This series of experiments show that, although Phestilla sp. 2 can feed on multiple species of Goniopora, G. fruticosa is its most favorite species in terms of diet and settlement substrate. During surveys of coral communities in Hong Kong in 2019, we discovered a species of Phestilla feeding on Goniopora spp. and laying eggs on the skeleton of this genus of corals (Fig. 1A). In this paper, we provide morphological description of this species, Fig. 1. Phestilla goniophaga sp. nov. A, A colony of Goniopora sp. with two adults of P. goniophaga sp. nov. and their coiled egg ribbons. B, Holotype (BU-Mol-20-001). C, Paratype (BU-Mol-20-002). Scale bars: A–C = 10 mm. All photographs were taken when the nudibranchs were alive. page 2 of 9Zoological Studies 59:62 (2020) © 2020 Academia Sinica, Taiwan and report a molecular phylogenetic analysis of its relationship with other congeneric species. MATERIALS AND METHODS Sample collection Samples of the Phestilla goniophaga sp. nov. were collected by SCUBA diving from Sharp Island (22°21'32.9"N 114°17'47.8"E, water depth ~2 m) and Chek Chau (22°30'04"N 114°21'32"E, water depth ~ 2 m) in June 2019 and June 2020, respectively. The specimens were preserved either in 95% ethanol for molecular analysis or in 4% formaldehyde in seawater for morphological analysis. All specimens examined in this study are deposited in Hong Kong Baptist University (BU-Mol-20-001 to BU-Mol-20-006). Morphological analysis The external morphological characteristics were examined under a Motic SMZ-171 stereomicroscope, and photos were taken using a Canon Mark IV camera. The veligers were examined under a Motic BA210 compound microscope. To extract the radula and the jaws, the buccal masses from two specimens were soaked in 10% bleach solution for 10 min at room temperature (~22°C) to dissolve soft tissues, and then rinsed in deionized water four times to remove the bleach. The radula and jaws were dried in an oven, goldplated, mounted on a stub, and examined under a LEO 1530 FESEM scanning electron microscope (SEM). Specimens BU-Mol-20-002 and BU-Mol-20-005 were also dissected under the stereomicroscope to examine the reproductive system. Molecular analysis Foot tissues of three specimens (BU-Mol-20-001 to BU-Mol-20-003) were used for molecular analyses. Genomic DNA was extracted using the TaKaRa MiniBEST Agarose Gel DNA Extraction Kit Ver.4.0 (TaKaRa Biotechnology, Dalian, China). Concentration and purity of the DNA samples were determined using a NanoDrop ND-1000 spectrophotometer (Thermo Fisher Scientific, Wilmington, DE, USA), and DNA integrity was checked using electrophoresis on 1.0% agarose gel. Polymerase chain reaction (PCR) reactions were then conducted using the extracted DNA as templates to amplify the COI, 16S rRNA and H3 genes. The primers used were identical to those adopted by Ekimova et al. (2017): HCO2198 and LCO1498 (Folmer et al. 1994), 16SarL (Palumbi et al. 1991), 16SR (Puslednik and Serb 2008), H3AF and H3AR (Colgan et al. 1998). PCR programs were identical to those in Wang et al. (2019) and Hu et al. (2020). PCR products were sent to BGI Hong Kong for sequencing on an ABI 310 Genetic Analyzer. All new sequences were deposited in GenBank (Table S1). To understand the phylogenetic relationship between the new species and other related species, fragments of COI, 16S rRNA and H3 genes from 28 nudibranch species belonging to 11 genera (MolluscaBase 2020) were downloaded from GenBank (Table S1). The species names used in this paper conformed to those adopted in WORMS (MolluscaBase 2020). In some cases, informal names used in previous studies such as Phestilla sp. 2 (Faucci et al. 2007) were also used in this study. The COI, 16S rRNA and H3 sequences were aligned separately using MUSCLE v.3.8.31 (Edgar 2004) under default settings. For each species, the sequences of the three genes were concatenated using SequenceMatrix v.1.7.8 (Vaidya et al. 2011). Phylogenetic analyses were conducted using the Maximum Likelihood (ML) method and the Bayesian inference (BI) method based on the concatenated dataset. The ML analysis was conducted using raxmlGUI v.1.3.1 (Silvestro and Michalak 2012) with bootstrap supports estimated from analyses of 1000 pseudoreplicates. The GTR+I+G nucleotide-substitution model was selected using jModelTest v. 2.1.1 (Darriba et al. 2012) as the best model for the phylogenetic analysis, based on Akaike information criteria (AIC). The BI analysis was conducted using MrBayes v 3.2 (Ronquist and Huelsenbeck 2003) with random starting trees and 1x 107 generations. Diagnostics were calculated every 1000 generations with a 25% burnin to calculate posterior Probability (PP). Phylogenetic trees were rendered using FigTree 1.4.0., edited and annotated by Adobe Illustrator CC 2019 (Adobe, USA). RESULTS Morphology and Life-history Characteristics Class GASTRAPODA Order NUDIBRANCHIA Family Trinchesiidae F. Nordsieck, 1972 Phestilla goniophaga sp. nov. (Figs. 1–4) urn:lsid:zoobank.org:act:11EB8D-E89B-43AA-B439A5A61507C9E2 Type material: Holotype BU-Mol-20-001, live specimen 30 mm in length, ethanol preserved specimen 20 mm in length. ParatypesBU-Mol-20-002 to BUpage 3 of 9Zoological Studies 59:62 (2020) © 2020 Academia Sinica, Taiwan Mol-20-006, live specimens 8–15 mm in length, ethanol preserved specimens 7–17 mm in length. BUMol-20-001 to BU-Mol-20-004 were collected from Sharp Island in June 2019. Specimens BU-Mol-20-005 to BU-Mol-20-006 were collected from Chek Chau in June 2020. Type locality: Eastern Hong Kong waters. Etymology: The species epithet goniophaga, composed of parts of the Latin genus name “Goniopora” and “phaga” (eat), describes the predator-prey relationship between this new species and corals from the genus Goniopora. Geographical distribution: The type specimens were collected from eastern Hong Kong waters including Sharp Island and Chek Chau. It has been found at Tung Ping Chau Marine Park and Wui Pai in north east waters and Bluff Island in eastern waters in our recent surveys, therefore should be a widely distributed species in Hong Kong. Comparing the COI sequences (with query cover of 90.0% and percent identity of 100.0%) deposited in NCBI shows that it is conspecific to Phestilla sp. 2 in Palau (Faucci et al. 2007). Thus, P. goniophaga sp. nov is expected to be widely distributed in the Indo-Pacific region where Goniopora spp. are found. Habitat: 2–5 m water depth, associated with colonies of Goniopora spp., including G. columna, G. djiboutiensis and G. lobata. External morphology of holotype: Mature live specimen length up to 3 cm (Fig. 1B). Body color brownish to pale reddish. Body excluding cerata elongate, dorsoventrally flattened. Notum with transverse wrinkles. Foot broad with rounded edge. Oral veil rounded, extending about 1/3 to 1/2 of the length of oral tentacles. Oral tentacles and rhinophores smooth, digitiform. New born individuals with digitiform cerata, older ones terminally more oblate. Ceras cylindrical, each with a subterminal constriction. Cerata arranged in transverse rows with each row being situated on top of a ridge. Numbers of cerata on left and right sides not always identical. Cerata 16 rows, each row with 4 to 23 cerata; anterior and posterior rows with fewer and shorter cerata than midbody rows. Length of cerata 1 to 8 mm. From eighth row to posterior, distance of lateral rows progressively smaller. Behind eighth row, pericardium forms a white rounded hump, almost halfway down the body (Fig. 1A). Anus and reproductive opening both on right side of body. Anus acleioproctic, located between sixth and seventh rows of cerata. Reproductive opening located between fourth and fifth rows of cerata. Variations in external morphology: Body length of live and ethanol preserved paratypes vary from about 16 mm to 30 mm, and 8 mm to 17 mm, respectively. There are 10 to 14 rows of cerata and up to 9 to 20 cerata per row. The maximum number of cerata occurs in the seventh and eighth row near the hump. The longest cerata appear in the eighth row, about 9 mm in length. Less rows of cerata and fewer number of cerata per row present in juvenile specimen (Fig. 2B). Internal morphology of holotype: Body color of preserved specimen quite opaque. Digestive gland extending into cerata (Figs. 1B–1C, 3A). Color of cerata Fig. 2. Phestilla goniophaga sp. nov. A, Egg masses. B, Veliger with well-developed velum. C, Juvenile with under-developed cerata (paratype BUMol-20-003). Scale bars: A = 4 mm, B = 50 µm, C = 5 mm. page 4 of 9Zoological Studies 59:62 (2020) © 2020 Academia Sinica, Taiwan and digestive gland identical to that of the dinoflagellate Cladocpium sp. (Fig. 3B) in its host coral (Zhang et al. 2019). Glandular region without nematocysts (Fig. 3A). A pair of jaws present in buccal mass, with a radula folded inside. Jaws thin and triangular, without denticles along masticatory edge (Fig. 3C). Radula formula 35 × 0.1.0. Each tooth with a strong central cusp tapering to a tip, six or seven primary denticles on each side, long and slender, slightly different in length (Fig. 3D). Base of central cusp and primary denticles with small secondary denticles. Reproductive system diaulic with both female and male ducts (Fig. 3E). Ovotestis large, consisting of a number of lobules located posterior in coelom. Penial bulb elongated. Penis simple, with a small stylet inside penial sac. Prostate spherical, connecting to a swollen, carrot-like ampulla. Female gland big. Hermaphroditic duct connects to ovotestis. Vas deference narrow and winding. Bursa copulatrix spherical, connecting to vagina. Eggs, egg masses and larvae: The newly laid eggs are orange in color. The developing embryos are light orange. The egg masses are ribbon-shaped, coiled, each ~1 cm in diameter (Fig. 2A). Inside the egg mass, eggs Fig. 3. Internal structures of Phestilla goniophaga sp. nov. A, Glandular region in the tip of a ceras. B, An enlarged part of ceras showing some intact Cladocpium sp. cells. C, A SEM micrograph of the jaw (lateral view). D, Radula. E, Reproductive system. am, ampulla; bc, bursa copulatrix; fgm, female gland mass; pb, penial bulb; pg, penial gland; ps, penial sac; vd, vas deferens. Scale bars: A = 100 μm; B = 10 μm; C = 200 μm; D = 20 μm; E = 5 mm. page 5 of 9 Zoological Studies 59:62 (2020) © 2020 Academia Sinica, Taiwan are arranged in a radical pattern. Each egg mass has thousands of eggs (> 6000) coated with a translucent membrane (Fig. 2A). Development from egg to planktonic veligers with a well-developed swimming velum (Fig. 2B) takes approximately five days. Ecology: Like other species from the genus Phestilla, Phestilla goniophaga sp. nov. possess cerata that mimic the color and shape of Goniopora spp. tentacles (Fig. 1A–1C). It is among the largest species of Phestilla. Individuals of this species together with their orange egg masses are usually found under Goniopora spp. colonies. This new species has been observed to feed on Goniopora columna, G. djiboutiensis and G. lobata, but has not been found to associate with any other genus of corals in Hong Kong. Two to several individuals are often found to live on the same coral colony. During the productive season, they produce up to 20 egg masses on one coral colony, and the egg ribbons are glued tightly on the coral skeleton. Molecular Analysis Nine gene sequences were obtained from three specimens of P. goniophaga sp. nov. Removing the low-quality sites resulted in a 613bp COI fragment, a 410-bp 16S rRNA fragment and a 306-bp H3 fragment. The concatenated DNA sequences of the three genes were used for phylogenetic analysis based on both the Maximum Likelihood (ML) method and the Bayesian inference (BI) method (Fig. 4). The topology of the two analyses were almost identical and most branches had high support values, except with several polytomies where the support values are quite low (ML < 50, PP < 0.9). All the Phestilla species with available sequences except P. sibogae formed a Fig. 4. Phylogenetic tree of nudibranchs from the family Trinchesiidae constructed based on the concatenated sequences of COI, 16S rRNA and H3 genes, inferred by Maximum Likelihood (ML) analysis and Bayesian Inference (BI) analysis. Only ML bootstrap values > 50 and PP value > 0.9 are shown. page 6 of 9Zoological Studies 59:62 (2020) © 2020 Academia Sinica, Taiwan monophyletic clade (posterior probabilities [PP] = 1). Phestilla sibogae is nested inside a clade of several Trinchesia species, rather than with other species of Phestilla. These results are congruent with the results of several previous studies (Cella et al. 2016; Ekimova et al. 2017; Mehrotra et al. 2020; Wang et al. 2020). The three specimens of P. goniophaga sp. nov. form a monophyletic clade with Phestilla sp. 2 from Palau (Faucci et al. 2007) and were deeply nested within the Phestilla clade. Phestilla goniophaga sp. nov. was sister to P. melanobrachia, a predator of corals from the genus Tubastraea (Harris 1968 1975; Salvini-Plawen 1972). Phestilla lugubris, together with the former mentioned species formed a clade that was sister to a clade consisting of P. subodiosa, P. viei and P. fuscostriata. Phestilla poritophages, P. minor and P. chaetopterana were progressively more distantly related to the clade. Uncorrected minimum p-distance among the three individuals of P. goniophaga sp. nov. was small – only 0.3% for COI, 0.8% for 16S rRNA and 0.0% for H3. However, the distances between the new species and its most closely related species P. melanobrachia are much larger – 14.3% for COI, 5.9% for 16S rRNA and 3.9% for H3 (Table S2). DISCUSSION Our analyses showed that the divergence in COI, 16S rRNA and H3 sequences between Phestilla goniophaga sp. nov. and other Phestilla species ranged from 14.3 to 22.6%, 5.9 to 15.5% and 3.9 to 9.7%, respectively (Table S2). These values are much higher than the intraspecific sequence divergences in most molluscan species (i.e., < 2% in COI, Layton et al. 2014), which supports the recognition of the new species. Phylogenetic analyses showed that P. goniophaga sp. nov. is nested within a clade that contains P. melanobrachia, P. lugubris, P. minor and P. poritophages, as well as an undescribed species Tenellia sp. 3 (Cella et al. 2016). Among these species, P. goniophaga sp. nov. is sister to P. melanobrachia. This clade is sister to a clade comprising of P. subodiosa, P. viei, P. fuscostriata and P. chaetopterana. Among Phestilla spp., P. melanobrachia has been known to form obligate association with the scleractinian coral Tubastraea Lesson, 1830 and Dendrophyllia Blainville, 1830 (Harris 1975; Robertson 1970; Ritson-Williams et al. 2003; Fritts-Penniman et al. 2020), while P. lugubris, P. panamica, P. minor and P. poritophages are associated with corals from the genus Porites Link, 1807 (Rudman 1979 1981 1982). Phestilla fuscostriata (Metrotra et al. 2020) and Phestilla subodiosa (Hu et al. 2020) is a predator of Pavona explanulata (Lamarck, 1816) and Pavona decussata (Dana, 1846), respectively. Phestilla viei is a predator of Montipora Blainville, 1830 (Wang et al. 2020). Therefore, our phylogenetic analysis support the hypothesis that this host-specificity together with the monophyly of Phestilla support the conclusion of previous studies indicating corallivory in these nudibranchs has evolved only once, consisting with that in previous studies (Cella et al. 2016; Fritts-Penniman et al. 2020), although since then there has been host shift from one genera to another (Faucci et al. 2007). There has been some confusion in the taxonomy history and diet of Phestilla sibogae. This species was originally described as a predator of Porites. However, Rudman (1981) considered it to be identical with P. lugubris. Gosliner et al. (2018) found that this species (as Tellelia sibogae) is a predator of hydroides from the genus Sertularella. The sequence of P. sibogae we used was taken from Cella et al. (2016), who found it to be a predator of hydroids (as Tenellia sibogae) by Gosliner et al. 2018. In the phylogenetic tree, it was nested with Trinchesia species which are known to feed on hydroids, outside the Phestilla clade. Our results therefore indicate that the sequence of P. sibogae used by Cella et al. (2016) was not likely from P. sibogae or P. lugubris, but a species of Trinchesia. Although no Goniopora-eating nudibranch species was formally named prior to this study, our analysis showed that Phestilla sp. 2 collected from Goniopora in Palau (Faucci et al. 2007) should be conspecific of P. goniophaga sp. nov. based on their high similarity in COI (100%) and 16S rRNA (99.75%) sequences. As such, we believe that P. goniophaga sp. nov. is widely distributed in the tropical and subtropical Indo-Pacific together with its host coral Goniopora (Veron 2000). Morphologically, P. goniophaga sp. nov. can be distinguished from its congeneric species in five aspects. First, with up to 16 rows of cerata and 23 cerata in a single row, this new species has the largest number of cerata. Several species of this genus, such as P. fuscostriata, P. chaetopterana, P. viei and P. subodiosa have only a small number of cerata, likely due to paedomorphosis – the retention of larval trait of only few cerata in the adult. Several other Phestilla species possess a large number of cerata, but not numerous as in P. goniophaga sp. nov. For instance, P. panamica has up to 13 rows of cerata, and each row has up to 18 cerata (Rudman 1982). Phestilla lugubris has up to 15 rows of cerata, and each row has up to 18 cerata (Rudman 1981). Phestilla melanobrachia has up to 15 rows of cerata, and each row has up to 15 cerata (Harris 1968). Second, with the exception of P. melanobrachia, species in the clade that includes P. panamica, P. lugubris, P. minor and P. viei share a common characteristic page 7 of 9Zoological Studies 59:62 (2020) © 2020 Academia Sinica, Taiwan that the pericardium forms a swollen hump on the notum (Rudman 1979 1981 1982). In P. lugubris, the hump situates at about anterior one-third of the notum (Rudman 1981), but in P. goniophaga sp. nov., P. minor and P. panamica, it is located in roughly the middle of the notum (Rudman 1982). Other species of Phestilla do not have an apparent hump on the notum. Third, the radula morphology of P. goniophaga sp. nov. is also unique. Among the recognized species of Phestilla, only P. lugubris and P. melanobrachia also possess a radula with a strong central cusp flanked by six to seven long, slender, and pointed primary denticles (Rudman 1981; Korshunova et al. 2017). However, unlike P. lugubris whose secondary denticles of the central cusp are quite large (~1/2 length of central cusp), those of P. goniophaga sp. nov. are usually very small (~1/5 length of central cusp). Fourth, the reproductive system of P. goniophaga sp. nov. resembles that of P. panamica and P. lugubris to some extent (Rudman 1981 1982), with respect to the shape of the organs and their relative position. However, in P. melanobrachia, its closely related species, the shape of ampulla and its relative position to female gland mass are different. Fifth, the color and shape of the egg mass of P. goniophaga sp. nov. are unique. No other Phestilla species lays orange color and coiled ribbon-shaped egg masses. CONCLUSIONS We report Phestilla goniophaga sp. nov. – the first formally named predatory nudibranch of the scleractinian coral Goniopora spp. This new species has a white hump on the notum and long cerata that resembles the polyp mouth and tentacles of its host coral, respectively. It can be distinguished from other species of Phestilla by the large numbers and rows of cerata, the small secondary denticles on the central cusp of its radula, and the orange colored and coiled egg masses. Analyses of the COI, 16S rRNA and H3 gene sequences confirmed that P. goniophaga sp. nov. is a member of Phestilla, and it is distinct from other congeneric species. Although there have been studies in the biodiversity of nudibranchs in the northern South China Sea (Su et al. 2009), little is known about the diversity of coral-eating Phestilla. Together with Hu et al. (2020), our present study indicates that coral-eating nudibranchs in this region may have been overlooked in previous field surveys of nudibranchs. Given that their feeding activities may pose a threat to coral heath that is already deteriorating due to various human stressors (Mehrotra et al. 2020; Xie et al. 2020), it is essential to further discover their diversity and experimentally determine their potential ecological impact. Acknowledgments: This work and the new species name were registered with ZooBank under urn:lsid:zoobank.org:pub:07A24203-37DD-47C3ADB3-1D2951AE73A0. This study was supported by Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0404, GML2019ZD0404, L20190005), and Environment and Conservation Fund (34/2019) and General Research Fund (12102018) from the government of the Hong Kong Special Administrative Region of China. We thank Mr. Yu Zhao for taking some of the photographs used in this paper, and Ms. Yip Hung Yeung, Mr. Keith Kei and Ms. Sheena Chung for collecting some of the samples. Authors’ contributions: JWQ initiated the study. JH conducted the morphological analysis and drafted the manuscript, JH and YZ conducted the molecular analysis, SKFY conducted the ecology survey, YJX first discovered and collected some of the samples. All authors revised the manuscript. Competing interests: JH, YZ, SKFY, YJX and JWQ declare they have no conflict of interest. Availability of data and materials: Nine DNA sequences of Phestilla goniophaga sp. nov.—three from the COI gene, three from the 16S rRNA gene and three from the H3 gene—are deposited in GenBank (accession numbers in Supplementary Material Table S1). The type specimens are deposited in Hong Kong Baptist University (catalog numbers BU-Mol-20-001 to BUMol-20-006). Consent for publication: All of the authors agreed to publish the paper. Ethics approval consent to participate: Not applicable. REFERENCES Cella K, Carmona L, Ekimova I, Chichvarkhin A, Schepetov D, Gosliner TM. 2016. A radical solution: the phylogeny of the nudibranch family Fionidae. 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Supplementary materials Table S1. GenBank accession numbers of the sequences used to reconstruct the phylogeny of Phestilla spp. and their related genera/species. (download) Table S2. Uncorrected COI/16S rRNA/H3 p-distances (%) among all species of described Phestilla with available sequences. (download) page 9 of 9Zoological Studies 59:62 (2020)