Phestilla fuscostriata, Hu & Zhang & Xie & Qiu, 2020

Hu, Juntong, Zhang, Yanjie, Xie, James Yang & Qiu, Jian-Wen, 2020, Fig. 3 in Fig. 21. Sesarmops mora n in Paralbunea dayriti, Zoological Studies (Zool. Stud.) 59 (30), pp. 1-11 : 3-7

publication ID	https://doi.org/ 10.6620/ZS.2020.59-30
persistent identifier	https://treatment.plazi.org/id/5A24306B-FFBB-0E47-FC2E-0F14FAF0F3E8
treatment provided by	Felipe
scientific name	Phestilla fuscostriata
status

Treatment

Family Trinchesiidae F. Nordsieck, 1972 View in CoL

Phestilla fuscostriata sp. nov. ( Figs. 1–5 View Fig View Fig View Fig View Fig View Fig ) urn:lsid:zoobank.org:act:3C3AE4E9-414F-4D05-A674-E4580B7F1ED2

Type material: Holotype SWIMS-Mol-19-001, living specimen 8 mm in length; Paratypes SWIMS- Mol-19-002 to SWIMS-Mol-19-007, living specimens 2–8 mm in length. All type specimens were collected on 26 October 2018 from the surface of Pavona decussata colonies cultured in an aquarium at Hong Kong Baptist University. The coral colonies were originally brought back from Sharp Island, Hong Kong on 21 August 2018 by James Xie.

Type locality: Sharp Island, Hong Kong.

Etymology: The species epithet fuscostriata , from the Latin fuscus (brown) and striatus (streaky), refers to the brown stripes on the body, which is a morphological character of the new species.

Geographical distribution: This species is currently known from eastern Hong Kong waters including the type locality Sharp Island (this study) and Chek Chau ( Wong et al. 2017) in Hong Kong. Given the wide distribution of its host coral P. decussata in the Indo-Pacific region ( Veron 2000) including Hong Kong ( Xie et al. 2017 2020), we expect this species to have a wider distribution, especially along the northern coasts of the South China Sea.

Habitat: Shallow water, 1–4 m water depth, on the surface of P. decussata colonies ( Fig. 1 View Fig ).

External morphology: Living specimens 2 mm to 8 mm in length ( Fig. 2A–F View Fig ). Body excluding cerata elongate, dorsal-ventrally flattened. General body colour white with dense brown pigmentation on dorsal side of head, tentacles, body and cerata. Ethanol preserved specimens white due to loss of brown pigmentation.

Oral tentacles and rhinophores digitiform; in adults the former approximately twice as the length and twice the diameter of the latter ( Fig. 2A–D View Fig ). A very small eye present behind rhinophore ( Fig. 2F View Fig ). Cerata digitiform, swollen distally, and arranged in seven transverse rows in holotype, each row consisting of 1 to 6 cerata attached laterally on a distinctly raised ridge on each side of the body, with number of cerata decreasing from anterior to posterior. Less rows of cerata and fewer number of cerata per row present in juveniles ( Figs. 2E– F View Fig , 4F View Fig ). Within a row, one single pair of dorsal cerata and zero to several pairs of ventral cerata present. In holotype, dorsal cerata seven pairs, ventral cerata six rows with 5, 5, 4, 3, 2 and 1 pair from first row to the sixth row, respectively. Longest cerata on the second row, approximately 1.5 times as long as body width. A translucent glandular region present at the tip of each ceras ( Fig. 2A–F View Fig ). Comparing adult and juvenile specimens ( Figs. 2A–F View Fig , 4F View Fig ) indicates that dorsal cerata develop earlier than ventral cerata.

Anus acleioproctic, located dorsally on right side of body between third and fourth rows of cerata. Reproductive opening located anterior to first row of cerata, on right side of body.

Internal morphology: Some internal structures such as white-coloured buccal mass, digestive gland, and oocytes clearly visible through translucent body wall ( Figs. 2B View Fig , 3A–C View Fig ). Digestive gland with branches extending to each oral tentacle and ceras throughout body ( Figs. 2A View Fig , 3A–B View Fig ). Digestive gland within cerata brown throughout the length, with a translucent outer body wall and apex ( Fig. 3B View Fig ). Colour of digestive gland and cerata reflects that of cells of the dinoflagellate Cladocpium sp. ( LaJeunesse et al. 2018), presumably ingested from the coral host; many such dinoflagellate cells remain intact and clustered ( Fig. 3C View Fig ). Glandular region contains large glandular cells but lacks nematocysts ( Fig. 3B View Fig ).

A pair of jaws present in buccal mass, with a radula inside. Jaws thin and triangular, with no distinct serrations present along masticatory border ( Fig. 3D View Fig ). Radula formula 18 × 0.1.0. In each row tooth sizes different, with a medium-sized central cusp, three to four smaller secondary denticles on each side of central cusp, and seven to eight large primary denticles with one to two secondary denticles between them ( Fig. 3E View Fig ).

Reproductive system diaulic with both female and male ducts ( Fig. 3F View Fig ). Ovotestis large, consisting of a number of lobules located posterior in coelom ( Fig. 2B View Fig ). Hermaphroditic duct connects to ovotestis on one end and swollen ampulla on the other end. Prostate swollen and elongate. Vas deference narrow and winding. Penis simple, with a small stylet inside penial sac. Bursa copulatrix spherical.

Eggs, egg masses and early juveniles: Eggs white, 0.2 mm in diameter, clearly observable through translucent body wall on ventral side ( Fig. 2B, 2D View Fig ). Eggs masses crescent-shaped, ~ 0.25 cm in diameter ( Fig. 1 View Fig ), with a translucent membrane enclosing around 20– 50 eggs ( Fig. 4A View Fig ). At ~ 24°C, eggs would develop into veligers and break through membrane in 2–3 weeks ( Fig. 4B and C View Fig ). Veliger with a pair of black eyes and a welldeveloped swimming velum ( Fig. 4D View Fig ). Newly settled juveniles more elongate, velum lost, but oral tentacles or cerata not yet developed ( Fig. 4E View Fig ). After roughly one week, juveniles resemble adults, with black eyes, but with tentacle and cerata, although at this stage cerata few and small ( Fig. 4F View Fig ).

Ecology: Phestilla fuscostriata sp. nov. resembles its host coral P. decussata in the coloration pattern, therefore exhibiting excellent camouflage. We were unaware of its presence in our aquarium system until this nudibranch built up a dense population on P. decussata , which eventually killed some of the colonies. The only known food source for the new species is P. decussata . When other species of scleractinian corals such as Platygyra carnosa and Acropora digitifera were also present in the same aquarium, the nudibranch was found only on P. decussata , which indicates its host specificity. During reproduction, this nudibranch deposits egg masses and glues them tightly on the surface of the coral colonies ( Fig. 1 View Fig ).

Molecular Analysis

Six new gene sequences were obtained for three genes based on DNA extracted from two P. fuscostriata sp. nov. specimens. Removing the low-quality sites at the two ends of the raw sequences resulted in a 671- bp COI fragment, a 448-bp 16S rRNA fragment and a 349-bp H3 fragment. The concatenated DNA sequences of the three genes were used for phylogenetic analysis based on the Maximum Likelihood ( ML) method ( Fig. 5 View Fig ). Exploratory analyses were also conducted for each gene. The topology of the COI tree is similar to that of figure 5, although the former has lower bootstrap support values at several nodes ( Fig. S1 View Fig ). The 16S rRNA and H3 trees also support the conclusion that P. fuscostriata sp. nov. is a distinct species, but compared with the COI tree their topology is more dissimilar with that of figure 5.

The two concatenated sequences of the new species formed a monophyletic clade and were deeply nested within the clade containing most of the Phestilla sequences ( Fig. 5 View Fig ), which is in accordance with the results of previous studies ( Cella et al. 2016; Ekimova et al. 2017; Mehrotra et al. 2020; Wang et al. 2020). Phestilla fuscostriata sp. nov. is sister to P. viei , which is associated with another species of Pavona (i.e., P. explanulata ) ( Mehrotra et al. 2020). Except for P. sibogae , all recognized Phestilla species are included in a clade, but the maximum likelihood bootstrap value (62) was not very high. Phestilla sibogae , however, is different from all other Phestilla spp. in that it is nested in a clade with some Trinchesia species, as well as several undescribed species assigned to Tenellia by Cella et al. (2016); this topology is consistent with the results of previous studies ( Cella et al. 2016; Ekimova et al. 2017; Mehrotra et al. 2020; Wang et al. 2020).

The results of the uncorrected pairwise distances for the three genes showed that the minimum interspecific distances of COI, 16S rRNA and H3 were 12.5%, 4.8% and 3.2%, respectively, which are substantially larger than the intraspecific differences between the two sequenced specimens of P. fuscostriata sp. nov. (0.5% for COI, 1.5% for 16S rRNA and 0.6% for H3 ( Table S2). Among these intraspecific distances, the smallest values for the three genes came from P. viei , which is consistent with figure 5 in identifying P. fuscostriata sp. nov. and P. viei as sister species.