Phestilla subodiosa Wang, Conti-Jerpe, Richards & Baker, 2020

Yiu, Sam King Fung & Qiu, Jian-Wen, 2022, Fig. 1 in Parascorpaena poseidon Chou and Liao 2022, Zoological Studies 61 (59), pp. 1-12 : 5-8

Phestilla subodiosa Wang, Conti-Jerpe, Richards & Baker, 2020 ( Figs. 2–4 )

Materials examined: SCSMBC030983 -86, collected from M. peltiformis colonies at Bluff Island (22°19'30.0"N, 114°21'14.8"E), Eastern Hong Kong waters, at 2–6 m water depth.

External morphology ( Fig. 3 ): Mature live specimens are elongated, measuring up to 5 mm in length and 0.5 mm in width. Body is white, with patches of rusty pigments inside cerata and the tissue next to cerata. Both rhinophores and oral tentacles are short digitiform. One pair of light black eyes are located slightly posterior to the base of rhinophore. There are 5 to 6 rows of cerata, and each row comprise one to three pairs of cerata, with larger individuals having more pairs. Cerata of bigger individuals have swollen bulbs that are spherical. Cerata tip lacks a cnidosac.

Internal morphology ( Fig. 4 ): Inside the cerata and the body next to the cerata, there are many symbiotic dinoflagellate cells which give the rusty coloration of the nudibranch. Jaws are translucent and thin, around 0.3 mm in width in a 3 mm long individual. Radula are located inside the jaw, with a formula of 8 × 0.1.0. There are three to four primary denticles on each side of radula, and the denticles are of similar lengths. Reproductive system consists of a large female gland mass, a small penial gland, a vas deferens bridging penile gland and female gland, and a tubular ampulla on the opposite side of the penial gland.

Egg mass and veliger larvae: Egg masses are oval shaped, roughly 1 mm × 0.5 mm in size, with a translucent membrane enclosing around 20 eggs. Each egg is light yellow in colour, roughly 0.2 mm in diameter ( Fig. 5A ). At 24°C, eggs developed into embryos ( Fig. 5B ) and then veliger larvae ( Fig. 5C ) in 2 days without feeding. Veliger larvae have a pair of black eyes and a well-developed swimming velum. After 8–10 days, the veliger larvae lost the velum and shell, and metamorphosed into more elongated juveniles that do not yet have tentacle or cerata ( Fig. 5D ).

Feeding rate on host coral: Twenty individuals of P. subodiosa consumed 0.57 ± 0.24 cm 2, 1.48 ± 0.95 cm 2, 2.70 ± 1.48 cm 2 and 3.80 ± 2.03 cm 2 of M. peltiformis fragment when measured after 24 h, 48 h, 72 h, and 96 h of exposure, respectively ( Fig. 6 ). The mean feeding rate was 0.95 ± 0.51 cm 2 per day.

Non-host coral test: All nudibranchs transferred to the surfaces of the other three coral species were found dead within one hour, but the method of nudibranch killing appeared to differ. Pavona deccussta killed the nudibranch by extruding the mesenterial filaments entangling the prey ( Fig. 7A ). Porites lutea killed the prey using the tentacles outside the body, then slowing digested the prey after it became a slurry ( Fig. 7B ). Duncanopsammia peltata killed the prey after secreting mucus to trap the nudibranch ( Fig. 7C, D ).

Larval metamorphosis: There were no or very few metamorphosed larvae in the first five days, and the metamorphic rates were not significantly different among the three treatments ( Fig. 8 , Tables S5–S 6). Starting from day 6, there were significant differences among the three treatments, with the metamorphic rate in the M. peltiformis conditioned seawater being significantly higher than in the other two treatments. By the end of the experiment at day 10, the metamorphic rate in the M. peltiformis conditioned seawater reached 31.11%, compared to only 11.11% in the P. lutea conditioned seawater, and 6.06% in the control seawater.

Figures

Fig. 2. Selected photographs showing Phestilla subodiosa in the field. A, several adult individuals of Phestilla subodiosa feeding on a fragment of Montipora peltiformis. White arrows indicate the nudibranchs, yellow arrows indicate their egg masses. B, One juvenile and one adult individuals of Phestilla subodiosa feeding on a fragment of Montipora peltiformis. White arrows indicate the nudibranchs, yellow arrows indicate their egg masses. C, A juvenile individuals of Phestilla subodiosa feeding on a fragment of Montipora peltiformis. White arrows indicate the nudibranchs. Scale bars:A– C = 1 mm.

Fig. 3. Selected photographs showing Phestilla subodiosa. and its coral host. A, several individuals of Phestilla subodiosa. feeding on a fragment of Montipora peltiformis, with a clear feeding scar along the lower edge. B, Dorsal view of SCSMBC030984. C–D, Dorsal view and ventral view of SCSMBC030985, respectively; E, Dorsal view of SCSMBC030986. Scale bars:A = 5 mm; B–E = 1 mm.

Fig. 4. Internal morphology of Phestilla subodiosa. A, Close-up of a section of the dorsum and two cerata; note that there is no cnidosac at the tip. B, dinoflagellates inside the cerata and the body (indicated by white arrows indicated). C, Jaw. D, Radula. E, Drawing of reproductive system; fm: Female gland; am: ampulla; pg: penile gland; vd: vas deferens. Scale bars: A = 500 µm; B = 250 µm; C = 150 µm; D = 25 µm; E = 120 µm.

Fig. 5. Early development in Phestilla subodiosa. A, Newly laid egg mass attached to denuded coral skeleton. B, Developing embryos in the egg mass. C, A veliger. D, a post-metamorphosis juvenile. Scale bars: A–B = 500 µm; C = 200 µm; D = 300 µm.

Fig. 6. Feeding rate of Phestilla subodiosa on Montipora peltiformis. Each datum represents mean ± SD of four replicates.

Fig. 7. Photographs showing the individuals of Phestilla subodiosa were killed by non-host coral species. A, Pavona deccussata. B, Porites lutea. C– D: Duncanopsammia peltata.

Fig. 8. The metamorphic rate of larvae response to chemical cue released by host and non-host coral species. Three replicates of each treatment were scored for cumulative metamorphosis. Error bars represent ± 1 SD.