The Nudibranch superfamily Fionoidea is a highly diverse group of marine Sea Slugs, containing 20 families, 52 genera and 324 species. ike many other nudibranchs, however, there has been some controversy with the systematics and phylogeny of Fionidea. In the family Trinchesiidae, the majority of species are free-living, though some form obligate association with their prey species on which they settle, feed and lay eggs. Among the most well-known examples of such obligate relationship are those between the genus Phestilla, and certain Acroporid, Agariciid, Poritid and Dendrophylliid Coral species: the Nudibranchs live on these Corals, feed on them, and their larvae show host specificity in settlement. Different from other genera of Trinchesiidae, Phestilla does not possess a cnidosac (an anatomical feature in which the Sea Slug stores cnidocytes, Cnidarian stinging cells harvested from its prey and used for its own defence) at the tip of its cerata, but has a glandular region in this location. The genus Phestilla currently has nine recognized species, among which, eight are obligate corallivores. Field observation and laboratory studies have revealed the specificity between Corals and their Nudibranch predators, with a particular species of Phestilla feeding on only one species or genus of Coral, which implies host shifts are associated with speciation.
In a paper published in the journal Zoological Studies on June 2020, Juntong Hu of the Department of Biology at Hong Kong Baptist University, Yanjie Zhang, also of the Department of Biology at Hong Kong Baptist University, and of the Southern Marine Science and Engineering Guangdong Laboratory, James Yang Xie, again of the Department of Biology at Hong Kong Baptist University, and Jian-Wen Qiu, once again of the Department of Biology at Hong Kong Baptist University, and of the Southern Marine Science and Engineering Guangdong Laboratory, describe a new species of Phestilla from the Coral Pavona decussata, a structure forming Agariciid species in the South China Sea.
Colonies of the Scleractinian Coral Pavona decussata were collected from Sharp Island, Hong Kong, at depths of about 2 m, in August 2018 by SCUBA diving and then cultured in the laboratory in an aquarium system. Nudibranchs and their egg masses, found on the surface of the Pavona decussata in October 2018, were collected from the Coral surface. The specimens were preserved either in 95% ethanol for molecular study or in 4% formaldehyde in seawater for morphological analysis. All specimens examined werer deposited in the collection of the Swire Institute of Marine Science, at the University of Hong Kong.
The new species is named Phestilla fuscostriata, from the Latin 'fuscus' meaning 'brown' and 'striatus' meaning 'streaky', refers to the brown stripes on the body, which is a morphological character of the new species.
A colony of the Coral Pavona decussata showing three adults of the Nudibranch Phestilla fuscostriata (indicated by red arrows) and many crescent-shaped egg masses of the Nudibranch on the Coral surface. Scale bar is 1.0 cm. Hu et al. (2020).
Living specimens of Phestilla fuscostriata are 2 mm to 8 mm in length. The body excluding the cerata is elongate and dorsal-ventrally flattened. The general body colour is white with dense brown pigmentation on dorsal side of head, tentacles, body and cerata. Ethanol preserved specimens are white due to loss of brown pigmentation.
The oral tentacles and rhinophores are digitiform; in adults the former are approximately twice as long and twice the diameter of the latter. A very small eye is present behind the rhinophore. The cerata are digitiform, swollen distally, and arranged in seven transverse rows in the holotype, each row consisting of 1 to 6 cerata attached laterally on a distinctly raised ridge on each side of the body, with the number of cerata decreasing from anterior to posterior. Fewer rows of cerata and fewer cerata per row are present in juveniles. Within a row, one single pair of dorsal cerata and zero to several pairs of ventral cerata are present. In the holotype, there are seven pairs of dorsal cerata, and six rows of ventral cerata with 5, 5, 4, 3, 2 and 1 pair from first row to the sixth row, respectively. The longest cerata on the second row is approximately 1.5 times as long as the body width. A translucent glandular region present at the tip of each ceras. Comparing adult and juvenile specimens indicates that the dorsal cerata develop earlier than the ventral cerata.
The anus is acleioproctic, located dorsally on right side of body between the third and fourth rows of cerata. The reproductive opening is located anterior to the first row of cerata, on right side of body.
Living specimens of Phestilla fuscostriata. (A) Holotype, SWIMS-Mol-19-001, dorsal view. (B) Holotype, SWIMS-Mol-19-001, ventral view; (C) Paratype, SWIMS-Mol-19-002, dorsal view. (D) Paratype SWIMS-Mol-19-002, ventral view. (E) Paratype, SWIMS-Mol-19-005, dorsal view. (F) Paratype, SWIMS-Mol-19-005, ventral view. Scale bars are 1.0 mm. Hu et al. (2020).
Eggs of Phestilla fuscostriata are white, 0.2 mm in diameter, and clearly observable through translucent body wall on the ventral side. Egg masses are crescent-shaped, about 0.25 cm in diameter, and have a translucent membrane enclosing around 20–50 eggs. At about 24°C, eggs develop into veliger larvae and break through membrane in 2–3 weeks. Veligers have a pair of black eyes and a well-developed swimming velum. Newly settled juveniles are more elongate, the velum is lost, but the oral tentacles or cerata have not yet developed. After roughly one week, juveniles resemble adults, with black eyes, but with tentacle and cerata, although at this stage cerata few and small.
Early developmental stages of Phestilla fuscostriata. (A) Embryos inside egg membrane. (B) Rudimentary veligers without well-formed shells inside egg membrane. (C) Veligers with well-formed shells. (D) Hatched veliger with well-developed velum for swimming. (E) Postlarva, dorsal view. (F) Early juvenile, with the head turned to the left when the photograph was taken. Scale bars: (A)–(C), (F) 500 μm; (D)–(E) = 40 μm. Hu et al. (2020).
Phestilla fuscostriata resembles its host coral P. decussata in the coloration pattern, therefore exhibiting excellent camouflage. Hu et al. were unaware of its presence in our aquarium system until this nudibranch built up a dense population on Pavona decussata, which eventually killed some of the colonies. The only known food source for Phestilla fuscostriata is Pavona decussata. When other species of Scleractinian Coral such as Platygyra carnosa and Acropora digitifera were also present in the same aquarium, the Nudibranch was found only on Pavona decussata, which indicates its host specificity. During reproduction, this Nudibranch deposits egg masses and glues them tightly on the surface of the Coral colonies.
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