Plocamopherus tilesii Bergh, 1877
Class: Gastropoda Cuvier, 1797
Subclass: Heterobranchia J.E. Gray, 1840
Clade: Euthyneura Spengel, 1881
Clade: Nudipleura Wägele & Willan, 2000
Order: Nudibranchia Cuvier, 1817
Suborder: Euctenidiacea Tardy, 1970
Infraorder: Doridacea Thiele, 1931
Superfamily: Polyceroidea Alder & Hancock, 1845
Family: Polyceridae Alder & Hancock, 1845
Subfamily: Triophinae Odhner in Franc, 1968
Genus: Plocamopherus Rüppell & Leuckart, 1828
Species: Plocamopherus tilesii Bergh, 1877
This species has a maximum reported length of 12cm. The body is oval shaped with the anterior side rounded. It has a translucent, yellowish background color and the whole body is homogeneously speckled with regular black and white spots, some of which are ocellate. There are yellow patches of irregular in size distributed over the entire body, usually associated with papillae. There are also large diffuse brownish stains in fewer numbers. The oral veil is wide and flattened, the margin lined with a yellow or orange coloration, and bears 12-15 black, short and somewhat flattened papillae, where the outermost are the largest, with a series or large spots on the base of each papilla, which seem to be exclusive for this species. Rhinophores are bent, triangular, with a lamellate club, brownish with a white dorsal keel and white tips. The rhinophoral sheaths are short and have large, irregular, black spots on the margin. It has five tripinnate branchial gills standing out on the centre of the back, colored brown internally and decorated externally with black and yellow dots. There are three pairs of short lateral appendages, with the last pairs, flanking the gills, having a prominent globular structure that is white in color. The anterior part of the foot sole is brown, while the rest is yellowish. There are tiny black dots along the foot margin. The rear portion of the foot is short and flattened forming a keel that has a small crest with small marginal papillae. Between the foot sole and the body wall there is a series of transverse very narrow lamellae, most distinct anteriorly and almost indistinguishable posteriorly, that seems to occur only in this species. The anal papilla is prominent, with a papillose yellow tip. The genital opening, which is not prominent and located in the anterior third of the body (close to the head veil) is surrounded by three black spots.
The feeding habits of Plocamopherus tilesii have never been studied in depth, although Rudman and Darvell (1990) reported that this species feeds on bryozoans. Behrens (2006 @ The Slug Site) suggests it may feed on branching hydroids. However a later work by Nakano, R. (2016) concludes that (in Japan) this species feeds on bryozoans (Amastigia rudis, Caberea boryi, C. climacina or C. lata, and Bugula neritina) and ophiura (Ophiothrix exigua), the later predation observed in bigger specimens only. It also has the ability to burrow itself in the sand, that suggests that it may be a soft-bottom dwelling nudibranch, the enlarged oral veil probably used to sense food. It has the ability to swim away when disturbed: it apparently controls the altitude with its broad flat head, tipping it back at 45 degrees until it reaches about 0.5m above the bottom, then setting it into a flatter posture and continued swimming by to producing vigorous lateral movements using its large flat tail (Holmes, 2002 @ Sea Slug Forum). The knob shaped paired appendages on the rear side of the body may, at least in Plocamopherus imperialis and Plocamopherus ceylonicus, flash a bright luminescent light when disturbed. How the light is produced, whether by light emitting bacteria or some other process is unknown. According to Vallés and Gosliner (2005) it is important to note that it may not have been observed in the wild because although a stimulation provokes the animal into swimming and emitting light, when the animal is stressed or receives a continuous mechanical stimulation it stops emitting light, suggesting that there is some kind of product that it stores, that is used for the light emission and therefore its availability is limited (Wilbur & Yonge, 1966). It is possible that since these animals emit light only when disturbed and for a short span of time, the role could be to temporally distract the attention of potential predators (Wilbur & Yonge, 1966).
- Plocamopherus. From Greek “plokamos”, hair tress + “-pherus”, from Latin “fero”, a combining form meaning “bearing”, “producing”, “yielding”, “containing”, “conveying” used in the formation of compound words.
- Tilesii. In honor of the German physician, naturalist and artist Dr. Wilhelm Gottlob Tilesius von Tilenau (1769-1857), who had followed von Krusenstern on the first Russian circumnavigation (and being ennobled to von Tilenau by the czar after that).
It is distributed throughout the western Pacific, where it is known from Japan, China, Phillippines, Papua New Guinea, Eastern and Western Australia (Bergh, 1877; Rudman & Darvell, 1990; Wells & Bryce, 1993; Rudman, 1998; Kohler, 2002; Debelius & Kuiter, 2007) also in the Arabian Sea. There’s also a report from the gulf of Iskenderun, Turkey (Yokeş et al., 2012), the first for the Mediterranean Sea. The absence of this species in the Western Indian Ocean malacofauna suggests that ship-mediated transportation from Western Pacific to Eastern Mediterranean seems to be most possible way of introduction (Yokeş et al., 2012).
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