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Author: (Kelaart, 1858)
Order: Nudibranchia Family: Polyceridae
Maximum Size: 55 mm
Sightings: Sunshine Coast
Plocamopherus ceylonicus (Kelaart, 1858)
Within the nudibranch Polyceridae family there are some similar slugs grouped as a subfamily under the name of Triophinae. Probably the best known genera in that subfamily include: Plocamopherus, Kaloplocamus, Crimora and Limacia. Within that subfamily, Plocamopherus and Kaloplocamus are even more closely related with some species of both genera featuring the ability to emit bioluminescence (Defined as: the emission of visible light, produced through chemical reaction, by animals or (rarely) their symbionts).
Plocamopherus ceylonicus is a moderately sized species that we have recorded to 55 mm in length however, smaller examples are often found in intertidal pools. The body is elongate in form and high in profile.The mantle skirt is much reduced laterally to a narrow ridge down along the sides of the body however, anteriorly there is a substantial, expanded and rounded oral veil. The edge of the oral veil carries numerous papillae, many small, but also approximately six of a comparatively large size that are compound i.e. with many branches. The lateral notal ridge also carries papillae of which two pair of these are particularly large and bear brown to pink terminal globular knobs. One pair are situated anterior to the gill with the other pair posterior to the gill, this seemingly indicating a protective defensive nature to their positioning. These globular knobs are unique to the Plocamopherus genus. Small papillae are also scattered over the notum especially on the midline crest of the tail. This crest is formed by the junction of the lateral ridges posterior to the rear-most knob-bearing pair of papillae. The foot is wider than the notum and keel-like at the tail. The tail extends further posteriorly into a distinct flagellum of varying length.
The rhinophore stalks are substantial and carry clubs, with oblique lamellae. The clubs gradually taper to a small plain apical knob. The rhinophores can retract into pockets that have slightly raised rims carrying small papillae. The gill is large, expanded and unable to be retracted (phanerobranch). It consists of from four to five tripinnate branches surrounding the anal papilla.
There are a number of colour forms throughout the Indo-Pacific region mostly related to the degree of dark brown pigment present. The background colour is cream with a brown patterned reticulation that may vary in density (giving rise to the so-called light and dark forms). Distinctive and characteristic yellow/orange spots are present upon the sides of the body, the dorsal surface of the foot and tail and also at times scattered over the notum. The rhinophore stalks are translucent with brown/orange specks and the clubs brown often with white pigment to the edge of the lamellae. The gill is coloured similarly to the body with the centreline of each gill branch carrying a white line. Viewed upon its typical habitat it is extremely cryptic.
The two pair of globular knobs surrounding the gill in Plocamopherus ceylonicus possess a light emitting capability. This bioluminescence is only known in two genera of the dorids – Plocamopherus and Kaloplocamus however, thus far, not all species have been recorded with the ability. It has been postulated that because the light is emitted as a flash rather than a glow it acts as a temporary startle distraction against a potential predator. The flashing knob may, apart from startling a predator, also lead it towards a portion of the body that can be sacrificed and away from the gill. Specimens are observed at times with missing knobs or in the process of regeneration. Tissue of the body wall is also capable of producing bioluminescent flashes but not of the same intensity. The bioluminescence process of Plocamopherus is not reliant upon symbiotic bacteria but is rather the result a chemical reaction outside the cells hence it is classified as extracellular bioluminescence.
Plocamopherus ceylonicus is well-known for its swimming action when disturbed. This is made possible by the formation of the tail portion into a keel-like paddle by contracting and laterally flattening into a paddle shape, not unlike the caudal fin of a fish, combined with strong lateral flexions of the whole body length. Direction and efficiency of the swimming is relatively ineffectual but no doubt aids escape from a predator. It has been recorded maintaining this action for up to 15 minutes. Perhaps the action of lifting off the substrate into the water column is to take advantage of current or surge to put distance between itself and a predator.
Plocamopherus ceylonicus feeds on arborescent bryozoans.
The spawn is yellow and laid as a coil, on edge, of at least three whorls.
Distribution is broad in the Indo-Pacific region from the east coast of Africa across to Hawaii and from southern Japan to central New South Wales, Australia
Originally described as: Polycera (?) ceylonica – (Contrary to WoRMS, which lists the original combination incorrectly as Polycera zeylanica.) The (?) in the name by Kelaart relates to him not being entirely sure of the genus stating: “ I have placed this species, very doubtfully, under the head of Polycera.”
David A. Mullins – October, 2021
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– MolluscaBase eds. (2021). MolluscaBase. Plocamopherus ceylonicus (Kelaart, 1858). Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=404968 on 2021-10-12.
– MolluscaBase eds. (2021). MolluscaBase. Polycera zeylanica Kelaart, 1858. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=531272 on 2021-10-12.