Janolus hyalinus (Alder & Hancock, 1854)
Class: Gastropoda Cuvier, 1797
Subclass: Heterobranchia J.E. Gray, 1840
Clade: Euthyneura Spengel, 1881
Clade: Nudipleura Wägele & Willan, 2000
Order: Nudibranchia Cuvier, 1817
Suborder: Dexiarchia Schrödl, Wägele & Willan, 2001
Infraorder: Cladobranchia Willan and Morton, 1984
Parvorder: [unassigned] Cladobranchia Willan & Morton, 1984
Family: Proctonotidae J.E. Gray, 1853
Genus: Janolus Bergh, 1884
Species: Janolus hyalinus (Alder & Hancock, 1854) [Antiopa]
- Antiopa hyalina Alder & Hancock, 1854 (original)
- Antiopella hyalina (Alder & Hancock, 1854)
- Janolus flagellatus Eliot, 1906
Medium sized body, it can grow up to 30mm in length, relatively wide and with a short tail. The color of the body is translucent yellowish white, with numerous reddish brown spots and some white spots. The central area of the dorsum is intensely pigmented in brown. The cerata are not smooth, but instead have their surface covered with small warts. The cerata are tappered and are arranged all around the animal (also in the front and back sides) forming transverse rows of 4 or 5 ceras, of which the interior ones are longer and reduce their length to the outside until they end up as mere tubercles; all of them are transparent, with some white and brown pigment on their surface and show, near their base, the dark brown digestive gland that runs up to halfway of the cerata. This gland is not seen in front nor rear cerata. Sometimes the cerata hide the animal’s back. The brown rhinophores have a truncated apex and numerous elongated, rib-like lamellae, obliquely oriented in relation to the rhinophore, with numerous “warts” that concentrate yellowish-white spots. The eyes, small, are located at the base of each rhinophore. The inter-rhinophoric caruncle is very apparent, it is elongated, relatively large and follows the chromatic pattern of the animal. The foot is relatively broad, with the anterior margin somewhat wavy and cleft in the middle part; lacks any propodial lobes or tentacles. The pedal sole is whitish and allows the internal organs to be seen through by transparency. The sides of the head have a “ribbon” that extends downwards, towards the margins of the foot, forming some kind of crest running along these margins. There is a small fold around the head that forms a small oral veil that extends on the sides to form two differentiated oral tentacles. The anus is in a dorsal posterior position.
It apparently feeds on bryozoans such as Bugula fastigiata (Cornet and M. Marchad, 1951), Scrupocellaria spp. (Picton, 1984) or Tricellaria inopinata (Faasse, M. com.pers.). The spawn consists of a wavy spiral arranged around its food and with relatively small eggs compared, e.g. with Janolus cristatus. It is an extremely cryptic species on the substrate so it has been rarely observed; in fact, when it is at rest it can be easily confused with an anemone. It is found from the low tide down to about 20 meters deep. In the Atlantic Ocean it is usually found under rocks with abundant bryozoans’ growth, whereas in the Mediterranean it seems to be more frequently found in the Posidonia oceanica meadows (Schmeckel, 1968; Haefelfinger, 1960). The specimens of J. hyalinus often readily detach their cerata if disturbed. These detached cerata continue to move for a while to distract the predator and allow the animal to escape, but they do so with a vertical movement, and not in a circle as in many aeolidaceans. This species is very similar to Proctonotus mucroniferus but they are distinguished basically by the presence of the inter-rhinophoric caruncle, typical in Janolus and absent in Proctonotus.
- Janolus is referred to the Greek god Janus, the one with two heads, god of beginnings and transitions (the month January is devoted to him), protector of homes (hence he’s represented with a key).
- Hyalinus, word derived from Greek: ὑάλινος meaning “transparent”.
It is present on the European coasts of the Atlantic Ocean, with many reports around the British Isles and on the coasts of The Netherlands and some from French Brittany. It is also present in the Mediterranean, where it has been cited in Villefranche sur Mer, France (Haefelfinger, 1960), Banyuls sur Mer, France (Vayssière, 1913; Thompson and Brown, 1976) and Naples, Italy (Schmeckel, 1968). The first report for the Iberian Peninsula took place in Cantabria (Ortea, 1978) with subsequent reports from Galicia (Urgorri & Besteiro, 1983), the Strait of Gibraltar (García Gómez et al. , 1989), where some authors report it in Morocco, as it was found in the Spanish autonomous city of Ceuta; in Spanish Levantine Coast (Templado, 1984) and Catalonia (Ballesteros et al., in press). Reports of this species in Portugal (Cervera et al., 2004) attributed to Calado (2003) correspond in fact to Janolus cristatus, and no mention is made of the presence of J. hyalinus in Portuguese waters until the Ph.D. thesis by Da Costa, M.D.N. (2008) citing this species in the estuary of Aveiro (NW Portugal). New Zealand reports (Green, A. in Sea Slug Forum, 2001) are most likely a different species because in Green’s photos it is clearly seen that the digestive gland reaches the tip of the cerata (Picton, 2001 in Sea Slug Forum) something that does not happen in the European species.
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: Enric Madrenas
: João Pedro Silva
: Bernard Picton
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