Janolus hyalinus

Janolus hyalinus (Alder & Hancock, 1854)

Janolus hyalinus @ Etang de Thau, France March 2021 by Sylvain Le Bris










































Janolus hyalinus  (Alder & Hancock, 1854)

 Classification according to Bouchet et al. (2017)
Taxonomic source: World Register of Marine Species (AphiaID: 140857).

  • 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, where it is common in the British Isles and the Netherlands, as well as in French Brittany. It is present in the Mediterranean, where it has been reported in France, Italy and Spain. The citations for Morocco come from the study of Templado (1984) in the Spanish autonomous city of Ceuta. The Portuguese reports attributed to Calado (2003) actually correspond to Antiopella cristata, but Da Costa (2008) cites J.hyalinus in the Aveiro estuary (NW Portugal). Reports from New Zealand (Green, A. in Sea Slug Forum, 2001) are most likely a different species because the photos clearly show that the digestive gland reaches the tips of cerata, which is not the case for the European species. (Picton, 2001 at Sea Slug Forum).

Known georeferenced records of the species: Janolus hyalinus
: GROC 2010-2011
: Enric Madrenas
: João Pedro Silva
: Bernard Picton
: Manuel Ballesteros.
: M@re Nostrum
: Altres fonts
: Marine Regions

References for the species: Janolus hyalinus

Similar species
Proctonotus mucroniferus, of Atlantic distribution, does not have a sensory caruncle between the rhinophores, which are wrinkled and have no lamellae.


    Western Mediterranean:1 out of 5 stars
    Eastern Mediterranean:0 out of 5 stars
    Atlantic Ocean:2 out of 5 stars

This chart displays the monthly observation probability for Janolus hyalinus based on our own records.


Janolus hyalinus @ Baie d’Arcachon, France by Jean Pierre Segonnes

Janolus hyalinus by Pascal Girard


More pictures


Further reading

Cite this article as:

Pontes, Miquel, Manuel Ballesteros, Enric Madrenas (2021) "Janolus hyalinus" in OPK-Opistobranquis. Published: 16/05/2012. Accessed: 02/08/2021. Available at (https://opistobranquis.info/en/P5Nui)

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