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

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: ★☆☆☆☆
    Eastern Mediterranean: ☆☆☆☆☆
    Atlantic Ocean: ★★☆☆☆

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



More pictures


    Alder J, Hancock A. 1845. A monograph of the British nudibranchiate Mollusca: with figures of all the species. Ray Society, London.
    Alder J, Hancock A. 1854. A monograph of the British nudibranchiate Mollusca: with figures of all the species. Ray Society, London. Available from www.biodiversitylibrary.org/item/131598#page/200/mode/1up.
    Aquenal Pty Ltd. 2001. Exotic marine pests survey Port of Launceston, Tasmania. Aquenal Pty  Ltd., Aquatic Environment Analysts. 104 pp p. Available from http://data.brs.gov.au/mapserv/marinepest/html/portsurvey_tas.pdf.
    Australian Museum Business Services. 2002. Port survey for introduced marine species – Sydney Harbour, final report, viii + 62 pp. Appendix 12 - Mollusca survey results, pp 84-91. Australian Museum and Royal Botanic  Gardens database search for all marine species from Sydney Harbour, 38 unnumbered pages.
    Ballesteros M. 1980. Contribución al conocimientos de los Sacoglosos y Nudibranquios (Mollusca: Opisthobranchia). Estudio anatómico, sistemático y faunístico de las especies del mediterráneo español. Tesis Doctoral. Barcelona: Universitat de Barcelona.
    Ballesteros M, Madrenas E, Pontes M. 2016. Actualización del catálogo de los moluscos opistobranquios (Gastropoda: Heterobranchia) de las costas catalanas. Spira. 6:1–28.
    Ballesteros M, Pontes M, Madrenas E. 2019. Els nudibranquis del mar català. Figueres: Brau Edicions. 192 p.
    Ballesteros M, Madrenas E, Pontes M. 2024. OPK - Opistobranquis. Available from https://opistobranquis.info/.
    Bergh LSR. 1907. The Opisthobranchiata of South Africa. Transactions of the South African Philosophical Society 17(1):1- 144, pls. 1-14, In: Marine Investigations of South Africa 5(1).
    Bergh LSR. 1904. Malacologische Untersuchungen. In: Reisen im Archipel der Philippinen von Dr. Carl Gottfried Semper. Zweiter Theil. Wissenschaftliche Resultate, Vol. 9, Chapter 6, Part 1: 1-56, pls. 1-4.
    Bergh LSR. 1882. Beitrage zur Kenntniss der Aeolidiaden. VII. Verhandlungen der koniglich-kaiserlich Zoologisch-botanischen Gesellschaft in Wien (Abhandlungen) 32:7-74, pls. 1-6.
    Bergh LSR. 1908. Malacologische Untersuchungen, In: Reisen im Archipel der Philippinen von Dr Carl Gottfried Semper. Zweiter Theil. Wissenschaftliche Resultate. Band 9, Theil 6, Heft 3; pp 119-181, pls. 9-12.
    Bielecki S, Cavignaux G, Crouzet JM, et al. 2011. Des limaces de rêve.
    Brown GH, Picton BE. 1979. Nudibranchs of the British Isles — a colour guide. Underwater Conservation Society, Manchester, by R. Earll. 30 p.
    Burn RF. 1958. Further Victorian Opisthobranchia. J. Malacol. Soc. Australia (2):20-36, pls. 6-7.
    Calado G, Malaquias M, Gavaia C, et al. 2003. New data on opisthobranchs (Mollusca: Gastropoda) from the southwestern coast of Portugal. Boletín Instituto Español de Oceanografía 19 (1-4): 199-204.
    Cattaneo-Vietti R. 1986. Alcune considerazioni sui molluschi opistobranchi del Mar Ligure. Lavori Societa Italiana di Malacologia 22:85-96.
    Cattaneo-Vietti R, Barletta G. 1984. Elenco preliminare dei molluschi opistobranchi viventi nel Mediterraneo (Sacoglossa, Pleurobranchomorpha, Acochlidiacea, Aplysiomorpha, Nudibranchia). Bollettino Malacologico (Pubblicazione Mensile Edita dalla Societa Italiana di Malacologia) 20(9-12):195-218.
    Cattaneo-Vietti R, Thompson TE. 1989. Mediterranean opisthobranch molluscs: a zoogeographic approach. Bollettino Malacologico (Pubblicazione Mensile Edita dalla Societa Italiana di Malacologia) 25(5-8):183-204.
    Cattaneo-Vietti R, Chemello R, Giannuzzi-Savelli R. 1990. Atlas of Mediterranean Nudibranchs. Rome, Italy: La Conchiglia. 264 p.
    Cervera JL, Templado J, García-Gómez JC, et al. 1988. Catálogo actualizado y comentado de los Opistobranquios (Mollusca, Gastropoda) de la Península Ibérica, Baleares y Canarias, con algunas referencias a Ceuta y la isla de Alborán. Iberus, supl. 1: 1-84.
    Cervera JL, Calado G, Gavaia C, et al. 2004. An annotated and updated checklist of the opisthobranchs (Mollusca: Gastropoda) from Spain and Portugal (including islands and archipelagos). Boletín Instituto Español de Oceanografía, 20 (1-4): 1-111. L.
    Cockerell TDA, Eliot CNE. 1905. Notes on a collection of Californian nudibranchs. Journal of Malacology 12(3):31-53, pls. 7-8.
    Cornet R, Marchad M. 1951. Inventaire de la faune marine de Roscoff Mollusques. Travaux de la Station Biologique de Roscoff, Suppl 5:1-80.
    Da Costa MDN. 2008. Colonização e sucessão de macrofauna em substratos artificiais. Universidade de Aveiro.
    De Fez S. 1974. Ascoglosos y Nudibranquios de España y Portugal. Valencia, Spain: Consejo Superior de Investigaciones Científicas. 325 p.
    Debelius H, Kuiter RH. 2007. Nudibranchs of the world. Frankfurt: IKAN- Unterwasserarchiv. 360 pp. p.
    Eales NB. 1967. The littoral fauna of the British Isles: a handbook for collectors, 4th ed; pp i-xviii, 1-297, pls. A-H. University Press, Cambridge, England.
    Eliot CNE. 1906. Notes on some British nudibranchs. Journal of the Marine Biological Association of the United Kingdom 7 (3): 333-382 giugno.
    Eliot CNE. 1910. A monograph of the British nudibranchiate Mollusca: with figures of the species; Figures by the late Joshua Alder and the late Albany Hancock, and others. Ray Society, London.
    Farran GP. 1909. Nudibranchiate Mollusca of the trawling grounds of east and south Coasts of Ireland. Department of Agriculture & Technical Instruction for Ireland Fisheries Branch. Dublin. Scientific Investigations, 1907, (6):1-18.
    Fischer MA. 2006. Opisthobranchs from the Chilean coast, a taxonomic, morphological and  histological study of nudibranch species. 173 p.
    Garcia FJ, Bertsch H. 2009. Diversity and distribution of the Gastropoda Opisthobranchia from the Atlantic Ocean: A global biogeographic approach. Scientia Marina. 73(1):153–160. https://doi.org/10.3989/scimar.2009.73n1153.
    García-Gómez JC, Cervera JL, García FJ, et al. 1989. Resultados de la Campaña Internacional de Biología Marina “Ceuta-86”: Moluscos Opistobranquios. Bollettino Malacologico. 25:223–232.
    Long SJ. 2006. Bibliography of Opisthobranchia 1554-2000. Bayside Books & Press, Tustin, CA, U.S.A. 672p.
    Marcus EG. 1958. On western Atlantic opisthobranchiate gastropods. American Museum Novitates (1906):1-82. Available from http://digitallibrary.amnh.org/dspace/bitstream/2246/5128/1/N1906.pdf.
    Marcus EG. 1955. Opisthobranchia from Brazil. Boletim da Faculdade de Filosofia, Ciencias e Letras, Universidade de Sao Paulo, Zoology, 207, 20:89-261, pls. 1-30.
    McDonald G. 2009. Bibliographia Nudibranchia. 2nd Online Edition, Annotated. 1072 pp  Institute of Marine Sciences, University of California, Santa Cruz. Available from http://escholarship.org/uc/item/8115h0wz.
    Miller MC, Willan RC. 1986. A review of the New Zeland arminacean nudibranchs (Opisthobranchia: Arminacea). New Zealand Journal of Zoology 13 (3): 377-408.
    Moro L, Ortea J, Bacallado JJ. 2016. Nuevas citas y nuevos datos anatómicos de las babosas marinas (Mollusca: Heterobranchia) de las islas Canarias y su entorno. Revista de la Academia Canaria de las Ciencias., 48: 9-52.
    Nordsieck F. 1972. Die europäischen Meeresschnecken : (Opisthobranchia mit Pyramidellidae, Rissoacea) : vom Eismeer bis Kapverdem, Mittelmeer und Schawarzes Meer. Stuttgart: Gustav Fischer. XIII, 327 p. p.
    Nunn JD. 2006. The marine Mollusca of Ireland 3. Lough Hyne, Co. Cork. Bulletin of the Irish  Biogeographical Society (30):114-292.
    Ortea J. 1978. Cinco opistobranquios nuevos para la fauna ibérica (Gastropoda: Opistobranchia) colectados en Asturias. Suplemento de Ciencias del Boletín del Instituto de Estudios Asturianos 23: 107-120.
    Ortea JJ, Espinosa. 2000. Nueva especie del género Janolus Bergh, 1884 (Mollusca: Nudibranchia) de Cuba y Costa Rica. Avicennia 12/13: 79-83.
    Ortea J, Llera EM. 1988. Una nueva especie de Janolus Bergh, 1884 (Mollusca: Nudibranchia) dedicada a la memoria de Fausto Gonzalez. Publicações Ocasionais da Sociedade Portuguesa de Malacologia 11: 33-38.
    Ortea JA, Quero A, Rodriguez G, et al. 1989. Estudio de Limacia clavigera Muller, 1776 (Mollusca: Nudibranchia) con nota sobre su distribución geográfica y la válidez de género Laila MacFarland, 1905. Revista de la Facultad de Ciencias de la Universidad de Oviedo 7: 99-107.
    Picton BE, Morrow C. 1994. Janolus hyalinus accessed through: Nudibranchs of the British Isles on 2014-12-14. Available from http://www.habitas.org.uk/marinelife/species.asp?item=W14320#.
    Picton BE, Morrow C. 1994. A Field guide to the Nudibranchs of the British Isles. London: Immel Publishing Ltd. London: 143 pp.
    Pola M, Hallas J, Gosliner TM. 2019. Welcome back Janolidae and Antiopella: Improving the understanding of Janolidae and Madrellidae (Cladobranchia, Heterobranchia) with description of four new species. J Zool Syst Evol Res. 57 : 345–368. https://doi.org/https://doi.org/10.1111/jzs.12257.
    Pruvot-Fol A. 1954. Mollusques opisthobranches. Paris, France: Paul Lechevalier (Faune de France). 460 p.

    Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

Cite this article as:

Pontes, Miquel, Manuel Ballesteros, Enric Madrenas (2023) "Janolus hyalinus" in OPK-Opistobranquis. Published: 16/05/2012. Accessed: 14/04/2024. Available at (https://opistobranquis.info/en/?p=556)

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