Hydatina physis

Hydatina physis (Linnaeus, 1758)

Hydatina physis by Sonja Ooms

































Hydatina physis  (Linnaeus, 1758)

 Classification according to Bouchet et al. (2017)
Taxonomic source: World Register of Marine Species (AphiaID: 215423).
Taxonomic note: Odhner named Hydatina stromfelti the Atlantic form of this species, later synonymized with Hydatina vesicaria, a thesis supported by Voskuil (1995) and currently accepted by WoRMS (2016),  however, according to Rudman (Sea Slug Forum, 2010) it seems there are no recognisable anatomical differences and no consistent shell differences between them and Hydatina physis so he concluded that they were all a single species with a circumtropical distribution.


  • Aplustrum virgatum Mörch, 1852
  • Bulla atrolineata Schröter, 1804
  • Bulla physis Linnaeus, 1758 (original)
  • Bulla quoyana d’Orbigny, 1845
  • Bulla staminea Menke, 1835
  • Hydatina filosa Schumacher, 1817

This species can grow up to 60 mm (Kensley, 1973) but there are reports of bigger animals up to 92mm (Pittman & Fiene, 2016) in Hawai’i, (total length extrapolated from the shell’s size). The body colour varies from pale rose, almost white, to reddish brown, with the foot and cephalic shield lobes displaying a vivid iridescent blue coloured margin. The shell is globose, of ‘bubble’ type, thin, fragile, colored translucent white, and convoluted in three turns. The bigger (or body) turn is large enough to cover the other turns. The shell has noticeable brown to black spiral lines, usually two slightly broader lines bordering one or more narrower lines. The aperture of the shell is very large, but the complete body can barely retract into the shell, and they seldom do so. The operculum is missing, the columella and columellar muscle are very reduced. The head bears two black eyes located between the cephalic shield lobes. Close to them are the Hancock’s Organ (a chemosensory organ in primitive cephalaspidean sea slugs), that consist of a row of folds or ‘flaps’. The front of the head shield is well developed into a pair of large lobe-like tentacles. The cream coloured gill extends out on the right side of the animal, from the mantle cavity within the shell and slightly behind the genitalia. It has a very dark and long ‘proboscis-like’ muscular oral tube that seems adapted to be inserted down the tubes of the worms it feeds on. The foot is very large and broad, extends beyond the shell when the animal is in motion, and it has fleshy wing-like flaps, called parapodia.

This species is commonly found in shallow waters (usually not deeper than 5 m) crawling over or through sandy substrate in sand flats and rock pools. It’s main food source are cirratulid polychaete worms (Kilburn & Rippey, 1982), a group of worms that live in crevices or in sand burrows and feed by extending long sticky tentacles out over the nearby sand and rock to pick up detrital food particles. Hydatina physis captures its preys by everting the long ‘proboscis’ oral tube down the burrow of the worm to get it. It is a restricted and eurythermal reproducer, as it does not reproduce round the year and spawning occurs during the season of relatively low temperature, covering a long period of four months, from October to mid of February in the tropics (Zehra & Perveen, 1992). A similar relationship between temperature and spawning has also been established by Amio (1963). The migration and aggregation of adults (sometimes they occur in large numbers, up to 20 specimens per square meter) from their subtidal habitats, prior to the onset of breeding season is an indication of migratory as well as aggregative behavior of breeding. The spawn consists on a twisted egg ribbon that is completely produced on the animal’s parapodia before attaching it to the substrate by a mucus thread. The eggmass anchoring is achieved by the animal turning itself upside down on the substrate so the foot faces upwards (Zehra & Perveen, 1992). The milky white eggs are enclosed in a single layer of hyaline capsules (4-6 eggs per capsule). On an average, a single egg ribbon contains more than 10.000 eggs at the peak of the breeding season. Development to hatching takes 13-14 days at 25-26°C (Zehra & Perveen, 1992). Only 62% of total hatchings emerged as free swimming veligers in laboratory conditions, but this situation may vary in nature. It is a nocturnal species that buries itself in sand during the day. It follows a 12 h nocturnal circadian rhythm mediated by light intensity and modulated by food availability. The mating behaviour seems to be primarily influenced by the lunar cycle (Murugan et al., 2011). As previously stated Hydatina physis is barely capable of hiding in its shell for protection, so it relies on distasteful acid glands in their skin and burrowing in the sand to protect itself from possible predators.


  • Hydatina – from Greek ὕδωρ – hýdōr, “water”
  • Physis, from Greek: φύσις is usually translated into English as “nature” or “natural form of a thing”, but also refers to the principle of growth or change in nature, to the nature as the source of growth or change or to something that grows, becomes, or develops.

Hydatina physis is a circumglobal species of warm and temperate waters (Rippingale & McMichael, 1961). It has been found in the shallow tropical waters of the Atlantic and Indo-Pacific oceans (Kilburn & Rippey, 1982; Wells & Bryce, 1986; Wirtz, 1999). In the Indo-Pacific it has been reported from the waters of Japan (Rudman, 1972), the Philippines (Satyamurti, 1952), Hawai’i (Rudman, 1972), Australia (Springsteen & Leobrfra, 1986; Short & Potter, 1987; Sethi, 2013), New Zealand (Rudman, 1972) and South Africa (Sundaram, 1969; Rudman, 1972; Voskuil, 1995; Venkatraman & Venkataraman, 2012). In the Atlantic it has been reported from Faial Island, Azores (Yoshiyama & Darling, 1982; Wirtz, 1999), São Tiago Island, Cape Verde (Wirtz, 1999), and in the Canary Islands (Odhner, 1931 as H. stromfelti; Wirtz, 1999).

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


    Western Mediterranean:0 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 Hydatina physis based on our own records.

More pictures


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Further reading

Cite this article as:

Ballesteros, Manuel, Enric Madrenas, Miquel Pontes (2021) "Hydatina physis" in OPK-Opistobranquis. Published: 21/12/2014. Accessed: 21/01/2021. Available at (https://opistobranquis.info/en/q9H7C)

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