Author: (Linnaeus, 1758)

Order: Acteonoidea  Family: Aplustridae

Maximum Size: 75 mm

Sightings: Sunshine Coast, Whitsundays

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Hydatina physis  (Linnaeus, 1758)

Some sea slugs belong to a group commonly known as “Bubble Snails”. This name was coined due to the possession of a greatly inflated shell that is thin-walled and consequently light and fragile – a “Bubble Shell”. Some belong to the headshield slugs of the Cephalaspidea Order and others to the Acteonoidea Order. They are all considered primitive sea slugs due to the presence of this external shell, among other features.

Hydatina physis belongs to the Order Acteonoidea in the Family Aplustridae. Although the possession of a shell is considered a primitive feature in the sea slugs the shell of Hydatina physis is greatly modified, by being inflated and thin walled. The large animal cannot retract entirely into the shell so it also lacks the protective operculum, used by those species that rely predominantly upon a thickened shell for protection, to seal off the aperture when the animal has entirely withdrawn. This reduction in the reliance upon the shell for defence is an important marker in understanding the eventual entire loss of the shell in the true nudibranchs and development along the way of other, predominately chemical, defences.

When moving out in the open this is a relatively large animal (up to 75 mm) with a flamboyant display. The sight of one out hunting on the substrate is a delight to observe. The body, parapodia, headshield, infrapallial lobe and foot are reddish-pink in colour and ruffled along all the edges. Those edges carry a thin white line to the border but also a slightly wider light blue band submarginally that diffuses into the red. There is a range of intensity to the colours among specimens, even within the one locality. The parapodia (lateral extensions of the foot) are large and capable of enveloping the shell but instead are more often held out to the side with the thin foot also extending a reasonable distance posteriorly but laid flat on to the substrate. The headshield is developed anteriorly into lateral triangular lobes with well-developed tentacular corners but divides into two large lobes that form siphonal folds and then extend further posteriorly to cover the anterior of the shell. The close-set eye spots are visible in the gap between the two halves. The sensory Hancocks Organ may sometimes be observed, as a series of flaps, located behind the siphonal folds of the headshield, positioned thus to test the water flowing over the head. The cream coloured gill is large and attached at a single point such that it is free to move somewhat freely. Normally it is protected under the shell on the right side but at times, depending upon the animal’s movement/position, may become visible. The rear of the shell is covered by an extension of the mantle –  the infrapallial lobe. The shell, as previously mentioned, is thin and inflated with the aperture starting as narrow posteriorly but flaring out widely anteriorly. The spire is depressed. Externally the shell colour is light fawn with many narrow closely spaced dark brown to black spiral lines of irregular thickness, but white internally with the external markings showing through.

Distribution is circumtropical extending into temperate regions. It is found intertidally and in shallow waters on both soft sandy/muddy or hard broken rocky/coral bottoms and is able to burrow for both hunting purposes and as a method of escape. It is known to be a predator and specialised feeder upon cirratulinid polychaete worms. Possession of an extremely long and muscular oral tube (or proboscis) enables extraction of worm prey from down in their deep burrow and crevices. The prey are stored in a crop until the sea slug is ready to undertake digestion. From its prey, toxic compounds are sequestered and concentrated in a series of defensive glands. Classed as a nocturnal hunter, inactive and buried in the substrate during the day, it can nevertheless, at times, be seen out in the open during daylight hours.

Aggregations of Hydatina physis are sometimes observed, mating and spawning. Mating most often is one on one but threesomes, forming a mating triangle, have been reported. Like all the sea slugs it is a hermaphrodite. The spawn of Hydatina physis is a white convoluted ribbon gathered together into a mass and can be, for comparative purposes, not unlike the size and shape of its shell. The whole ribbon is extruded completely and formed into a mass on the right parapodial fold to be carried about until a suitable substrate is located and then being anchored to the substrate by a mucous thread. The eggs develop and hatch as free-swimming veligers.

Originally described as Bulla physis.

Sometimes the use of a “common name” of Rose-petal Bubble Shell may be encountered.

David A. Mullins – December 2020

References:

– Rudman, W. B., (1972). The anatomy of the opisthobranch genus Hydatina and the functioning of the mantle cavity and alimentary canal. Zoological Journal of the Linnean Society, 51: 121-139.

– Rudman, W. B., (1998 December 18). Hydatina physis (Linnaeus, 1758). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/hydaphys and associated messages.

– Hamel, J. & Mercier, A. (2007). Development and movement of the opisthobranch, Hydatina physis, in the Solomon Islands. SPC Trochus Information Bulletin #13

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – NudiNotes Column, Issue: #384 (October 2020) by David A. Mullins.