Author: G. B. Sowerby, I, 1833

Order: Anaspidea  Family: Aplysiidae

Maximum Size: 25 mm

Sightings: Sunshine Coast

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Aplysia concava G. B. Sowerby, I, 1833.

Aplysia concava is one of a group of Sea Hares often referred to as Dwarf Sea Hares due to their small size. Differentiation between them can be problematic due to intraspecific variation and interspecific similarities. Fortunately, two recent papers (Golestani, et al, 2019 and Nimbs & Wilson, 2021) have gone a long way to untangling the complexity that had arisen around the species name Aplysia parvula and into which, many were lumped.

Aplysia concava is small (to 25 mm) and possesses what could be called the “archetypical sea hare shape” with a distinct head and tail separated by a wider central or visceral region. The eyespots are distinctive with the rolled, tapered-tip rhinophores just posterior, sitting erect and sited on top of the head. Located most anteriorly on the head are the two rolled cephalic tentacles that usually project forwards and laterally. The parapodia do not meet anteriorly or over the mantle, leaving the mantle exposed, but do join posteriorly. The exposed mantle has a large foramen on its left side through which the large shell is revealed. In this species the rim of the foramen is not raised. The parapodia form a characteristic mid-fold both sides. The siphon is visible on the right side posterior to the mid-fold, its height varying to even project above the parapodia.

The intraspecific variation in colour is wide. Specimens may be various shades of red, brown or cream. The dark brown and red specimens aren’t otherwise marked but the cream specimens exhibit white spots. All variations have markings to the margins. The very edging of the parapodia and foot carry a narrow white line then a black submarginal band. The red specimens may have a grey marginal line in lieu of white whilst in the lighter coloured examples the black band may be discontinuous. The rim of the mantle foramen is most often black but it too may be discontinuous or even lacking in the lighter coloured specimens. Rhinophores and cephalic tentacles have black margins that may be discontinuous and often there is is a concentration of black at the tips.

The shell of Aplysia concava is described as ovate-auriculate, large relative to the animal size and deeply concave as alluded to in the specific epithet. The protoconch sits perpendicular to the shell proper.

Current distribution is recorded as the Southwestern Pacific bounded by New Caledonia, New Zealand, Tasmania and the east coast of southern Queensland and New South Wales.

(Acknowledgement: Much of the descriptive information in the previous paragraphs has been acquired from Nimbs & Wilson, 2021 with personal observations from images I have taken of live specimens incorporated.)

To complicate matters further there is another species cryptic within Aplysia concava that has been identified through molecular sequencing and awaiting description – Aplysia sp. – (Nimbs, 2021 – and see Aplysia sp. 01 on this website).

About the name:
When the name Aplysia concava was originally conferred, it was done so merely as one in a short list of names appended beneath a few general paragraphs about a genus, entitled Aplysia. There was no accompanying description of the animal or even of the particular shell just a black and white sketch of a single aspect of the animal’s shell. Back then the focus of study was upon the shells of the molluscs – Conchology – and any details about the animals (if known) were only added if thought interesting enough.

This provision of a name but lacking of descriptive information about the animal (or even the shell) naturally has caused immense difficulty for those who follow. Today the emphasis of study is malacology, or the study of all aspects of the molluscan animal. In these instances then, this requires identification of the shell to match it unequivocally with an existing name and then relating that shell to the animal, to the exclusion of all other relatives. Unfortunately the original shell upon which the “description” was based, such as it was, cannot be located. Fortunately there is a historical succession of shell illustrations and some text in the literature (1833, 1870, 1895/6, & 1899) plus a brief 1877 description of a collected shell from Tasmania. All of this is laid out in Nimbs & Wilson, 2021. The title of their paper, “Saved by the Shell”, is certainly on the mark.

Timeline:
Originally described as Aplysia concava, G. B. Sowerby, I., 1833.
Synonymized with Aplysia parvula, Eales, 1960.
Aplysia concava became taxon inquirendum, Golestani, et al, 2019.
Resurrected as a good species, Aplysia concava, Nimbs & Wilson, 2021.

Occasionally the genus name Tethys or Laplysia may pop up in old references to sea hares. Tethys was in use alongside Laplysia and Aplysia as a genus name for sea hares. Tethys was also in use as a genus of Tethydidae (the “Melibe-like”) nudibranchs. Due to the confusion, in 1954, Opinion 200 was issued by the ICZN. To simplify (or perhaps oversimplify)
(1) the name Laplysia Linnaeus, 1767 was emended to Aplysia Linnaeus, 1767,
(2) Aplysia Linnaeus, 1767 with the type species Aplysia deplanes Gmelin, 1791 be used for the sea hare types and
(3) Tethys Linnaeus,1767 with the type species Tethys leporine Linnaeus, 1767  be used for the Tethydidae nudibranchs.

David A. Mullins – April 2021

References:
– Tryon, G. W., continuation by Pilsbry, H. A. (1895-1896). Manual of Conchology; Structural and Systematic. Series 1, Vol. 16. Conchological Section Academy of Natural Sciences of Philadelphia.

– Tenison-Woods, J. E. (1877). Census; with brief descriptions of the marine shells of Tasmania and the adjacent islands. In Papers & Proceedings and Report of the Royal Society of Tasmania; 1877; pp. 26–57.

– ICZN (1954). Opinion 200. Validation, under the Plenary Powers, of the accustomed usage of the generic names Tethys Linnaeus, 1767, and Aplysia Linnaeus, 1767 (Class Gastropoda). Opinions and declarations rendered by the International Commission on Zoological Nomenclature, 3: 241-265.

– Eales, N. B. (1960). Revision of the world species of Aplysia (Gastropoda, Opisthobranchia). In Bulletin of the British Museum, Zoology; British Museum (Natural History): London, UK; Volume 5, pp. 267–404.

– Nimbs, M. J., Willan, R. C., & Smith, S. D. (2017). Is Port Stephens, eastern Australia, a global hotspot for biodiversity of Aplysiidae (Gastropoda: Heterobranchia)? Molluscan Research. 37, 45–65.

– Golestani, H., Crocetta, F., Padula, V., Camacho-García, Y., Langeneck, J., Poursanidis, D., Pola, M., Yokes ̧, M. B., Cervera, J. L., Jung, D.-W., et al. (2019).The little Aplysia coming of age: From one species to a complex of species complexes in Aplysia parvula (Mollusca: Gastropoda: Heterobranchia). Zoological Journal of the Linnean Society, 187, 279–330.

– Nimbs, M. J. & Wilson, N. G. (2021). Saved by the Shell: Molecular Analysis Detects the Cryptic Sea Hare, Aplysia concava G. B. Sowerby I, 1833 (Mollusca: Heterobranchia: Aplysiidae), from Oceania, with a Redescription. Taxonomy 2021, 1, 48-59.

– Nimbs, M. J. (2021). Sea slug research – Identifying dwarf sea hares from eastern Australia and New Zealand (ex. Aplysia parvula). Available online at https://mattnimbs.wixsite.com/matt-j-nimbs/identify-sea-hares  (Accessed on 13 April 2021).