Author: (d’Orbigny, 1835)

Order: Pteropoda  Family: Cavoliniidae

Maximum Size: 9 mm – Shell length

Sightings: Sunshine Coast

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Cavolinia uncinata (d’Orbigny,1835)

Belonging to a group of sea slugs in the order Pteropoda, Cavolinia uncinata is commonly called a sea-butterfly due to the distinctive flapping action of its foot, the anterior portion being modified into a pair of “wings” that protrude, together with the foot-lobes, from the shell aperture. It is a holoplanktonic species (as with all the pteropods) spending its whole lifecycle in the water column.

The small thin fragile shell (constructed of aragonite rather than calcite) is bilaterally symmetrical and may be colourless or brownish with a maximum height of 9 mm. The shell bears three posterior “points”: two lateral spines, well-developed and pointing essentially in a downwards direction and the centre-most caudal spine carrying the protoconch that is strongly curved backwards, or dorsally, with the proximal portion being straight (To help in differentiation, Cavolinia gibbosa has a short straight central spine.) The dorsal surface is flat with five moderately developed ribs, while the ventral surface has a globose protruding bulge carrying transverse striae anteriorly and exhibiting a hammered appearance to much of the remaining ventral surface. The aperture is almost as wide as the shell being covered by the dorsal lip that curves strongly over giving a flattened triangular appearance not unlike a turret when viewed ventrally. The animal can withdraw completely into the shell. There is no operculum present.

The mantle covers the entire shell and is complex in arrangement. Four sets of mantle lobes are present outside of the shell, extending into the water – anterior, medial, ventral and dorsal, some divided into appendages but are difficult to observe in situ as the animal is on the move as are all the lobes and their appendages, most of which are transparent. The most easily distinguished are the two long trailing central  appendages of the medial mantle lobes.  These are brown in colour for almost their entire length except for a short proximal section. In captive animals the lobes are most often reduced or withdrawn. The mantle lobes play an important role in buoyancy control and feeding. Another aid to buoyancy is the presence of a transparent and gelatinous temporary pseudoconch, or temporary shell, secreted by the mantle and sited in the space between the dorsal mantle lobes and the shell. It is not permanently attached and may be discarded if disturbed or the mantle withdrawn.

Feeding is effected by the production of an extremely large transparent mucus web (up to ten times the animal size) deployed overhead that traps planktonic prey such as dinoflagellates, radiolaria, foraminifera, diatoms and the like – our knowledge of diet being currently limited to examination of the stomach contents of captured animals. This web can be deployed in less than 2.5 minutes and ingested between one to three minutes. Prey is trapped in the mucus, the web drawn down onto the foot-lobes and into the mouth, ingested and the prey reduced by radula and gizzard action. The actual ingestion process is quite involved. The anterior mantle lobe forms a funnel around the foot-lobes and wings and the retrieval of the mucus feeding web is channeled through. Sorting of captured prey occurs here with rejected material directed away from the mouth, dorsally between the wings, by a ciliated pathway formed by the junction of the dorsal mantle lobes. Some sort of sensory organ must be involved in the sorting process. The rejected material, or pseudofaeces, join the faeces in forming strings below the animal. The web may be cast off if the animal is disturbed.

Respiration is believed to be effected via the large surface area of the wings but also by a secondary gill in the mantle cavity.

As with all pteropods it is a protandric hermaphrodite, each specimen maturing as a male stage first, then passing through a hermaphroditic phase, to finally a mature female stage.

The species is neutrally buoyant, meaning at rest it does not sink, and therefore does not expend energy maintaining depth. The flapping wings are mainly used for avoidance or as an escape mechanism if the animal is threatened.

Note: Many internet images of pteropods are incorrectly identified/labelled.

Nomenclature: This name is often credited to (Rang, 1829) but therein the name Hyalea uncinata was given, without a description, as an example of one group of species in the genus Hyalea; the species is figured on plate 2 fig 2 but without a connection to the name. The name is available from d’Orbigny (1834) p. 93-94, pl. 5 figs 11-15. [MolluscaBase eds. (2022)]

David A. Mullins – December 2022

References:
– Rang, S. (1829). Manuel de l’histoire naturelle des mollusques et de leurs coquilles, ayant pour base de classification celle de M. le baron Cuvier. Paris: Roret. iv + 390 pp., 8 pls. Paris, 1829. (330-361).

– Orbigny, A. D. d’. (1834-1847). Voyage dans l’Amérique méridionale (le Brésil, la république orientale de l’Uruguay, la République argentine, la Patagonie, la république du Chili, la république de Bolivia, la république du Pérou), exécuté pendant les années 1826, 1827, 1828, 1829, 1830, 1831, 1832 et 1833. Tome 5(3) Mollusques. pp. i-xliii, 1-758, 85 plates [pls 1-2, 1834; pp. 1-104, pls 3-7, 10, 12, 1835; pp. 105-184, pls 8-9, 11, 13-23, 25-28, 1836; pls 24, 29-43, 41bis, 45-46, 1837; pp. 185-376, pls 44, 47-52, 55, 1838; pls 54, 56-65, 1839; pl. 66, 1840; pp. 377-488, pls 53, 67-77, 80, 1841; pp. 489-758, 1846; pls 78-79, 82-85, 1847].

– Lalli, C. M. & Gilmer, R. W. (1989) Pelagic Snails – The Biology of holoplanktonic Gastropod Mollusks. Stanford University Press Stanford, California.

– Newman, L. J. (1990). Holoplanktonic Molluscs (Gastropoda; Thecosomata, Gymnosomata and Heteropoda) from the waters of Australia and Papua New Guinea: their taxonomy, distribution and biology. Zoology Dept., University of Qld.

– Newman, L. (1998). Order Thecosomata Pp 980-985 in P.L. Beesley, G.J.B. Ross, and A. Wells, eds., Mollusca: The Southern Synthesis. Fauna of Australia. 5, Part B. CSIRO Publishing, Melbourne.

– Poppe, G. T. (2010). Philippine Marine Molluscs Vol. III. ConchBooks.

– Pierrot-Bults, A. C. & Peijnenburg, K. T. C. A. (2015). Pteropods. Encyclopedia of Marine Geosciences. Springer Science.

– Gershwin, L., Sutton, C., Gowlett-Holmes, K., and Kloser, R. (2018). The Pelagic Snails. In: Pelagic Invertebrates of the Tasman Sea and Great Australian Bight: A practical field guide. CSIRO Oceans and Atmosphere: Hobart.

– Janssen, A. W., Bush, S. L. & Bednarsek, N. (2019). The shelled pteropods of the northeast Pacific Ocean (Mollusca: Heterobranchia, Pteropoda). Zoosymposia 13: 305-346.

– Ponder, W. F. & Lindberg, D. R., with illustrations by Ponder, J. M., (2020). Biology and Evolution of the Mollusca, Volume One & Two. CRC Press, Taylor & Francis Group.

– Pittman, C. & Fiene, P. (2022) Cavolinia uncinata (Rang,1829) [In] sea Slugs of Hawaii. Available from: http://seaslugsofhawaii.com/species/Cavolinia-uncinata-a.html. (Accessed Nov. 2022)

– Poppe, G. T. & Poppe, P. (1994 – 2022) Cavolinia uncinata [In] Hardy’s Internet Guide to Marine Gastropods. Available from: https://www.conchology.be/?t=263&family=CAVOLINIIDAE&fullspecies=Cavolinia%20uncinata&shellID=81657 (Accessed Nov. 2022).

– MolluscaBase eds. (2022). MolluscaBase. Cavolinia uncinata (d’Orbigny, 1835). Accessed through: World Register of Marine Species (WoRMS) at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=139031 (Accessed Nov. 2022)