Identifying dwarf sea hares from eastern Australia and New Zealand
(ex. Aplysia parvula)
Molecular analysis of dwarf sea hares from eastern Australian waters carried out as part of my PhD project revealed that there are three species of dwarf sea hares present, two named species: Aplysia nigrocincta von Martens, 1880, Aplysia concava G. B. Sowerby I, 1833 and an as yet undescribed Aplysia sp. At least one, A. concava is found in New Zealand.
All three species are diminutive as adults, rarely exceeding 60mm crawl length. They all exhibit a contrasting colour pattern. The body one colour, the parapodial margins, mantle margin, foot margin, rhinophore and oral tentacles tips another.
Aplysia nigrocincta
The most readily identifiable is Aplysia nigrocincta.
The 'typical' colour form is a dark body with white 'trim' - the reverse of most other dwarf sea hares (Fig. 1 below).
Fig 1. Aplysia nigrocincta, typical colour form. Raja Ampat, Indonesia, Photo: davidr (iNat). Used under CC license.
This less-common colour form was found at North Solitary Island, NSW. It was confirmed as A. nigrocincta by molecular analysis (Fig. 2 below). It still has the contrasting margins but the intrusion of patches of colour into the body adds some confusion.
Fig 1. Aplysia nigrocincta, non-typical colour form. North-Solitary Island, NSW, Australia. Photo: Matt Nimbs.
Aplysia concava
see Nimbs & Wilson, 2021. here
Aplysia concava requires an image of the mantle cavity to identify.
It exhibits the 'usual' dwarf sea hare colour pattern of a lighter coloured body with dark 'trim' (Fig 3. Below) however the intraspecific colour palette is broad. As such it may look like many of the other species in the ex. Aplysia parvula sensu lato group, however these other species are not found in e. Australian & New Zealand waters.
Aplysia concava has a large, highly-concave shell for its size and, consequently, the parapodia are not able to meet over the swollen mantle (contrast with A. nigrocincta in Fig. 1). The large shell means that the mantle aperture (opening that exposes the shell) is often visible. The aperture is circular and can be quite large. The mantle margin is not raised. (Fig. 4). In caramel-brown specimens there is a white patch at the base of the siphon and this may extend along the length of the siphon in some specimens (Fig 4.).
Fig 3. Aplysia concava, caramel-brown colour form. Sandy Beach, NSW, Australia. Photo: Matt Nimbs
Fig 4. Aplysia concava, enlargement of part of Fig. 3 showing identifying characteristics. Photo: Matt Nimbs
Aplysia concava has other colour variants. Dark-red specimens do not exhibit a white patch at the base of the siphon (Fig. 5) and light-brown animals exhibit the main diagnostics but they may be more difficult to distinguish (Fig. 6)
Fig 5. Aplysia concava, dark-red form, Korff's Islet, Coffs Harbour, NSW, Australia. Photo: Matt Nimbs
Fig 6. Aplysia concava, light-brown form, Newcastle, NSW, Australia. Photo: Matt Nimbs
Aplysia sp.
Aplysia sp. was detected using molecular analysis. It is generally cryptic with A. concava (Fig. 7), however it may be identified by examination of the mantle cavity. In Aplysia sp. the mantle aperture is small, round, raised and 'volcano-like' (Fig. 8)
Fig 7. Aplysia sp. Bermagui, NSW, Australia. Photo: Matt Nimbs
Fig 8. Enlargement of part of Fig. 7, showing raised, volcano-like mantle aperture.