Aldisa zetlandica

Aldisa zetlandica (Alder & Hancock, 1854)

Aldisa zetlandica by Klas Malmberg

Taxonomic Note: First described by Alder & Hancock (1854) as Doris zetlandica.  Millen & Gosliner (1985) redescribed it after a detailed study of its morphology. Recent genetic analysis (Valdés et al., 2017) confirms its placement within the genus Aldisa, distinguishing it from the closely related species A. albatrossae.

Synonyms

• Doris zetlandica Alder & Hancock, 1854 (original)

Description
Body is an elongate oval with high sides and a maximum length of up to 35 mm. The mantle is typically white or translucent white with scattered white specks, though some specimens exhibit a greenish-grey hue (Picton & Morrow, 1994). The pale-cream to reddish-brown viscera are visible through the dorsum. The dorsum itself is covered with a number of large, conical tubercles, varying in size, typically larger in the centre of the dorsum and smaller on the mantle edge. Gill composed of 7 bipinnate white branchial leaves, arranged in a circle around the anus, the gills being fully retractable into a dorsal pocket. Rhinophores with 11-12 lamellae. The anterior foot border is grooved but not notched, and the mouth area has two blunt lateral expansions without oral tentacles.

Biology
This is a sublittoral species originally discovered in Shetland Islands from deep-water dredging (Alder & Hancock, 1854). It inhabits depths ranging from 25 to 1,900 meters but is is recorded in literatura between 70 and 320 meters (Millen & Gosliner, 1985), 845 meters (Malaquias, 2001) and 138 to 529 meters (Valdés et al., 2017) at temperatures ranging from 2.5 to 4ºC. Recent observations associate it with Axinellid sponge-rich habitats, where it feeds on sponges such as Hymedesmia paupertas and Hymedesmia jecusculum.

Similar Species

• Cadlina laevis, much flatter in profile and very short dorsal tubercles. With white or lemon-yellow glands situated towards the mantle margin. Occasionally with a yellow border around the mantle edge. Feeds on sponges Halisarca dujardini and perhaps Dysidea fragilis.
• Adalaria proxima, with blunter rhinophores and pointed dorsal tubercles. Feed on bryozoan Electra pilosa.
• Adalaria loveni, with flatter dorsal tubercles. Feed on bryozoan Membranipora membranacea.
• Onchidoris muricata: lack a true gill pocket and have separately contractile (not retractile) gills. Feeds on Membranipora membranacea on the lower shore or on Securiflustra securifrons in the sublittoral.

Etymology

• Aldisa. Probably related to an Iceland viking saga of the same name.
• Zetlandica. Originary of the Shetlands, United Kingdom, where this species was described.

Distribution
This species is reported from the Shetland Islands (Alder & Hancock, 1854) to the Azores (Malaquias, 2001), with records in western Scotland, western Ireland, southern Norway (Picton & Morrow, 1994), Finland, and Iceland (Millen & Gosliner, 1985; Schiøtte, 2005). However, it apparently avoids Arctic waters, raising questions about a reported record from North Greenland (Koltun, 1964). In the northwestern Atlantic, it has been recorded from the Flemish Cap and Grand Bank, Terranova, Canada  (Valdés et al., 2017).

More pictures

Bibliography

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Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

• Biodiversity Heritage Library
• Encyclopedia of Marine Life of Britain and Ireland
• Flickr pictures
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• World Register of Marine Species

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