Bubble shells Cephalaspidea
Cephalaspidea is an Order of marine gastropods molluscs with approximately 650 living species worldwide. The systematic composition of the group comprises 12 families and about 40–50 genera worldwide. The first studies about these molluscs in Norway date back to the first half of the 19th century and presently (2014) it is estimated that between 40 to 45 species occur in the country.
The typical spiralled gastropod shell is absent in the cephalaspids; most species have a bubble-shaped shell (because only one whorl is completely visible) with a short or involute spire. Some groups even lack a shell (e.g. Philinoglossidae) or have a reduced internal shell (e.g. Aglaja, Chelidonura, Navanax). The shell can be strongly calcified (e.g. Scaphander) but often it is thin, fragile, and translucent (e.g. Philine).
Most species are smaller than 1 cm, yet several genera have in average larger species (e.g. Bulla, Scaphander) and in Norway it is possible to find the largest species in the World – Scaphander lignarius – with a shell that can reach up to 7 cm.
The body of these animals varies from whitish to brownish but some families, particularly in tropical regions, have members with vibrant colours (e.g. Aglajidae, Gastropteridae, Haminoeidae).
Cephalaspidean snails are hermaphroditic with both male and female genital systems but they practice cross-fertilization with mating involving the participation of two animals. These snails live from shallow costal areas down to the deep-sea and inhabit soft bottoms of mud and sand, seagrass meadows, and hard substrates with algae. Most cephalaspids use their shovel shaped head to burrow in the sediment where they search for food and all have a strong muscular gizzard equipped with three hard plates to crash and grind their prey. The diet is formed mostly by diatoms, filamentous algae, foraminiferans, polychaete worms, and smaller molluscs.
Bouchet P og Warén A (1979). The abyssal molluscan fauna of the Norwegian Sea and its relation to other faunas. Sarsia 64: 211-243.
Brattegard T og Holthe T (2001). Distribution of marine, benthic macroogranisms in Norway. Research report for DN-2001-3. Directorate for Nature Management.
Eilertsen ME og Malaquias MAE (2013). Systematic revision of the genus Scaphander (Gastropoda, Cephalaspidea) in the Atlantic Ocean with a molecular phylogenetic hypothesis. Zoological Journal of the Linnean Society 167(3): 389-429. http://onlinelibrary.wiley.com/doi/10.1111/zoj.12013/abstract. DOI: 10.1111/zoj.12013.
Harry HW (1967). A review of the living tectibranch snails of the genus Volvulella, with descriptions of a new subgenus and species from Texas. The Veliger 10: 133-147.
Høisæter T (2009). Distribution of marine, benthic, shell bearing gastropods along the Norwegian coast. Fauna Norvegica 28: 5-106.
Lemche H (1948). Northern and Arctic Tectibranch Gestropods – I. The larval shells II. A revision of the Cephalaspid species. Det Kongelige Danske Videnskabernes Selskab, Biologiske Skrifter 5: 136.
Lemche H (1956). The anatomy and histology of Cylichna (Gastropoda Tectibranchia). Spolia Zoologica Musei Hauniensis 16: 1-278.
Ohnheiser LT og Malaquias MAE (2013). Systematic revision of the gastropod family Philinidae (Mollusca: Cephalaspidea) in the northeast Atlantic Ocean with emphasis on the Scandinavian peninsula. Zoological Journal of the Linnean Society 167(2): 273-326. DOI: 10.1111/zoj.12000.
Ohnheiser LT og Malaquias MAE (2014). The family Diaphanidae (Gastropoda: Heterobranchia: cephalaspidea) in Europe, with A redescription of the enigmatic species colobocephalus costellatus M. Sars, 1870. Zootaxa 6(3774): 501-522. http://zoobank.org/References/1C4C791C-09D7-4711-9D05-1ABE3DB24916 DOI: 10.11646/zootaxa.3774.6.1.
Sars GO (1878). Mollusca regionis Arcticae Norvegiae. Oversigt over de i Norges arktiske region forekommende bløddyr. Bidrag til kundskaben om Norges arktiske fauna: 1. Brøgger, Christiania.
Sneli J-A, Schiøtte T, Jensen KR, Wikander PB, Stokland Ø, Sørensen J (2005). The Marine Mollusca of the Faroes. Fróðskaparrit Supplementum 42: 15-176.
Thompson TE (1976). Biology of Opisthobranch Molluscs. The Royal Society: London. 203 sider.
Thompson TE (1988). Molluscs: Benthic Opisthobranchs. In: Brill EJ og Backhuys W. Leiden. Mollusca, Gastropoda: keys and notes for the identification of the species. Vol. 8. New York, Copenhagen, Cologne. 356 sider.