A selenizone (from the Greek "selene" meaning "moon", and "zone" meaning " girdle") is an anatomical structure that exists in the shells of some families of living

sea snail Sea snails are slow-moving marine (ocean), marine gastropod Mollusca, molluscs, usually with visible external shells, such as whelk or abalone. They share the Taxonomic classification, taxonomic class Gastropoda with slugs, which are distinguishe ...

s: the slit shells, the little slit shells and the

abalone Abalone ( or ; via Spanish , from Rumsen language, Rumsen ''aulón'') is a common name for any small to very large marine life, marine gastropod mollusc in the family (biology), family Haliotidae, which once contained six genera but now cont ...

, which are marine

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s from ancient lineages. It is a spiral band of crescentric growth lines or threads (lunulae) on the shell surface due to the semicircular end of a notch or slit on the outer lip. A structure of the same type exists in several fossil groups of mollusks, including all the fossil families of slit shells, as well as three superfamilies of what may have been gastropods, but may possibly have been

monoplacophora Shell of Monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic class of molluscs with a cap-like shell, inhabiting deep sea environments. Extant representatives were not recognized as such until 1952; previously they wer ...

ns or paragastropods instead. The function of the holes and slits in living sea snails is to allow for exhalant water circulation, which is important for respiration and other functions. It has also been suggested that the selenizone may serve to reinforce the shell against catastrophic breakage during predation attempts. A new shell-morphological term 'sutsel' has been introduced by Daniel Geiger for the area between the SUTure and the SELenizone.

In living gastropods

In the Pleurotomariidae (the slit shells), and the Scissurellidae (the little slit shells), the selenizone is a groove on the periphery of the shell which forms as the shell grows by the gradual sealing up of a slit that extends back from the edge of the aperture of the shell. In the Haliotidae, the

, the selenizone takes the form of a series of holes in the shell (reaching back from the leading growth edge of the aperture) which form as the shell grows. The older holes are gradually sealed up as the shell grows and new holes form. Each abalone species has a typical number of holes in the selenizone which remain open, and this feature is diagnostic for the individual species.

In fossil groups

A selenizone is also present in several superfamilies of fossil mollusks. Not surprisingly it is seen in the numerous fossil gastropod families within the Pleurotomarioidae. The selenizone also exists in the Bellerophontoidea, which may be gastropods or

ns; in the Euomphaloidea, which are probably gastropods, but may be monoplacophorans; and in the Macluritoidea, which are either archaeogastropods or Paragastropoda Linsley & Kier, 1984, (paragastropods are a group of mollusks that superficially resemble gastropods, but were untorted).

Taxa

A list of superfamilies and families (both living and fossil) which show this structure, based on the taxonomy of Bouchet ''et al'' 2005.Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdes A. & Warén A. 2005. ''Classification and nomenclator of gastropod families''. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. . ISSN 0076-2997. 397 pp

/ref> The taxa that are entirely extinct are marked with a dagger † : Extant taxon, Extant taxa: * Haliotoidea ** Haliotidae *

Pleurotomarioidea Pleurotomarioidea is a superfamily of small to large marine gastropods included in the order Pleurotomariida of the subclass Vetigastropoda. These are the slit shells, originally named Pleurotomariacea, in keeping with the convention for na ...

** Pleurotomariidae * Scissurelloidea ** Anatomidae ** Scissurellidae

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

taxa: Gastropods * Haliotoidea ** † Temnotropidae * Pleurotomariacea ** † Family Catantostomatidae ** † Family Kittlidiscidae ** † Family

Phymatopleuridae Phymatopleuridae is an extinct family of fossil sea snails, marine gastropod mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxo ...

** † Family Polytremariidae ** † Family Portlockiellidae ** † Family Rhaphischismatidae ** † Family Trochotomidae ** † Family Zygitidae Gastropods or monoplacophorans or paragastropods * † Bellerophontoidea ** † Bellerophontidae ** † Bucanellidae ** † Bucaniidae ** † Euphemitidae ** † Pterothecidae ** † Sinuitidae ** † Tremanotidae ** † Tropidodiscidae * † Euomphaloidea ** † Euomphalidae ** † Helicotomidae ** † Lesueurillidae ** † Omphalocirridae ** † Omphalotrochidae * † Macluritoidea ** † Macluritidae

References

External links

Selenizone mentioned here

Paper on the selenizone and breakage in fossil shells

Information on paragastropods here
{{Gastropod anatomy Mollusc shells Gastropod anatomy