A selenizone (from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

"selene" meaning "moon", and "zone" meaning "

girdle A belt, especially if a cord or rope, is called a girdle if it is worn as part of Christian liturgical vestments, or in certain historical, literary or sports contexts. Girdles are used to close a cassock in Christian denominations, including th ...

") is an anatomical structure that exists in the shells of some families of living sea snails: the slit shells, the little slit shells and the abalones, which are marine gastropod

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

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 monoplacophorans 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 Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

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 Dr. Geiger for the area between the SUTure and the SELenizone.

In living gastropods

In the Pleurotomariidae (the slit shells), and the

Scissurellidae Scissurellidae, sometimes known by the common name little slit snails, are a taxonomic family of minute sea snails, marine gastropod molluscs or micromolluscs in the clade Vetigastropoda (according to the taxonomy of the Gastropoda by Bouchet & ...

(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 In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

of the shell. In the

Haliotidae ''Haliotis'', common name abalone, is the only genus in the family Haliotidae. This genus once contained six subgenera. These subgenera have become alternate representations of ''Haliotis''. The genus consists of small to very large, edible, ...

, the abalones, 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 Bellerophontoidea, common name "bellerophonts",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 Malaco ...

, which may be gastropods or monoplacophorans; 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 **

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 namin ...

** Pleurotomariidae * Scissurelloidea ** Anatomidae **

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 preserved ...

taxa: Gastropods * Haliotoidea ** † Temnotropidae * Pleurotomariacea ** † Family Catantostomatidae ** † Family Kittlidiscidae ** † Family Phymatopleuridae ** † Family Polytremariidae ** † Family Portlockiellidae ** † Family Rhaphischismatidae ** † Family Trochotomidae ** † Family Zygitidae Gastropods or monoplacophorans or paragastropods * †

** † Bellerophontidae ** † Bucanellidae ** † Bucaniidae ** † Euphemitidae ** † Pterothecidae ** † Sinuitidae ** † Tremanotidae ** † Tropidodiscidae * † Euomphaloidea ** † Euomphalidae ** † Helicotomidae ** † Lesueurillidae ** † Omphalocirridae ** † Omphalotrochidae * † Macluritoidea ** †

Macluritidae Macluritidae is an extinct family of relatively large, Lower Ordovician to Devonian, macluritacean gastropods(?), hypserstrophically coiled, that is dextral while appearing sinsitral, of which the genus '' Maclurites'' is arch-typical. The base ...

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