Marginellona gigas is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of very large deepwater sea snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

, a marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod mollusk in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Marginellidae

Marginellidae

Marginellidae, or the margin shells, are a taxonomic family of small, often colorful, sea snails, marine gastropod molluscs in the clade Neogastropoda.- Taxonomy :...

. This species was originally thought to be a volute, in the family Volutidae

Volutidae

Volutidae, common name volutes, are a taxonomic family of predatory sea snails that range in size from 9 mm to over 500 mm, marine gastropod mollusks...

, but it is in fact a giant marginellid.

This is the only known species in the genus Marginellona, in other words, Marginellona is a monotypic genus.

Shell description

The shell can be as large 157 mm

Millimetre

The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....

. It is thin, translucent, porcellaneous, and narrowly ovate in shape. The protoconch

Protoconch

A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

 consists of 2-2½ rapidly expanding (diameter 0.5 mm to 11.5 mm in 2 whorls),
smooth, conical, glassy whorls, deflected from coiling axis of teleconch by up to 15º. The transition to teleconch is abrupt, marked by a growth line, and accompanied by the formation of thin parietal callus. The teleoconch has up to 3 smooth, inflated, convex, rapidly descending whorls. The suture has abutting whorls. The shell surface is smooth, glazed, and lacking spiral and axial sculpture. The aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 is ovate, narrow posteriorly, broad anteriorly. The outer lip is smooth.

The inner lip is smooth, with thin, whitish inductural overglaze in some specimens. The columella has a single sharp, axially-oriented columellar fold

Plait (gastropod)

A plait is an anatomical feature which is present the shells of some snails, or gastropods. This sculpture occurs often in the shells of marine gastropod mollusks in the clade Neogastropoda, but it is also found in some pulmonate land snails....

 and a sharp siphonal fold of nearly equal magnitude. The outer shell surface is uniformly tan to greenish-tan; the aperture is darker brown.

Type locality

West of Sombrero Channel, Nicobar Islands, Indian Ocean, 07º48'N, 92º07'E, in 805 m

Metre

The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

, coarse sand.

Distribution

This species has been collected in the eastern Indian Ocean

Indian Ocean

The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

 (Nicobar Islands

Nicobar Islands

The Nicobar Islands are an archipelagic island chain in the eastern Indian Ocean...

) and on the upper continental slope along the western margin of the South China Sea

South China Sea

The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...

. The bathymetric range is 380-1000 m.

Reports

W. of Pratas Reef, South of China Sea, 20º37'N, 115º43'E [380 m]

E. of Phan Thiet, Vietnam, South China Sea, 10º41'08"N, 109º53'08"E [495-500 m]

E. of Phan Ly, Vietnam, South China sea, 11º09'06"N, 110º02'00"E [700 m]

E. of Phan Thiet, Vietnam, South China Sea, 10º01'00"N, 109º55'00"E [460 m]

E. of Ba Ria, Vietnam, South China Sea, 10º40'08"N, 110º03'00"E [760-800 m]

E. of Phan Ly, Vietnam, South China sea, 11º10'00"N, 110º10'00"E [1000-1280 m]