Lacuna crassior 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 sea snail

Sea snail

Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....

, a marine 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...

 Littorinidae

Littorinidae

Littorinidae is a taxonomic family of over 200 species of sea snails, marine gastropod molluscs in the clade Littorinimorpha, commonly known as periwinkles and found world-wide.-Names:...

, the winkles or periwinkles.

Distribution

Lacuna crassior is a circumboreal and subarctic

Subarctic

The Subarctic is a region in the Northern Hemisphere immediately south of the true Arctic and covering much of Alaska, Canada, the north of Scandinavia, Siberia, and northern Mongolia...

 species that has Britain as its southern limit. It has though be found in the Channel Islands

Channel Islands

The Channel Islands are an archipelago of British Crown Dependencies in the English Channel, off the French coast of Normandy. They include two separate bailiwicks: the Bailiwick of Guernsey and the Bailiwick of Jersey...

 and Roscoff

Roscoff

Roscoff is a commune in the Finistère department of Brittany in northwestern France.The nearby Île de Batz, called Enez Vaz in Breton, is a small island that can be reached by launch from the harbour....

, the edges of its southerly range.

Description

Lacuna crassior is yellow in colour and can grow up to 14 × 10 mm in length. The maximum recorded shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 length is 14 mm. Its body is very similar in shape to the Lacuna vincta

Lacuna vincta

Lacuna vincta, common name the northern lacuna, is a species of sea snail, a marine gastropod mollusk in the family Littorinidae, the winkles or periwinkles....

, with a long, broad snout and slender head tentacles. Its foot is elongated, with a double-edged anterior margin and two short, white, metapodial tentacles projecting from below the operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

. These metapodial tentacles are less flattened than those of Lacuna vincta.

The Lacuna crassior has a strong shell, with six to seven turreted whorls

Whorl (mollusc)

A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...

. Its fine spiral striae and vertical costae are mostly obscured by thick periostracum

Periostracum

The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...

, but form a series of irregular folds on the shell's last whorl. The umbilicus is occasionally closed and the umbilical groove reduced with the aperture

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 drawn out and angulated at the base of the columella.

Habitat

It is uncommon and occurs at low water spring tide (LWST), the lower part of the beach that is uncovered only a few times a month during spring tides. It also occurs offshore at a depths greater than 30 metres and up to 90 m. It can be found among stones within mud, sand, gravel and coarser sediments.

Minimum recorded depth is 2 m. Maximum recorded depth is 176 m.