Pododesmus patelliformis or ribbed saddle-oyster 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 bivalve mollusc 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...

 Anomiidae

Anomiidae

Anomiidae is a family of bivalve molluscs related to scallops and oysters, and known as anomiids. It contains seven genera.The family is known by several common names, including jingle shells, saddle oysters and mermaid's toenails....

. It is found in the north east Atlantic Ocean.

Description

This oyster has a thin asymmetric shell up to four centimetres in length which is white, sometimes with brown markings. The outline is roughly circular and the beaks are central but set back from the margin. The upper valve is sculptured with up to sixty weak radial riblets, and the inside is glossy white or tinged yellowish-green with two distinct, furrowed muscle scars on the upper valve. The lower valve is flat, thin, transparent and hyaline

Hyaline

The term hyaline denotes a substance with a glass-like appearance.-Histopathology:In histopathological medical usage, a hyaline substance appears glassy and pink after being stained with haematoxylin and eosin — usually it is an acellular, proteinaceous material...

 and has one muscle scar.

Distribution

This species occurs on the north west fringes of Europe extending from Norway to the Mediterranean Sea. It lives on coarse sand and gravel, attached to rocks or other shells by its byssus

Byssus

Byssus means both a silky filament by which certain molluscs attach themselves to hard surfaces, and a rare fabric, also called sea silk and its fibre source.-Word:...

. It is found in intertidal zones and down to about two hundred feet. It is sometimes confused with P. squama and in fact Smith & Heppell (1991) considered them to be the same species.

Ecology

Older specimens of the ribbed saddle-oyster are usually covered by tubeworms

Serpulidae

Serpulidae is a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from the sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes when they withdraw into the tubes. In addition, serpulids secrete...

 and sponges. Other species often found living in the same vicinity include the parchment worm Chaetopterus variopedatus

Chaetopterus variopedatus

Chaetopterus variopedatus is a species of parchment worm, a marine polychaete in the family Chaetopteridae. It is found worldwide.Polychaetes, or marine bristle worms, have elongated bodies divided into many segments. Each segment may bear setae and parapodia...

, encrusting red algae

Red algae

The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species  of mostly multicellular, marine algae, including many notable seaweeds...

, the polychaete worm

Polychaete

The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...

 Pomatoceros triqueter

Pomatoceros triqueter

Pomatoceros triqueter is a species of tube-building annelid worm in the class Polychaeta. It is common on the north eastern coasts of the Atlantic Ocean and in the Mediterranean Sea....

and the fan worm Sabella pavonina

Peacock worm

The peacock worm is a marine polychaete worm belonging to the family Sabellidae. It occurs along the coasts of Western Europe and the Mediterranean. It is found in shallow, tidal waters with a bed of mud, sand or gravel. It is sometimes found on rocks or shipwrecks.It is 10-25 centimetres in...

. Scattered colonies of Alcyonium digitatum

Alcyonium digitatum

Alcyonium digitatum or dead man's fingers is a species of soft coral in the Alcyoniidae family. It is found around the coasts of the northern Atlantic Ocean.-Description:...

may be nearby along with the hydroid Bougainvillia ramosa, the barnacle

Barnacle

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

 Balanus balanus

Balanus balanus

Balanus balanus is a species of acorn barnacle in the Balanidae family. It is native to the colder seas of the northern hemisphere.-Description:...

, the hermit crab Pagurus bernhardus

Pagurus bernhardus

Pagurus bernhardus is the common marine hermit crab of Europe's Atlantic coasts. It is sometimes referred to as the common hermit crab or soldier crab. It is about long, and is found in both rocky and sandy areas, from the Arctic waters of Iceland, Svalbard and Russia as far south as southern...

and the squat lobster

Squat lobster

Squat lobsters are decapod crustaceans of the families Galatheidae, Chirostylidae and Kiwaidae, including the common genera Galathea and Munida. They are not lobsters at all, but are more closely related to porcelain crabs, hermit crabs and then, more distantly, true crabs...

 Munida rugosa

Munida rugosa

Munida rugosa, commonly known as the rugose squat lobster or plated lobster, is a species of decapod crustacean found in the north east Atlantic Ocean and the Mediterranean Sea.-Taxonomy:...

.