Terebratulina retusa is a plump, almost pear-shaped brachiopod up to 3.3 cm long and 2.5 cm wide. Characteristic of this phyla, the convex valves are inequivalve with the lower valve more convex than the upper valve. The upper valve tapers posteriorly and the lower valve has a large foramen close to the hinge. The cuticle-covered stalk (pedicle) extends from the foramen and attaches the animal to the substrata. The surface of both valves has distinct central growth lines and ca 11-14 numerous, coarse, ribs that radiate outwards from the hinge-line to the anterior edges of the valves. The ribs also form smooth rounded tubercles on the lateral margins towards the hinge but are more prominent on the lower umbo. The anterior margin of the shell can be rounded, straight or slightly concave. This species has a whitish or yellow shell but may be orange (in females) or cream (in males) during the breeding season due to the colour of the ripe gonads. The shells of live individuals are covered in a spongy, protein-rich layer.

Terebratulina retusa is the most common brachiopod in British waters but may be confused with the deep-living Terebratulina septentionalis. However, Terebratulina septentionalis has no pedicle and has 16-20 coarse radiating ribs on the outside of the valves.

 

Terebratulina retusa is gonochoristic with external fertilization and planktonic larvae present in the water column for up to 3 weeks. James et al. (1991) noted that reproductive cycles and fecundities varied relative to depth and food availability. Two populations of Terebratulina retusa were studied from the Firth of Lorn and Loch Fyne in Scotland at 200 and 30 meters depth respectively. Populations from Loch Fyne had numerous spawning events during the spring and summer, having five times greater planktonic productivity than those studied at depth from the Firth of Lorn, which had only a single spawning event in November.

 

Interestingly, brachiopods were present in the early Cambrian marine fauna. They are one of the few phyla represented throughout 550 million years of the Phanerozoic era, which extends from the first widespread appearance of organisms with calcified shells to the present day (James et al., 1991). In fact brachiopods were inaccurately named by the French palaeontologist Cuvier in 1805. Cuvier mistook the lophophore to be an organ of locomotion as opposed to feeding, hence the name brachion (upper arm) and podos (foot) in Greek (Moen & Svensen, 2004). 

Brachiopods are active ciliary suspension feeders trapping particulate organic matter from the water column. The use of mucus is confined to the transport of particles to the mouth and pseudofaeces production, and not the capture of food particles from the water column (James et al., 1992). The shells of Terebratulina spp. Consist of magnesium calcite, which is chemically very robust (James et al., 1992). 

 Terebratulina retusa is the most common brachiopod in British waters but may be confused with the deep-living Terebratulina septentionalis. However, Terebratulina septentionalis has no pedicle and has 16-20 coarse radiating ribs on the outside of the valves. 

Terebratulina retusa is gonochoristic with external fertilization and planktonic larvae present in the water column for up to 3 weeks. James et al. (1991) noted that reproductive cycles and fecundities varied relative to depth and food availability. Two populations of Terebratulina retusa were studied from the Firth of Lorn and Loch Fyne in Scotland at 200 and 30 meters depth respectively. Populations from Loch Fyne had numerous spawning events during the spring and summer, having five times greater planktonic productivity than those studied at depth from the Firth of Lorn, which had only a single spawning event in November.