Phylum Brachiopoda: lamp shells
Brachiopods are shelled marine invertebrates superficially resembling bivalve molluscs. Around New Zealand some may be encountered at low tide, but most are found on the continental shelf and into the deep sea down to abyssal depths (about 6,000 metres). Sometimes called lamp shells because of their fancied resemblance to a Roman oil lamp, they differ from bivalves in that the two valves of their shell are dorsal and ventral, not left and right. These valves may possibly also be regarded as anterior and posterior if the brachiopod body is interpreted as being folded during embryological development (Cohen et al. 2003). Brachiopods are solitary creatures, usually attached to the substratum by means of a muscular stalk (pedicle), but may be cemented (in which case they may resemble small oysters or limpets), free-lying, or in shallow burrows. Living species range in size from 2 to nearly 100 millimetres, including the pedicle. The shell is calcareous or sometimes phosphatic and, although always bilaterally symmetrical, the two valves are normally dissimilar in size, shape, and even ornamentation. Brachiopods are suspension feeders, filtering minute plankton and organic particles from the surrounding sea water. About 100 genera and 350 species of brachiopods are found in modern seas; these are a tiny fraction of the approximately 4,500 genera recognised (Kaesler 1997, 2000).