Sponge (phylum Porifera)
Sponges are distinguished from other metazoan animals in that they lack a true organisation of cells into body tissues, and by the presence of a unique aquiferous system. Most sponge cells are totipotent, meaning that one cell type can change into other cell types as the sponge requires. This is a distinct advantage: sponges can break into two or more individuals, each of which can regenerate into a new but genetically identical sponge, a form of asexual reproduction.
Sponges are sessile suspension feeders, and most species are marine. A sponge feeds by circulating seawater through its body’s aquiferous system, removing nutrients along the way before being expelled through the osculum. Some sponges have a rigid skeleton made of either calcium carbonate, silica or collagen.
The sponges associated with cold-water coral reefs are diverse: taxonomically, morphologically and ecologically. Over a hundred species inhabit the Lophelia reefs off western Scotland, and many new species are being discovered from cold-water coral reefs worldwide. Sponges often encrust the hard substrata within living reef frameworks and coral rubble habitats, and some species bore through the coral skeletal framework, which helps break down dead coral and contribute to mound formation. Large glass sponges (Class Hexactinellida) are particularly conspicuous on the reefs, forming delicate habitats themselves. Extensive areas of cold-water sponge reefs have recently been discovered off western Canada. The Porifera also exhibit a spectrum of bioactive chemicals with potentially pharmaceutical applications: the diversity they support and their socioeconomic value for humans must help to ensure the protection of both cold-water sponge and coral reef habitats.