This feature refers to the ability for most, if not all sponge cells to change form and function dictated by the particular needs of the individual. Totipotent cells (like stem cells) occur throughout the animal kingdom, but their ability to change usually occurs only in one direction: from a stem cell to another derived cell. In sponges, these changes can occur in both directions. There are hundreds of different types of cells in sponges, many whose functions are still not yet known. The most primitive are the Archaeocytes (amoeboid phagocytic cells that are the major transport cells for feeding, digestion, oxygen transport and body repair). In sponges, the archaeocytes can develop into any other cell type, and then back again as needed. Other cells include spherulous cells (with functions of excretion and chemical defence), spongiocytes (produce collagen), and scelerocytes (secrete spicules), amongst many others. Sponges have an intracellular metabolism: they catch, eat, digest food and excrete waste products within the cells, not within any common body cavity. The high mobility of cells is therefore critical to sponge survival.
Sponge cells in the mesohyl seen under transmission electron microscopy, showing the larger amoeba-like archaeocyte cells and the many smaller bacteria living inside the sponge body.
Dysidea sp., a thin section through the outer surface of the sponge under transmitted light microscopy, showing the continuous ectosomal cell layer and crust of sand at the surface (right), and the roving archaeocyte cells within the body mesohyl.
Sponges are also 'hotels' to a myriad of other life forms. These include shrimps, barnacles, worms and most significantly the other Domains of Life (Bacteria and Archaea). In many sponges there are often more bacteria cells than there are sponge cells, and some of these have co-evolved with sponges. These co-evolutionary partners include the cyanobacteria, which live in symbiosis with many sponges, especially in shallow waters, providing the sponge with nutrients through their photosynthesis.
Dysidea sp. with long filament-like cynanobacteria living in symbiosis with the sponge in its body (mesoyl), contributing to sponge nutrition through photosynthesis.
Collospongia auris, containing symbiotic cyanobacteria which produce photosynthetic by-products for sponge nutrition.
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