Sponges are the simplest animals known, lacking true tissue and muscle tissue. They feed by filtering water and are protected by spicules. They have at least eight cell types and over 5,000 species. They are found at every depth in the ocean and may have evolved from more complex ancestors.
“Poriferans” is the scientific term for sponges, members of the animal phylum Porifera, which in Greek means “bearer of pores”. Sponges are the simplest animals known. Unlike all other animal phyla, which have two- or three-layered (diploblastic or triploblastic) body plans, sponges have only a single (monoblastic) body layer and no true tissue. They have no appendages and no ability to make any movement, they lack muscle tissue. Sponges are exclusively aquatic.
Porifera feed by staying in one place, pumping water through themselves and filtering it for small organisms and food fragments that they digest. Sponges are protected from predators by their low nutrient content and irritating spines throughout their bodies, called spicules, which also act as a “skeleton”. Over 5,000 species are recognized by science and new species are discovered on a regular basis. That’s in part because the range of sponges is so wide—they’re found at every depth, from near shore to descending into oceanic trenches six miles deep. The simple body structure of the sponge lends itself to survival in oceanic pressures equal to tens of atmospheres.
For a long time sponges were thought to be the simplest organisms evolutionarily and the first animal phylum to exist. However, a landmark phylogenetic study in 2008 determined that sponges may be secondarily simplified (having evolved from more complex ancestors, likely with true tissue) rather than truly basal. In any case, it is common ground that the modern form of porifera is the simplest of all animal phyla.
Although porifera lack true tissue, they have cellular differentiation and display at least eight cell types, including choanocytes (“collar cells” with flagella that beat to pump water through the sponge), porocytes (tubular cells that make pores by pumping water), pinacocytes (which form the outer layer of cells), myocytes (muscle cells which open and close porocytes), archaeocytes (which can differentiate into other cells), sclerocytes (which secrete defensive spicules), spongocytes (which secrete the structural protein spongin) and collenocytes (which secrete other collagens).
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