Sponge

Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'). They are filter feeders that are typically bound to the seabed, being sessile. Sponges are one of the most ancient members of living seabed community (macrobenthos), with many species being important reef-builders. Some sponge species inhabit freshwater. Sponges are multicellular organisms consisting of jelly-like mesohyl sandwiched between two thin layers of cells, and usually have tube-like bodies full of pores and channels that allow water to circulate through them. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. They do not have complex nervous, digestive or circulatory systems. Instead, most sponges rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove waste products, usually via flagellar movements of the so-called "collar cells". Sponges may be the sister group to all other animals, although the evidence for this relationship remains inconclusive, and different studies may recover different placements between sponges and other animal clades. Fossil evidence of primitive sponges such as Otavia dates to as early as the Tonian period (around 800 Mya). The branch of zoology that studies sponges is spongiology. Historically, the skeletons of sea sponges were used as the cleaning tool of the same name, though this is no longer a common practice.

Etymology The term sponge derives from the Ancient Greek word σπόγγος spóngos. The scientific name Porifera is a neuter plural of the Modern Latin term porifer, which comes from the roots porus meaning "pore, opening", and -fer meaning "bearing or carrying".

Overview

Sponges are similar to other animals in that they are multicellular heterotrophs that lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs. Some of them are radially symmetrical, but most are asymmetrical. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where the water deposits nutrients and then leaves through a hole called the osculum. Their choanocyte cells are used to drive their water flow systems and capture most of their food. This, along with phylogenetic studies of ribosomal molecules, has been used as morphological evidence to suggest sponges are the sister group to the rest of animals. A great majority are marine (salt-water) species, ranging in habitat from tidal zones to depths exceeding 8,800 m (5.5 mi), though the freshwater sponges inhabit all continents other than Antarctica. All adult sponges are sessile, meaning that they attach to an underwater surface and remain fixed in place (i.e., do not travel). While in their larval stage of life, they are motile. Many sponges have a mesh-like endoskeleton composed of spicules made of either calcium carbonate or silicon dioxide (silica), embedded within spongin (a modified type of collagen). An internal gelatinous matrix called mesohyl functions as a binder, and it is the only skeleton in soft sponges that encrust such hard surfaces as rocks. More commonly, the mesohyl is stiffened by mineralized spicules, by spongin fibers, or both. The vast majority of extant sponges (over 90% of all known species) are demosponges, which have the widest range of habitats (including all freshwater ones); they use spongin, silica spicules, or both, and some species have calcium carbonate exoskeletons. Calcareous sponges (calcereans) have calcium carbonate spicules typically in high-magnesium forms of calcite or aragonite, and additional calcium carbonate exoskeletons in some species; they are restricted to relatively shallow marine waters where production of calcium carbonate is easiest. The fragile hexactinellids or glass sponges use "scaffolding" of silica spicules and are restricted to polar regions or ocean depths where predators are rare. Fossils of all of these types have been found in rocks dated from 580 million years ago. In addition Archaeocyathids, whose fossils are common in rocks from 530 to 490 million years ago, are now regarded as a type of sponge. The smallest class of extant sponges are homoscleromorphs, which either have calcium carbonate spicules like the calcereans or are aspiculate, and found in shaded marine environments like caves and overhangs. Although most of the approximately 5,000–10,000 known species of sponges feed on planktonic bacteria and other microscopic organic particles suspended in water, some become mixotrophic by hosting symbiotic photosynthetic microorganisms such as cyanobacteria and algae (zooxanthella) within the mesohyl, and these endosymbionts often produce more food and oxygen than they can consume themselves. A few species of sponges that live in food-poor environments have evolved as predatory carnivores that use fishhook-like spicules to snag benthic animals, mainly small crustaceans. Most sponges reproduce sexually, but they can also reproduce asexually. Sexually reproducing species release sperm cells into the water to fertilize ova released or retained by its mate or "mother"; the fertilized eggs develop into larvae which swim off in search of places to settle. Sponges are known for regenerating from fragments that are broken off, although this only works if the fragments include the right types of cells. Some species reproduce by budding. When environmental conditions become less hospitable to the sponges, for example as temperatures drop, many freshwater species and a few marine ones produce gemmules, "survival pods" of unspecialized cells that remain dormant until conditions improve; they then either form completely new sponges or recolonize the skeletons of their parents.

The few species of demosponge that have entirely soft fibrous skeletons with no hard elements have been used by humans over thousands of years for several purposes, including as padding and as cleaning tools. By the 1950s, though, these had been overfished so heavily that the industry almost collapsed, and most sponge-like materials are now synthetic. Sponges and their microscopic endosymbionts are now being researched as possible sources of medicines for treating a wide range of diseases. Dolphins have been observed using sponges as tools while foraging.

Distinguishing features

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