Sponge
This article is about the animal. For other uses, see Sponge (disambiguation). In addition to these three, a fourth class has been proposed:
Archaeocyatha. While these ancient animals have been phylogenetically vague for years, the current general consensus is that they were a type of sponge.
Although 90% of modern sponges are
demosponges, fossilized remains of this type are less common than those of other types because their skeletons are composed of relatively soft spongin that does not fossilize well. The fossil
Archaeocyantha may also belong here, though their skeletons are solid rather than separated into spicules. It has been suggested that the sponges are
paraphyletic to the other animals. Otherwise they are sometimes treated as their own subkingdom, the
Parazoa. Similar fossil animals known as
Chancelloria are no longer regarded as sponges. Sponges have holes throughout their bodies in addition.
One phylogenetic hypothesis based on molecular analysis proposes that the phylum Porifera is in fact
paraphyletic, and that members of Porifera should be split into two new
phyla, the
Calcarea and the
Silicarea.
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Fossil sponge Raphidonema farringdonense from the Cretaceous of England |
The fossil record of sponges is not abundant, except in a few scattered localities. Some
fossil sponges have worldwide distribution, while others are restricted to certain areas. Sponge fossils such as
Hydnoceras and
Prismodictya are found in the
Devonian rocks of
New York state. In
Europe the
Jurassic limestone of the Swabian Alps are composed largely of sponge remains, some of which are well preserved. Many sponges are found in the
Cretaceous Lower Greensand and
Chalk Formations of
England, and in rocks from the upper part of the
Cretaceous period in
France. A famous locality for fossil sponges is the
Cretaceous Faringdon Sponge Gravels in
Faringdon,
Oxfordshire in
England. An older sponge is the
Cambrian Vauxia.
Fossil sponges differ in size from 1 cm (0.4 inches) to more than 1 metre (3.3 feet). They vary greatly in shape, being commonly vase-shapes (such as
Ventriculites), spherical (such as
Porosphaera), saucer-shaped (such as
Astraeospongia), pear-shaped (such as
Siphonia), leaf-shaped (such as
Elasmostoma), branching (such as
Doryderma), irregular or encrusting.
Detailed identification of many fossil sponges relies on the study of thin sections.
Sponges are able to reproduce
sexually or
asexually. Most sponges are
monoecious. Sperm are formed from
choanocytes; and
oocytes derive from choanocytes in some sponges, and archaeocytes in others. Sperm are released into open water. Some sponges have oviparous fertilization where sperm and eggs meet in open water. In other species sperm are taken into the bodies of other sponges and fertilized inside; this is known as viviparous fertilization. The free-swimming larvae, known as parenchymula, serve as one of the main forms of dispersal for this largely sessile phylum. Asexual reproduction of sponges is through
budding, where a small piece of sponge falls off of the main sponge and grows into a new one. Many freshwater sponges form small structures known as
gemmules, which function as overwintering devices. These are similar to a
bacterium's
endospore. Gemmules are made up of
amoebocytes surrounded by a layer of
spicules and can survive conditions that would kill adult sponges. When the environment becomes less hostile, the gemmule resumes growing.
Modern sponges are predominantly marine, with some 150 species adapted to freshwater environments. Their habitats range from the inter-tidal zone to depths of 6,000 metres (19,680 feet). Certain types of sponges are limited in the range of depths at which they are found. Sponges are worldwide in their distribution, and range from waters of the polar regions to the tropical regions. Sponges are most abundant in both numbers of individuals and species in warmer waters.
Adult sponges are largely sessile, and live in an attached position. However, it has been noted that certain sponges can move slowly by directing their water current in a certain direction with myocytes. The greatest numbers of sponges are usually to be found where a firm means of fastening is provided, such as on a rocky ocean bottom. Some kinds of sponges are able to attach themselves to soft sediment by means of a root-like base. Sponges also live in quiet clear waters, because if the sediment is agitated by wave action or by currents, it tends to block the pores of the animal, lessening its ability to feed and survive.
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Natural Sponges in Tarpon Springs, Florida |
In common usage, the term
sponge is usually applied to the skeletons of these creatures alone, from which the animal matter has been removed by
maceration and washing. The material of which these sponges are composed is spongin.
Calcareous and
siliceous sponges are too harsh for similar use. Commercial sponges are derived from various species and come in many grades, from fine soft "lamb's wool" sponges to the coarse grades used for washing cars. The
luffa sponge, also spelled "loofah," commonly sold for use in the kitchen or the shower, is not related to any animal sponge but is derived instead from the locules of a pepo fruit (
Cucurbitaceae). Marine sponges come from fisheries in the
Mediterranean and
West Indies. The manufacture of
rubber,
plastic and
cellulose based synthetic sponges has significantly reduced the commercial sponge
fishing industry over recent years. Additionally, synthetic sponge products can be up to 10 percent more effective at retaining liquids, as compared to a natural sponge.
