- This article is about the animal. For other uses, see Sponge (disambiguation).
Sponges, or poriferans, are animals of the phylum Porifera. They are sessile, mostly marine, waterdwelling filter feeders, pumping water through their matrix and filtering out particulates of food matter. Sponges are among the simplest of animals, with partially differentiated tissues but without muscles, nerves, and internal organs. In some ways they are closer to being a cell colony than multicellular organisms. There are over 15000 modern species of sponges known, and more are being discovered every day. The fossil record of sponges dates back to the precambrian era. The structure of a sponge is simple: it is shaped like a tube, with the end stuck to a rock or other object. The open end is called the osculum, and the interior is the spongocoel. The walls are perforated with microscopic pores that allow water to flow through the spongocoel.
Sponges have only four types of cells:
- Choanocytes (also known as "collar cells"), which line the spongocoel and function as the sponge's digestive system, are remarkably similar to the protistan choanoflagellates.
- Porocytes are tubular cells that make up the pores.
- Flat epidermal cells line the outside of the sponge and form its skin.
- Amoebocytes live between the choanocytes and the epidermis. They carry out many of the sponge's functions, such as transport of nutrients, secretion of the spicules, and production of gametes.
Sponges have only three body types: asconoid, syconoid, and leuconoid. Although animals, sponges may also be considered multi-cellular colonies. A sponge can be placed in a blender and the remaining living cells will reform themselves into another sponge. If multiple sponges are blended together, each species will recombine independently.
It is thought that the earliest animal life on Earth resembled sponges. The earliest known multicelled animal fossils are sponges from China that are roughly 600,000,000 years old. Sponges have not been as extensively studied as some other phyla and there may be some surprises still to be found. For example, it has recently been shown that some sponges are not sessile and can move to more favorable locations as rapidly as few cm a day. Another sponge, the Venus Flower Basket has some newly discovered uses involving fiber optics.
Sponges are divided into classes based on the type of spicules in their skeleton. The four kinds of sponges are bony (Calcarea), glass (Hexactenellida), spongin (Demospongiae), and a fourth kind made up of a mixture of types (the Sclerospongiae). Although 90% of modern sponges are demosponges, their fossilized remains 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 can reproduce either asexually or sexually. During bad times, sponges can form small structures called gemmules, analogous to the endospore of bacteria. The gemmule, made up of a few amoebocytes surrounded a tough layer of spicules, can last through conditions that would kill adult sponges. When conditions are again favorable, the gemmule will resume growing. Sponges also reproduce asexually by budding, where a piece of a sponge falls off and grows into a new one. Sexual reproduction in sponges is relatively simple. Sperm from one sponge swims to the egg of another, producing a motile larval stage.
Modern sponges are predominantly marine, with some species adapted to freshwater environments, ranging 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. Their body are porous and they feed by filtering micro-organisms from the water. They lack any internal organs, a nervous system, or circulatory and digestive systems, such as are found in the higher invertebrate animals.
Adult sponges live in an attached position, and tend to lack any means of locomotion. The greatest numbers of sponges are usually to be found where a firm means of fastening is provided, such as on a rocky 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.
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. They come from the 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.
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 Farringdon Sponge Gravels in Farringdon, Oxfordshire in England.
Fossil sponges vary 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), 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.