There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered.
Sponges have several cell types:
Asconoid sponges are tubular with a central shaft called the spongocoel. The beating of flagella forces water into the spongocoel through pores in the body wall. Choanocytes line the spongocoel and filter nutrients out of the water.
Leuconoid sponges lack a spongocoel and instead have flagellated chambers, containing choanocytes, which are led to and out of via canals.
Syconoid sponges are similar to asconoids. They have a tubular body with a single osculum, but the body wall is thicker and more complex than that of asconoids and contains choanocyte-lined radial canals that empty into the spongocoel. Water enters through a large number of dermal ostia into incurrent canals and then filters through tiny openings called prosopyles into the radial canals. Their food is ingested by the choanocytes. Syconoids do not usually form highly branched colonies as asconoids do. During their development, syconoid sponges pass through an asconoid stage.
It should be noted that these 3 body grades are useful only in describing morphology, and not in classifying sponge species, although the asconoid and syconoid construction is present in Calcarea only.
Sponges of the family Cladorhizidae (order Poecilosclerida, class Demospongiae) are species usually found in deep water, but also in littoral caves in the Mediterranean (Asbestopluma hypogea), that have become carnivorous, using a strategy that has much in common with what is found in carnivorous plants such as sundew. When small crustaceans comes in contact with their surface, they get captured by a sticky substance, or in the case of the Mediterranean species by spicules modified into raised hook-shaped spines, and then digested by migrating cells which soon covers the prey. This lifestyle has caused the loss of their aquiferous system and the choanocytes, resulting in forms like the ping-pong tree sponge (Chondrocladia lampadiglobus), which don't look like typical sponges.
Sponges are traditionally divided into classes based on the type of spicules in their skeleton. The three classes of sponges are bony (Calcarea), glass (Hexactenellida), and spongin (Demospongiae). Some taxonomists have suggested a fourth class, Sclerospongiae, of coralline sponges, but the modern consensus is that coralline sponges have arisen several times and are not closely related. In addition to these four, a fifth, extinct 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.
Sponge taxonomy is an area of active research, with molecular studies improving our understanding of their relationship with other animals.
Either way, sponges have long been considered useful models of the earliest multicellular ancestors of animals.
The fossil record of sponges is not abundant. 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 Alb 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. Sponges have long been important agents of bioerosion in shells and carbonate rocks. Their borings extend back to the Ordovician in the fossil record.
Fossil sponges differ in size from 1 cm (0.4 inches) to more than 1 meter (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.
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.
Recent evidence suggests that a new disease called Aplysina red band syndrome (ARBS) is threatening sponges in the Caribbean. Aplysina red band syndrome causes Aplysina to develop one or more rust-coloured leading edges to their structure, sometimes with a surrounding area of necrotic tissue so that the lesion causes a contiguous band around some or all of the sponge's branch.
Asexual reproduction is through internal and external budding. External budding occurs when the parent sponge grows a bud on the outside of its body. This will either break away or stay connected. Internal budding occurs when archaeocytes collect in the mesohyl and become surrounded by spongin. The internal bud is called a gemmule, and this is seen only in the freshwater sponge family, the Spongillidae. An asexually reproduced sponge has exactly the same genetic material as the parent.
In sexual reproduction, sperm are dispersed by water currents and enter neighboring sponges. All sponges of a particular species release their sperm at approximately the same time. Fertilization occurs internally, in the mesohyl. Fertilized oocytes develop within the mesohyl. Cleavage stages are highly varied within and between groups, sometimes even within a single species. Larval development usually involves an odd type of morphogenetic movement termed an inversion of layers. When this occurs in some species (for example, in Sycon coactum), the larva flips into the choanocyte chamber, and then can emerge via the water canal system and out through the osculum.
Although sponges are hermaphroditic (both male and female), they are not self-fertile. Most sponges are sequential hermaphrodites, capable of producing eggs or sperm, but not both at the same time.
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