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Porifera are a Phylum of Sessile (permanently affixed) and mostly marine Invertebrate Animals. Poriferans are commonly known as 'Sponges'. They are one of the most primitive multicellular aquatic Animals. There are approximately 9,000 known Species of Porifera of which: about 400 are Glass Sponges; about 500 are Calcareous Species and the rest are Demosponges. Majority of Porifera Species are Marine with only a few Species found in fresh water. Fossils and molecular data put their origin at somewhere around 650 million years ago, nearly back to the dawn of Animal life. DNA analysis indicates that the Sponge’s immediate evolutionary predecessors are the 'Protistan Choanoflagellates'.
Sponges in temperate regions live for at most a few years, but some tropical species and perhaps some deep-ocean ones may live for 200 years or more. Early Europeans used Soft Sponges for many purposes, including padding for helmets, portable drinking utensils, municipal water filters, applicators for paints and ceramic glazes and discreet contraceptives. In modern times Sponges are used mostly in pottery and jewellery making, painting and decorating, and in surgical medicine. Most valuable Sponges are found in the eastern Mediterranean area. They also are harvested off the west coast of Florida and the Florida Keys, in the West Indies, off Mexico and Belize, and in some areas of the Philippines. Since many Sponges exist on Coral Reefs some are threatened by Reef destruction.
Scientific Classification
Domain - Eukaryota.
Kingdom - Animalia.
Phylum - Porifera.
1. Calcarea - These have skeletons made of Calcite. There are about 300 Species.
Subclass - Calcinea.
Subclass - Calcaronea.
2. Hexactinellida (Hyalospongiae) - They are also known as Glass Sponges. They have silicate spicules, the largest of which have six rays and may be individual or fused. There are about 500 Species.
3. Demospongiae - These have silicate spicules or spongin fibres or both within their soft tissues. Some Demospongiae also have massive external skeletons made of aragonite, another form of calcium carbonate. There are about 4,200 Species.
Sponges are built up from relatively few cell types. These are:
a) Choanocytes - These are vase shaped cells with a collar of fine fibrils connected by microvilli. It strains out the smallest food items from the water such as individual Bacteria. Extending from the centre of this collar is the single flagellum whose beating drives the water currents that keep the Sponge alive and healthy.
b) Pinacocytes - These form much of the epidermis of Sponges and are as close as a Sponge gets to having a tissue. Generally they cover the exterior and some interior surfaces. They can change their size (contractile) and can therefore change the size of the openings of the ostia thus controlling the flow of water through the Sponge. Pinacocytes are also implicated in the absorption into the Sponge of larger food items.
c) Amoebocytes - These come in several forms, they are alike in that they are mobile and move around within the Sponge body. Archaeocytes are the basis of some asexual reproductive gemmules. If an amoebocyte secretes the spongin fibres of the skeleton they are called a 'Fibroblast', if it secretes spicules it is called a 'Scleroblast' and if it is star shaped and secrete collagenous fibrils then it is called a 'Collencyte'.
d) Lophocytes - These are a type of amoebocyte, they are the most motile of the Sponge cells moving around relatively freely within the mesohyl where they are important in the secretion of fibrils.
Adult Sponges are Asymmetrical or Radially Symmetrical. Sponges come in a variety of shapes. Some form a crust on their rocky habitat. Some form a single straight tube. Others are vase shaped or cup shaped while others are massive clumps. Some others are fan shaped, while others have mitten-shaped or finger-like bulges on the body wall. Others are treelike or bushy.
Skeletal Arrangement in Porifera
a) Calcarea Sponges - They have spicules of calcium carbonate that have 1, 3 or 4 rays, a a skeleton that involves a single large lump of calcium carbonate rather than spicules.
b) Demospongiae - They have their spicules made from silica and they have 1, 2, or 4 rays.
c) Hexactinellida or 'Glass Sponges' - They have spicules made from silica that are 6 rayed.
Colour - Porifera come in a variety of colours. Deep-water Sponges are usually of neutral colour, drab or brownish. While shallow-water Sponges are frequently brightly coloured, ranging from red, yellow, and orange to violet and occasionally black. Most calcareous Sponges are white. Some Sponges like, the Spongillidae, are often greenish because green Algae live in a symbiotic relationship within them; others are violet or pinkish, because they harbour symbiotic blue-green Algae. These symbionts endow the Sponges with colour as long as light is available; the Sponges become white in the dark when no photosynthesis occurs and the Algal pigments utilized in photosynthesis are no longer produced.
Size - The height and width of Sponges ranges from less than one inch (a few millimetres) to about 6.5 feet (2 meters).
Diet - Most of Porifera feed on Bacteria and other food particles in the water, some host photosynthesising micro-organisms as Endosymbionts and these alliances often produce more food and oxygen than they consume. A few Species of Sponge that live in food-poor environments have become carnivores that prey mainly on small Crustaceans. They capture small Crustaceans with their spicules (Small pointed structures) which act like Velcro when they come in contact with the Crustacean exoskeletons. Cells then migrate around the helpless prey and digestion takes place extracellularly. Other Sponges harbour symbionts such as Green Algae, Dinoflagellates, or Cyanobacteria, from which they also derive nutrients.
Sponges are found worldwide, from the polar regions to the tropics. They occur in rivers and streams, from rock pools to the deep ocean depths, from frozen arctic seas to the warm tropical seas. The greatest numbers of Sponges are usually found on firm surfaces such as rocks. Sponges are more abundant but less diverse in temperate waters than in tropical waters, possibly because organisms that prey on Sponges are more abundant in tropical waters.
It is widely considered that Poriferans arose from Flagellated Protozoans. The primitive structure of the Porifera indicates affinities with certain types of Protozoan colonies; both lack integration of parts, mouths, and digestive systems, and both have a type of skeletal formation in which single elements are produced by a single cell or by a small group of cells. The earliest evidence of Poriferans in the fossil record consists of traces of 24-isopropylcholestane, a chemical formed by the breakdown of lipids in Sponges, that date back to the Cryogenian Period of the Proterozoic Era (about 635 million years ago). The first Sponge skeletons, however, appear in rocks that date to the Ediacaran Period (630 million to 542 million years ago). Calcareous Sponges appear in the Carboniferous Period (about 345 million to 280 million years ago). The first members of order Pharetronida from the Permian Period (about 280 million to 230 million years ago) had compact calcareous skeletons and spicules. Present day Sponges do not differ substantially from many groups in the Paleozoic Era.
Some Sponges start sexual reproduction when only a few weeks old, while others wait until they are several years old. Sponges reproduce by both asexual and sexual means, however they are only one gender at a time, being either Male or Female or neuter. Some Species like 'Halichondria moorei' change colour when they change sexes though most do not. Sponges have no permanent gonads, instead a number of areas of the Sponge will during the reproductive period become differentiated (changed) to produce either sperm or ova (eggs). Sperm are frequently 'Broadcast' into the water column, i.e. the sperm are created, concentrated and sent out the excurrent openings, sometimes in masses so dense that the Sponges appear to be smoking. These sperm are subsequently captured by Female Sponges of the same Species. Inside the Female, the sperm are transported to eggs by special cells called 'Archaeocytes'. Fertilization occurs in the mesenchyme and the zygotes develop into ciliated larvae. Some Sponges release their larvae, where others retain them for some time. Once the larvae are in the water column they settle and develop into juvenile Sponges. Sponges that reproduce asexually produce buds or, more often, gemmules, which are packets of several cells of various types inside a protective covering. Fresh water Sponges of the Spongillidae often produce gemmules prior to winter. These then develop into adult Sponges beginning the following spring.
Sponges have three asexual methods of reproduction:
i) After Fragmentation - Fragments of Sponges may be detached by currents or waves, and perhaps by predators. They use the mobility of their Pinacocytes and Choanocytes and reshaping of the mesohyl to re-attach themselves to a suitable surface and then rebuild themselves as small but functional Sponges over the course of several days.
ii) By Budding - Very few Species reproduce by budding. Budding is possible only if a Sponge fragment contains both collencytes to produce mesohyl and archaeocytes to produce all the other cell types.
iii) By Producing Gemmules - Gemmules are 'Survival Pods' which a few marine Sponges and many freshwater Species produce by the thousands when dying. Spongocytes make gemmules by wrapping shells of spongin, often reinforced with spicules, round clusters of archaeocytes that are full of nutrients. The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity. Freshwater gemmules often do not revive until the temperature drops, stays cold for a few months and then reaches a near-"normal" level. When a gemmule germinates, the archaeocytes round the outside of the cluster transform into 'Pinacocytes', a membrane over a pore in the shell bursts, the cluster of cells slowly emerges, and most of the remaining archaeocytes transform into other cell types needed to make a functioning Sponge. Gemmules from the same Species but different individuals can join forces to form one Sponge. Some gemmules are retained within the parent Sponge, and in spring it can be difficult to tell whether an old Sponge has revived or been recolonized by its own gemmules.
