SPONGES (Branches of the tree of life, BioMEDIA ASSOCIATES, Bea

Class HexactinellidaHexactinellid sponges are fairly common, but not so easy to see. To find them you mayneed to join a well-equipped expedition such as this. The team sets out to survey life atthe bottom, 200 meters below. What they discovered are huge reefs composed almostentirely of hexactinellid sponges. Core samples from the base of the sponge reef indicateover 9000 years of continuously growth. It’s entirely possible that individual spongesseen here at the top of the reef have grown for hundreds or even thousands of years.The tissue of hexactinellids it not cellular but a mass of cytoplasm containing thousandsof nuclei. The term of this is syncytial. These sponges are also called glass sponges.Their glass spicules are usually six- pointed, long and thin, tightly meshed around thesyncytial tissues. Remove the tissue from a glass sponge and you will be left with anintricate glass skeleton.Class DemospongiaClass Demospongia includes sponges with two different types of skeleton. You mayhave rubbed one skeleton type against your skin. Bath sponges were traditionally madefrom warm-water marine sponges, by chemically removing the cellular tissue. A thinsection reveals the skeleton – a matrix of linked fibres made of a flexible protein,collagen.The second group of demosponges have spicules made of silica, the same materialfound in glass; while HCl acid dissolves the spicules of calcereous sponges, it has noeffect on the glass spicules of demosponges.Class Demospongia is by far the largest group of sponges. Members of this group arethe most common incrusting species, and upright forms, such as the barrel sponges, areabundant on coral reefs.There are over 7000 known species of desmosponges, and it is estimated that there maybe just as many species yet to be described.EcologySponges are permanently attached to the ocean floor, so why aren’t they easy prey forhungry predators? The answer is simple: sponges have evolved remarkable abilities toproduce poisons. To most marine predators they probably taste and smell very bad. Butone group of predators is not easily foiled: nudibranchs or see slugs have two organs onthe top of the head that detect the smell of the sponge prey. The nudibranch not onlyeats the sponge, it stores the sponge poisons in its own tissue, thereby stealing thedefence of its prey. Many nudibranchs add another line of defence, looking so like theirsponge prey they become hidden from predators. This nudibranch even hides its eggs onthe surface of its food. But other sponge-eating nudibranchs seem advertise themselves–probably benefiting when predators learn they taste like poisonous sponge.This sponge has suffered severe wounding, but in a few months it will repair itself,regenerating lost tissue. Using the same power of regeneration, some sponges dispersefragments that grow into new individuals, a form of asexual reproduction. Another formof asexual reproduction involves the formation of gemmules by some marine spongesand most freshwater sponges. Gemmules are produced in the late fall and the spongetissue around them disintegrates. In spring, a new clone of the sponge grows up out ofthe gemmule.Occasionally, sponges reproduce sexually; when the stimulus is right, cells in thesponge become sperm, which are shed into the water. Of the millions of sperm released,some are taken in by a the receptive sponge, where they fertilize eggs inside the tissue.The larvae develop for a few days, and then break out. They swim for a few hours, andthen settle to the bottom, where they begin a new sponge.

The simplest living animals provide clues to the origin of the animal kingdom. In fact,rocks dated around 600 million years ago show fossils of simple sponges. Older rocksshow ancient Earth teaming with single-cell life, and single colonies, but nomulticellular animals. Did animals evolve many times from different single-celledancestors? Or did they evolve just once?Recent studies from genes from all animals groups show clearly that the answer is justonce, and that sponges are at the very base of the animal tree of life. So the first animalwas a sponge, and all other animals descended from this ancestor. But what about thesingle-celled ancestor of that ancient sponge. You can search for its living relatives in apond near you. These simple colonies, choanoflagellates, are made of cells that lookvery much like collar cells. Gene studies reveal that sponges and choanoflagellates arevery closely related, making choanoflagellates our nearest protist ancestors.Sponges are simple animals with no organs, and just a few cell types, including one thatis unique, the flagellated collar cell or choanocyte.Sponges are an ancient line of animals, a line that evolved at least 600 million years ago,and gave rise to the three main groups of living sponges: calcerea, hexactinellida anddemospongia. They’re found in all aquatic environments, but are far more diverse andabundant in oceans than in freshwater.Despite their simplicity, sponges are colourful and diverse, and they are very successful,with as many as 15000 species living through the world oceans and in freshwater.