Written by Ralph De Felice
Adapted From: Hooper, JNA. 1998. Sponguide, version April 1998. Queensland Museum, Australia.
WHAT ARE SPONGES?
Sponges are the most primitive of multicellular animals (metazoa). They have a cellular grade, which means they have no true tissues. Adults are asymmetrical or radially symmetrical. Sponges are exclusively aquatic (water dwelling), most are marine. They are found from deepest oceans to the edge of the seam and from the coldest oceans to the tropics. Sponges play important roles in so many marine habitats but we still know very little about their diversity, biology and ecology as compared with most other animal groups. In many benthic (sea bottom) habitats sponges are often the dominant animals.
Sponges have an amazing range of growth forms, best described as highly irregular and sometimes completely plastic, frequently altered by prevailing external conditions (currents, turbidity, salinity etc.). Sponges come in just about every color imaginable. Adult sponges are sedentary (sessile), attached to the seabed or other substrate for most of their lives, although many have larvae that motile, swimming or crawling away from their parent. Sponges have sexes that are separate, or sequentially hermaphroditic, although most population dispersal and recruitment is asexual (through budding, fragmentation from storm events, etc). Larvae are motile, incubated within the parent or broadcast into the seawater.
Sponges filter sea water to eat, breath and excrete waste products. Sponges often have complex water canal systems running throughout the body, with smaller inhalant (ostia) and larger exhalant pores (oscules). Sponges are able to actively pump up to 10 times their body volume each hour, making them the most efficient vacuum cleaners of the sea. Sponges appear to be very stable, long-lived animals, although growth rates vary enormously between different groups. Some sponges can grow centimeters in weeks, and may have shorter life spans. Others sponges, like the living fossil 'sclerosponges' are VERY slow growing, with the largest known individuals (up to 30cm diameter) thought to be around 5,000 years old (which makes them the oldest living individuals on the planet, if this is true!).
Sponges are a unique group of animals because they have unique collar-cells (choanocytes) which are surrounded by cilia with a central flagellum that moves to actively create a current pulling water in and out of the sponge. These collar cells line the walls of small chambers throughout the water vascular system. There may be 7,000-18,000 of these chambers per cubic millimeter of sponge, and each chamber may pump approximately 1,200 times its own volume of water per day!
Sponges have no tissues or sensory organs but they do have many different types of cells with many different functions that carry out normal bodily routines, including a primitive cell type (called an archaeocyte, an amoeboid-like cell) that is totipotent (able to change functions as required by the sponge [e.g. secrete the skeleton, form the epidermis, become feeding and reproductive cells etc.]) Outer and inner layers of cells (exopinacocytes, basipinacocytes) (="the skin") lack a basement membrane; middle layer.(mesohyl) is variable but always includes motile cells and usually some skeletal material. Mineral skeleton is present in most (but not all) groups of sponges composed of calcium carbonate, silicon dioxide, and/or collagen fibres. Skeletal elements (spicules) are diverse in their geometry and size.
Sponges are individuals, having a continuous "skin" (epithelium) that contains roving cells inside; they are not colonies (like corals and sea-squirts in which individuals animals group together). Sponges catch, eat, digest their food and excrete their waste products within cells, not within any common body cavity (like a stomach), unlike most multicellular animals. Some sponges (particularly those growing on coral reefs) have a unique symbiosis with cyanobacteria not found in any multicellular animal. These cyanobacteria (or blue-green algae) provide the sponge with nutrients from photosynthesis to supplement those obtained by the sponge from normal filter feeding activities. These extra nutrients greatly augments sponge growth rate and competitive ability in coral reef systems.
Sponges have been around for a long time. They were already well established during the Lower Cambrian (>550 million years ago), and were major reef builders during the Devonian (>370 million years ago) before the reef-building corals "took over" reef formation; most modern genera and species similar to those around today appeared at the beginning of the Cretaceous (>150 million years ago).
WHAT DO SPONGES DO?
Sponges don't appear to do much at all, but in fact they are very active. Sponges sit on the bottom of the sea actively pumping seawater through their bodies. This water, containing nutrients, is filtered through a series of sieve-like pores. Nutrients are actively carried across the cell wall, engulfed by special cells (archaeocytes), and subsequently transferred throughout the sponge. In addition to "regular food", this seawater also contains the toxic chemicals excreted by other plants and animals, such as corals growing on the coral reefs above. Sponges feed on these chemicals, modify and reuse them for their own purposes.
Many of these sponge-modified and sponge-produced chemicals have potent toxicity against human pathogens, cancer cells etc., or are useful against certain human ailments (e.g. anti-inflammatory, cardiovascular, respiratory, analgesic etc. properties).
Why do sponges need these chemicals?
Sponges are not mobile, and as such they cannot escape from predators (such as fishes, turtles, gastropods, echinoderms, flatworms). Perhaps they use these chemicals to defend themselves?
Sponges do not have arms or legs and so they cannot physically remove other animals and plants settling on their exterior surfaces and from within water canals. Perhaps they use these chemicals to repel parasites?
Sponges are generally slow-growing, easily out-competed for space and living resources by other faster growing animals and plants (such as corals and ascidians). Perhaps they use this arsenal of chemicals in a chemical warfare against these faster-growing species?
Sponges are often full of small animals and microbes, particularly embedded within body cavities and throughout the water canals. Perhaps many of these chemicals are antibiotic to these microbes?
Some sponges burrow into corals and use chemicals to eat away the calcium, eventually occupying the entire interior surface of the coral (with breathing tubes or fistules poking through the surface). Other sponges grow on top of corals, smothering and eventually eroding the dead coral, and some that bore into oysters and pearl oyster shells killing the animal inside. There are also many sponges that live in between coral branches, at the base of corals and in the substrata surrounding corals that bind the corals together. Perhaps some of these chemicals are important in the continual process of erosion of coral reefs and releasing calcium back into the system, and consolidation of dead coral producing stable reef structures?
Some sponges have intimate, symbiotic relationships with other animals (such as gastropods, hermit crabs, shrimps), or plants (blue-green algae or cyanobacteria), which is frequently species specific. Perhaps these chemicals produce recognition signals between the symbiotic partners?
Probably a combination of all these important factors are why sponges contain so many different chemicals.
HOW MANY SPONGES ARE THERE?
The magnitude of sponge biodiversity is not known yet. In some habitats sponges are the dominant macrobenthic life form; in others they are very scarce.
Some places in the world have a relatively well-known sponge faunas (e.g. Mediterranean, Caribbean, British Isles), basically reflecting the relatively long period of time scientists have worked on these faunas. Each of these regions contains about 500-800 species.
Some areas have moderately well known faunas (e.g. Madagascar, New Caledonia, New Zealand, Sri Lanka, Micronesia, Japan) mainly as a consequence of recent scientific exploration during the past 20 years or so. Each of these regions probably has between 400-600 species.
Other places have poorly known sponge faunas, and (worse still) most of our knowledge is from old expeditions (pre-1900s; which makes "data" even more dubious). These areas include Australia, Indonesia, Papua New Guinea and other parts of the Indo-Malay archipelago. However, it is well known from studies on other groups of animals that this region has the highest proportion of the world's marine biodiversity, and there is no reason to show us that this is different for sponges. In Australia there are about 1,400 species described in the scientific literature, but we estimate that there are probably at least 5,000 species living in continental and territorial waters.
It is thought that this Indo-Malaysian region may contain a very high diversity, perhaps in the order of 4000-6000 species, but it is probably also the least comprehensively documented fauna by modern standards given that most of our knowledge of the fauna derives from pre- and early 1900 literature (plus generally unreliable identifications made by collectors for chemical studies).
Hawaiian Islands: There are about 100 species described from Hawaii. Most work on Hawaiian sponges was conducted in the 1950s and early 60s, before the widespread use of SCUBA. Based on recent work, it is estimated there are between 200-300 species in Hawaii. More than half of these are probably endemic (found only in Hawaii).
Worldwide: In the literature there are about 7,000 "valid" species published worldwide, BUT we estimate that there are at least 15,000 living species in all the world's seas and lakes.
HOW ARE SPONGES IDENTIFIED?
Sponges are often soft bodied, many are fragile and colors are generally unstable. Many sponges are also harmful to humans, producing physical damage (e.g. from sharp spicules protruding through the surface) and/or with an irritating mucus and other chemicals, sometimes causing severe dermatitis. Consequently, special care is taken when collecting to minimize damage to both the sponge and collector.
Collections of sponges intended for identification should be accompanied by underwater photographs and adequate documentation (locality, habitat, surface features, color notes etc.). In many species both coloration and morphology may change dramatically following collection and preservation, and identifications, even by specialists, are often greatly facilitated if there are adequate color photographs of live material.
Sponge identifications are primarily based on morphology. Some of these morphological characters vary substantially between widely separated populations, or those living in different habitats, whereas other features are much more consistent between individuals. There are many morphological characters which can be used to aid in sponge identification including shape, distribution of surface pores, color, ornamentation of the surface, texture, structure and composition of the organic skeleton and water canal system, and the structure, composition, size and geometry of the inorganic skeleton (spicules).
Organic and inorganic skeleton of a sponge are usually the primary characters used for identification. The organic (spongin fiber) skeleton is composed of strands of collagen, and the patterns they form, and the material contained within the fibers are important characters used in classification. An inorganic (spicule) mineral skeleton, is often found within and outside spongin fibers. Spicules are constructed of either silica (glass) or calcite, and the shape, ornamentation, size, origin and arrangement of these spicules inside the sponge are also important characters used for classification.
Most sponges are not easy to identify, even for experts, requiring specialized techniques for preservation of the specimen, examination of spicules and thin sections of the sponge under a light microscope and/or a scanning electron microscope (SEM). Therefore most sponges cannot be identified with confidence without extensive and time consuming work in the laboratory.
WHAT IS THE ROLE OF TAXONOMISTS? AND WHY ARE MUSEUM COLLECTIONS IMPORTANT?
Documenting and describing biodiversity is a long, time consuming process that requires accuracy and patience, but it is an essential prerequisite to conservation and management of our marine resources. Collections and collection databases (containing accurate locality data, accurate descriptions of species' characteristics) are the main functions of museums and provide them with unique resources that other scientists don't have (usually no time to build collections and maintain them).
The taxonomist's role is to publish these species descriptions and distributions, with the (maybe impossible) aim of fully document regional biological resources in a systematic, scientific way.
Good taxonomy underpins every other branch of biological and biochemical science. Comprehensive, accurate descriptions, with recognizable illustrations and accurate locality data are essential for determining the relationships between species and regions (biogeography), how many species are shared between closer or more distant regions (species' dispersal potential, reproductive mechanisms etc.), habitat preferences (ecology), population variability (genetics) and chemical relationships (chemotaxonomy, marine natural products chemistry).
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