Sponges Most are typically hermaphroditic  (both male and  female) and form their gametes from choanocytes (sperm) and archeocytes (eggs). During spawning,  sperm enter the excurrent canals and are ejected into the water column where they are picked up by another sponge downstream. The fertilized eggs are typically larviparous, that is, they are retained and development proceeds internally until a free-swimming, lecithotrophic larva is released. In most demosponges (= most sponge species) the larva is called a  parenchymella. Some demosponges like the boring sponge Cliona are not larviparous and release the eggs soon after they are fertilized.

 

 

            from: www.bioweb.usc.edu/courses/2003-     

            fall/documents/bisc120-  

           13_CH_33(1)_Sponges.pdf

 

sponge Xestospongia muta releasing sperm

   from www.waterexplorer.com/il_cool02.htm

 Characteristics of the Phylum Porifera

See appended and detailed sponge chapter if this is new to you

 

Sponges are regarded as a sister group to all other Metazoans (Eumetazoans), often called the "Parazoa". An early branching event in the history of animals separated the sponges from other metazoans.They appear first in the fossil record and have, by far, the simplest metazoan body plan. Sponges are Metazoa at the cellular grade of construction; they have a very restricted cell repertoire and are without true tissues; the adults are asymmetrical or radially symmetrical.

 

Cells tend to be totipotent and be very flexible developmentally. Remarkably, they can reassemble after being dissociated into single-cell suspensions. If cells from different species are mixed, they will recognize themselves and form separate sponges.

 

Sponges have a feeding system unique among animals. They don't have mouths; instead, they have pore-bearing cells (porocytes) in their outer walls through which water is drawn. The large opening(s) at the top is the osculum (pl. oscula) through which water exits after it has passed through the sponge. Adults are sessile suspension feeders (typically), feeding primarily on detritus, small flagellates and bacteria. Fertilization typically occurs in the water, but the egg may be retained and brooded by some species. In either case, a larval stage, usually called a parenchymella, forms. More will be said about reproduction under the Zooplankton section.

        Sponges have specialized flagellated cells – choanocytes – that drive water through canals and chambers constituting the aquiferous system.

 

The choanocytes can be packaged in three levels of complexity: ascenoid, synconoid, leuconoid.

 

The middle layer -the mesohyl -  is variable in its composition, but always includes motile cells, connective tissue, and usually some inorganic skeletal material. The connective tissue is a form of collagen called spongin that is somewhat similar to that found in vertebrates. Skeletal elements, when present, are composed of microscopic spicules composed of either calcium carbonate, or more commonly, a form of silicon dioxide. The actual form of the mineral deposited is hydrated silicon dioxide, which has the general formula SiO2.(H2O)x and is similar to the the semi-precious gemstone called opal. Thus, sponge spicules and other biologically formed silica minerals are referred to as biogenic opal.

 

Sponges are enormously successful in benthic habitats, due in part to elaborate chemical defenses. Approximately 10,000 species, 80%-90% of which are placed in the Class Demospongiae. Almost all demosponges have glass spicules except for the commercially valuable species(see below). Calcitic spicules are made by sponges in the class Calcarea. Hexactinellid sponges class Hexactinellida make distinctive six-rayed glass spicules and are now mostly deep sea species, but at one time flourished in shallow water(see below).

The sponge below (left) is the yellow connective tissue remains of a demosponge that does not manufacture spicules. It is one of about 12 species in that class whose skeleton is composed entirely of resilient, highly absorptive and strong spongin fibers. These are the commercially valuable sponges and most of these do not grow in reef environments, but they are interesting nonetheless. There are 5 commercially valuable sponge species that grow in Florida. Their commercial value, however, has been diminished by the use of artificial sponges (below, right) which are made of cellulose, not spongin. Cellulose sponges have neither the strength nor the absorptive capacity of natural sponges, but you can buy 4 them for a dollar with your choice of colors. That is why the sponge fishery industry, which flourished in Tarpon Springs among other places in Florida until the 1940's, has now been relegated to a tourist attraction. Artisanal sponge fishing still exists in Florida and in the Bahamas, but does not appear to be ready for an economic comeback.

 

 

Sponge Plumbing

Many suspension-feeders modulate the flow rate of the water they filter to maximize capture of suspended particles and to minimize recirculation of filtered water. We briefly considered the plumbing architecture of sponges. Flow is generated by the beating of specialized ciliated choanocytes. Their beating moves water at approximately 50 microns/sec, a rate that allows efficient particle capture. The combined cross-sectional area of the filtering surface is much larger than that of the exhalent opening (the osculum) in the sponge. As water flows out of the relatively constricted exhalent opening, its velocity increases to 10 cm/sec which maximizes water flow out of the sponge..

 

Boundary Layer Flow and Bernouilli's Principle

As water flows over the seafloor, its flow rate diminishes to zero as it approaches the actual surface. This zone of diminished flow is called the BOUNDARY LAYER and, although it is often less than 1 cm deep, it is biologically important for suspension-feeding animals which must project above it for efficient food capture.

 

Many bottom living animals take advantage of the Boundary Layer to promote passive circulation of oxygenated water through their bodies (sponges) and/or burrows (infaunal worms) shells (abalone). Positioning one of the burrow openings above the boundary layer produces a greater water flow rate across this opening. According to Bernouilli's Principle, pressure will vary inversely with the velocity of the fluid so the upper exhalent opening (osculum) experiences lower pressure and, as a result, water passively flows in the lower opening and out of the top opening without the expenditure of energy by the organism. Bernouilli's Principle also explains the lift generated by curved upper surfaces of airplane wings and flatfish by the increased flow rates, and decreased pressure, generated over the dorsal surface.

 

Hexactinellid Reefs

Hexactinellid (glass sponge) reefs cover nearly 1000 square km of seafloor on the western Canadian continental shelf off British Columbia, between the Queen Charlotte Islands and the mainland. The sponge reef complexes began to grow about 10,000 years ago and today form structures more than 15 meters high on the seafloor, and kilometers wide.

 

The sponge reefs found off the B.C. coast are the only known example of Hexactinellid sponge reefs living today in the world. They were at one time widespread across much of the world. During the Jurassic, especially the Upper Jurassic, the most common reef type was not the coral reefs, which we see today, but was instead sponge reef. We can find the evidence of these extensive sponge reefs preserved in rocks from across much of southern Europe and even in North America. The total width of the Jurassic sponge reef belt was 7000 km (To compare the Great Barrier Reef is about 2000 km long). The main groups of siliceous sponges that formed these reefs - Hexactinellids - are very much unchanged in their basic form and, as far as we can determine, their lifestyle for millions of years. The reefs we see today off B.C. are formed by Hexactinellid sponges and are an analogue for the immense belt of sponge reefs that once stretched in a continuous zone from North America to Romania.

        Hexactinellids, now mostly deep sea sponges, have a very distinctive spicule type cellular architecture in that each individual is a giant syncitium. Unlike other sponges, they consist mostly of a single enormous cell with countless nuclei that is stretched over the sponge's skeleton. No cell membranes divide the interior to act as barriers. It's just one continuous cytoplasm from one end of the sponge to the other. Such cells are not unusual in the animal world--the axon of the giant squid is a well-studied example. So are our own muscle cells. What is unusual is for an entire adult animal to be virtually a single cell with many nuclei.

 

Boring Sponges and Chemical Defenses

Boring sponges are members of the order Demospongiae (as most species are), but the most common belong to the family, the Clionidae, an example of which is shown below. These play important role in the breakdown of carbonate substrates and are important to the process of bioerosion on coral reefs. In addition, boring sponges can kill living coral tissue as it grows.  Clionids are also unusual in that they often have zooxanthellae as symbionts as opposed to the more common zoocyanellae found in other groups of  sponges.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a red clionid, Cliona delatrix, shown boring into a scleractinian coral colony

 

Other less common families contain boring sponges that excavate large carverns deep within the substrate. The cavities are lined with sponge tissue and because there can be considerably more sponge biomass within the carbonate compared to the surface, the sponge may be provided with some protection from grazers, or may increase  the survival of non-motile larval stages. In either case, boring sponges have specialized archaeocytes called etching cells are responsible for the boring activity. Each etching cell chemically cuts into the substrate and mobilizes the fragment into its aquiferous system. The carbonate is released as sediment emanating from the osculum, a process that has been observed in the field.

Siphonodictyon coralliphagum bores deeply into carbonate substrate

including corals as shown (inset), and has prominent oscula perched on chimney-like

tissue that extends far above the substrate surface, a feature lacking in clionids.

 

Sponges are strong competitors for space in benthic habitats and many species have evolved the ability to produce suites of bioactive chemicals (allelochemicals) that are used as deterrents (against predators) and as offensive weapons (against space competitors). We are just beginning to understand this ecological/evolutionary scope of this process and sponges are now being extensively sampled by pharmaceutical companies for useful drugs.

Agelas sp. is the brown sponge in the foreground

 

This biosynthethic ability may well be central to the remarkable success these simple animals have attained in benthic habitats. Many sponge species are preyed upon by a very limited suite of predators; often single species of nudibranch snails that specialize on a single sponge species and have evolved elaborate detoxifying biochemical pathways to process the allelochemicals produced by that particular sponge. The chemical defenses of species of The Caribbean sponge Agelas produce a suite of secondary metabolites, the most conspicuous of which are brominated pyrrol alkaloids which are present in mg/ml-1 quantities and are distasteful to many potential predators.

 

Carnivorous sponges

Sponges have recently been discovered that completely lack choanocytes and capture prey by ensnaring them in "velcro-like" surface spicules, followed by their digestion. This feeding mode resembles that of some carnivorous land plants and this type of sponge is found most frequently in low nutrient habitats such as the deep sea and marine caves. Silicon dioxide spicules, jut out from the filaments like tiny shards of glass. The spicules act as hooks, so that small crustaceans are trapped as if the surface were Velcro. The cells of the sponge migrate as soon as the prey is trapped. After 24 hours, the prey is completely covered by sponge cells that phagocytize bits of meat, moving them into their cytoplasm and move away to start digestion.