Porifera

So he dissected sea sponges by night, winter night after winter night. . . adult and embryo human body parts by day, adult and larval sponge body parts by night.

Rebbeca Stott (2003) Darwin and the Barnacle, on the sponge doctor, Robert

Grant

Most of us have used a bath sponge, probably a synthetic replica of the real thing. But ancient peoples used sponge skeletons as an aid to bathing and possibly exfoliation in some of the world's earliest and most exclusive health farms. Most considered they were some form of plant until proper biological study in the mid and late 1700s suggested they were animals - and at first they were classified as corals. It was in fact Dr Robert Grant (17931874), one time mentor to Charles Darwin, who later established the Porifera as a unique group in its own right. The poriferans or sponges have a unique porous structure and a body plan based at the cellular level of organization; they are said to lack true tissues. Most lack symmetry, true differentiated tissues, and organs, although their cells, like those of the protists, can switch function. They reproduce both asexually (by budding) and sexually with different cells expelling clouds of eggs and sperm out through an opening; some are even viviparous, with the eggs hatching within the parent sponge, and larvae released into the water.

There are over 10,000 species of sponge. All are aquatic, and most are marine. Sponges are part of the sessile benthos, fixed to the seabed, pumping large volumes of water - in extreme cases over 1000 L per day - through their fixed but commonly flexible bodies, which act as filters for nutrients. The group has a remarkable range of morphologies; the more specialized, stalked forms live in deep-water environments and flattened, dumpy forms prefer shallower-water, high-energy environments. Despite the apparent simplicity of the sponges, the classification of the phylum has recently undergone considerable revision (Box 11.1). Some well-established calcified groups, such as the "chaetetids" and "sphinc-tozoans", are probably polyphyletic, merely representing convergence towards common grades of organization. The well-established and diverse Demospongea, the common sponges, may too be polyphyletic. Despite their relative simplicity, the complex relationships of "sponge-grade" animals have yet to be resolved.

Morphology: examining a typical sponge_

A typical sponge individual is not particularly complex or intellectually demanding to understand; it is nonetheless a remarkable organism. It is sac-shaped with a central cavity or paragaster, which opens externally at the top through the osculum (Fig. 11.1). The sponge is densely perforated by ostia, small holes marking the entrances to minute canals through which pass the inhalant currents. In simple terms, there are three main cell types: (i) flattened epithelial cells; (ii) collar cells or choanocytes, which occupy the internal chambers and move water along by beating their flagella; and (iii) amoeboid cells, which have digestive, reproductive and skeletal functions. Amoeboid cells can actually irreversibly change into other cell types with other functions. Nutrient-laden water is thus sucked through the ostia, flagellated by the choano-cytes and processed by the amoeboids. Waste products and spent water, together with reproductive products when in season, are ejected upwards through the paragaster into the water column.

osculum ostium -bud spongocel canal root tuft

Figure 11.1 Basic sponge morphology.

osculum ostium -bud spongocel canal root tuft

Ascon Sycon Leucon

Figure ll.2 Main grades of sponges.

Figure 11.1 Basic sponge morphology.

Box 11.1 Classification and spicule morphology of the sponges CLASSIFICATION OF THE SPONGES

The phylum Porifera was traditionally subdivided into four classes, the Demospongea, Calcarea, Sclerospongea and Hexactinellida, based mainly on the composition of the skeleton and type of spicules. Higher-level taxonomy is based exclusively on soft-tissue morphology. Some workers have suggested the exclusion of the glass sponges from the Porifera but this is poorly supported; rather they are closely related to the demosponges. However, the sclerosponges, with some additional calcareous skeletons, are now placed within the Demospongea. Thus three classes now comprise the phylum (Fig. 11.3).

Class CALCAREA (calcareous sponges)

• Sponges with calcitic spicules, usually simple, and/or porous calcareous walls. Marine environments

• Cambrian to Recent

Class DESMOSPONGEA (common sponges)

• Sponges with skeletons of spongin, a mix of spongin and siliceous spicules or only siliceous spicules. The spicules may be of two different sizes and the larger are represented by monaxons and tetraxons. Marine, brackish and freshwater environments. Living sponges previously assigned to the Sclerospongiae (coralline sponges) - sponges with a compound skeleton of siliceous spicules, spongin and an additional basal layer of laminated fibrous aragonite or calcite - are now also included here

• Cambrian to Recent

Class HEXACTINELLIDA (siliceous sponges)

• These are the glass sponges with complex siliceous spicules having six rays directed along three mutually perpendicular axes. Deep-water marine environments

• Precambrian (?) and Cambrian to Recent

However, two form-groups of sponge, the sphinctozoans (with a segmented chambered skeleton) and the chaetetids (with microscopic tubules) have representatives within the Calcarea and Demo-spongea; both were important reef builders.

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