“Sponges are the simplest multicellular animals. Because the cell is the elementary unit of life, the evolution of organisms larger than unicellular protozoa arose as an aggregate of such building units. […] There are many advantages to multicellularity as opposed to simply increasing the mass of a single cell. Since it is at cell surfaces that exchange takes place, dividing a mass into smaller units greatly increases the surface area available for metabolic activities. […] Thus multicellularity is a highly adaptive path toward increasing body size.”
Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Page 428-429.
Major Characteristics of Phylum Porifera
- Multicellular; body a loose aggregation of cells of mesenchymal origin;
- Body with pores (ostia), canals, and chambers that serve for passage of water;
- All aquatic; mostly marine;
- Symmetry radial or none;
- Epidermis of flat pinacocytes; most interior surfaces lined with flagellated collar cells (choanocytes) that create water currents; a gelatinous protein matrix called mesoglea contains amebocytes, collencytes, and skeletal elements;
- Skeletal structure of fibrillar collagen (a protein) and calcareous or siliceous crystalline spicules, often combined with variously modified collagen (spongin) fibrils;
- No organs or true tissues; digestion intracellular; excretion and respiration by diffusion;
- Reactions to stimuli apparently local and independent; nervous system probably absent;
- All adults sessile and attached to substratum;
- Asexual reproduction by buds or gemmules and sexual reproduction by eggs and sperm; free-swimming ciliated larvae.
Sponges are differentiated by the type of canal system (check your lab manual for diagrams):
- Asconoids – Small and tube shaped. Water enters through microscopic dermal pores into a large cavity called the spongocoel, which is lined with choanocytes. The choanocyte flagella pull the water through the pores and expel it through the single osculum.
- Syconoids – Syconoid sponges also have a tubular body and single osculum, but the body wall is thicker and more complex. Water enters through incurrent canals that deliver it to radial canals, which are lined with choanocytes. The radial canals then empty into the spongocoel, which is lined with epithelial cells and not with choanocytes.
- Leuconoids – Leuconoid sponges have the most complexity and are the best adapted for increase in sponge size. Most leuconoids form large colonial masses, each member of the mass having its own osculum, but individual members are poorly defined and often impossible to distinguish. Clusters of flagellated chambers are filled from incurrent canals and discharge water into excurrent canals that eventually lead to the osculum.
|TYPICAL SPONGE FEEDING
Water flowing through sponges provides food and oxygen, as well as a means for waste removal. This flow is actively generated by the beating of flagella. The water movement through some sponges is aided by ambient currents passing over raised excurrent openings. This moving water creates an area of low pressure above the excurrent openings that assists in drawing water out of the sponge. Sponges are capable of regulating the amount of flow through their bodies by the constriction of various openings. The volume of water passing through a sponge can be enormous, up to 20,000 times its volume in a single 24 hour period. In general, sponges feed by filtering bacteria from the water that passes through them. Some sponges trap roughly 90 percent of all bacteria in the water they filter. Other sponges, in particular hexactinellids, appear to be less efficient at capturing bacteria and may specialize in feeding on smaller bits of organic matter. Still other sponges harbor symbionts such as green algae, dinoflagellates, or cyanobacteria, from which they also derive nutrients.
THE CARNIVOROUS EXCEPTION
Sponges reproduce by both asexual and sexual means. Most poriferans that reproduce by sexual means are hermaphroditic and produce eggs and sperm at different times. Sperm are frequently “broadcast” into the water column. That is, 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.]
At one time, a diagnostic feature of the Porifera was the presence of spicules. As a result, certain fossil groups whose organization was consistent with that of living sponges were not placed within the phylum Porifera. In particular, groups with a solid calcareous skeleton such as the Archaeocyatha, chaetetids, sphinctozoans, stromatoporoids, and receptaculids were problematic. A great deal of insight into the phylogenetic affinities of these groups was gained with the discovery of more than 15 extant species of sponges having a solid calcareous skeleton. These species are diverse in form, and would be classified with the chaetetids, sphinctozoans and stromatoporoids if found as fossils. However, with the living material in hand, histological, cytological, and larval characteristics can be observed. This information suggests that these 15 species can readily be placed within the Calcarea and the Demospongia. This radically changes our view of poriferan phylogeny.
It is widely accepted among poriferan biologists that the Calcarea and the Demospongia are more closely related to each other than either is to the Hexactinellida. With the discovery of living chaetetids, stromatoporoids, and sphinctozoans, a fourth class was erected for these so-called sclerosponges. However, the Sclerospongia is not a natural monophyletic grouping and is thus being abandoned. The abundant fossil chaetetids, stromatoporoids, and sphinctozoans are probably part of the classes Demospongia and Calcarea, though some uncertainty still remains. The Archaeocyatha pose a special case. No living representative of this group has been discovered. Their organization is consistent with that of living sponges. The one phylogenetic analysis (carried out by Reitner) that included archaeocyaths with other sponges, grouped them as sisters to the demosponges. Therefore, although the taxonomic term Archaeocyatha is often accorded phylum status it is likely a sub-clade of the phylum Porifera, thereby violating the ranking system.