Phylum Porifera (Sponges) Lecture Notes PDF

**Phylum Porifera (Sponges)** **[Learning Aims]** At the end of this lecture, you are required to know: 1. The position of these phylum in the animal kingdom and their biological contributions 2. The classification and characteristics of this sub kingdom and its taxa 3. The form and...

**Phylum Porifera (Sponges)** **[Learning Aims]** At the end of this lecture, you are required to know: 1. The position of these phylum in the animal kingdom and their biological contributions 2. The classification and characteristics of this sub kingdom and its taxa 3. The form and the functional morphology of the sponges 4. Their phylogeny and adaptive radiation 5. Their ecological features The lowest Metazoan consist of three phyla: Metazoa, Placozoa and Porifera. These phyla differ from other Metazoa in their cellular layers and their patterns of development. Only Porifera will be dealt with in details. The level of organization is cellular in comparison with Eumetazoa, where organization is at tissue level. There are no organs or tissues. For most physiological purposes, sponges can be regarded as loose aggregations of cells, which function independently and uncoordinated. The most important cell components are the choanocyte lining the spongocoel. Ensure to know what choanocyte is. The Porifera may be regarded as a branch outside the line of evolution from Protozoan to the pother Metazoa and they are therefore called Parazoa (para -- beside + zoa -- animal). What is the reason for the success of sponges as a group? **[Position in the animal kingdom ]** Porifera belongs to the Parazoa branch of Metazoa. Although Parazoa are multicellular, they do not fit into the general plan of other phyla. The poriferans are considered to be aberrant, that is, deviating widely from standard patterns, hence the name "Parazoa", which means the "beside animals" **[Biological contributions of the porifera ]** 1. They are said to belong to the cellular level of organization. 2. They are more complex than the Metazoa and Placozoa, with several types of cells for various functions. 3. Some of these are incipient tissues of a low level of integration. 4. The sponges have developed a unique system of water current on which they depend for food and oxygen. **[General characteristics of the porifera]** For the sake of comparison, these characteristics can be summarized under the following headings: - Form and size. Multicellular, form may be flat, flasklike, round or branched. - Symmetry. Radial or none - Germ layers. Two but layers are not homologous to those of Eumatazoa. - Organs and tissues. No organs or true tissues. Body with pores, canals and chambers for passage of water. - Mode of living. All free living and aquatic, mostly marine. - Locomotion. All adults are sessile and attached to substratum. - Body covering and skeleton. Epidermis of flat pinacocytes. (make sure you understand what pinacocyte means). Endoskeleton of spincules or organic fibres (spongin) or both. - Nutrition. Intracellular digestion - Nervous system. Absent; loose bipolar and multipolar cells are found. - Excretion. No specialized organs or structures; excretion by diffusion through water in canals and chambers. - Transport system. None - Habitat. Always aquatic, mostly marine. - Reproduction. Asexual -- buds or germules; Sexual -- eggs and sperms; free swimming ciliated larvae. **[Classification of the phylum porifera]** The following is a summary of the characteristics of the four poriferan classes. It is important to know these: 1. **Class Calcarea**: this group is known as calcareous sponges because their spicules are composed of calcium carbonate. The spicules have one ray (monaxons) or three or four rays, commonly bound together into a glasslike network, and the trabecular net. Body small, surface bristly and mostly drab in color. Tubular or vase-shaped. *Leucosolenia* is an example. 2. **Class Hexactinellida**: this group is known as glass sponges. Skeleton is six-rayed siliceous spicules. Single spicules (*Hyalonema*) and spicules in a network are also found. No surface epithelium, choanocytes in finger -- shaped chambers. Body vase -- shaped or funnel-shaped. Example is *Euplectalla*. 3. **Class Desmospongiae**: this group comprises 95 percent of living sponges. Skeleton of siliceous spicules and or spongin fibres. Spicules never six-rayed. e.g *Spongilla*. 4. **Class Sclerospongiae**: representative of this group are always found in association with coral reefs. They secrete a massive external calcareous skeleton with siliceous spicules and spongin fibres. Living tissue occur as a thin layer on the skeleton. **[Form and functions of poriferans]** The forms and functions of porifera are well described in many textbook, and they are important. Sponges possess a central cavity (spongocoel) which opens to the outside through the osculum for water outlet. Many ostia or tiny pores in the body wall facilitate the incoming flow of water. The opening are connected by a specialized canal system, lined with flagellated choanocytes for phagocytic absorption of food. Can you identify the three layers of the body wall sponges? The canal type can be classified into three types, based on their structure. Please note: - The types of canal systems: you must be able to draw the following three thypes of canal systems found in the phylum: - Asconoid system (flagellate spongocoels) - Synconoid system (flagellate canal) - Leuconoid system (flagellate chambers) - Types of cells: note the structure and function of the three different cell types: - Pinacocytes - Choanocytes - Archaeocytes - Sclerocytes - Spongocytes - Collencytes - Lophocytes - Types of skeleton: the skeleton types are important for the identification of the different classes of poriferans. - Physiology of sponges: note the following: - The procurement of food and the function of the pump mechanism in sponges. - The functions of the pinacocyte and the choanocytes. - Intracellular digestion and the role of archaecytes. - Respiration and excretion mechanisms. - Possible communication between cells. - Reproduction: you must be able to discuss reproduction in sponges. How are sponges able to survive periods of drought or freezing? Discuss asexual and sexual reproduction. Please note budding, gemmules, the formation of sperm and oocytes from choanocytes, ciliated larvae and phagocytosis of sperm by choanocytes which act as carrier cells. Note the difference in development between the Porifera and the other metazoans. Please note the tremendous regenerative ability of sponges. What is somatic embryogenesis? Discuss the morphological adaptation of sponges to their specific mode of living? Note how they produce food as sessile animals. Are sponges of any economic importance? **[Phylogeny and adaptive radiation ]** - Relationship between sponges and protozoan as shown by phagocytic nutrition and resemblance of their flagellated larvae to colonial protozoa. - The possibility that sponges arose from choanoflagellates. Why is this theory not widely accepted? - The possibility that sponges arose from a hollow, free swimming colonial flagellate that's volvox. Their sessile existence may have caused the development of water canals and the formation of choanocytes. - The early divergence of sponges to be regarded as a "dead-end" phylum. What is the justification for this statement? The adaptive radiation in this phylum is clearly shown by the large variety within this group. This diversification is particularly evident from the variety and degrees of complexity of the water canal systems.