Porifera

Porifera

Alexa Wright

The Phylum **Porifera** is composed of sponges. Sponges, which are on the branch Parazoa of the phylogenetic tree, are the closest related to the colonial choanaflagellates, which gave rise to the animal kingdom. **Sponges** are sessile (permanently attached to a base [JE]) animals that appear so still that the ancient Greeks believed them to be plants. They have no nerves or muscles, but their individual cells have the ability to sense and react to stimuli in the environment.

A sponge in its habitat. (EK)

** Diagnostic Characteristics **  Sponges range in height between 1 cm to 2 cm. There are approximately 9,000 species of sponges. Of these 9,000 species, only about 100 live in fresh water environments while the rest live in marine environments. Sponges have been characterized as sessile metazoans, or multicellular organisms. (EK3). All known living sponges can remold their bodies as most types of their cells can ,move within their bodies and a few can change from one type to another.(17DO) **Structure ** A sponge’s body is a porous sac like structure. Sponges are either radially symmetrical or asymmetrical. The body consists of two layers of cells separated by a gelatinous region called the mesohyl. The outer layer consists of tightly packed epidermal cells, which function as a protective surface. More complex sponges have folded body walls and branched water channels containing many oscula, a large opening apparent in all sponges. These animals are sessile, meaning they are permanently fixed to their environment and are not free movers. Cells known as amoebocytes form tough skeletal fibers in the mesohyl. Amoebocytes, in addition to aiding food intake and circulation, also act like unspecialized cells, meaning they can develop to transform into any cell required. This allows for growth, repair, and reproduction of the sponge. (4 AS) Some groups of sponges have sharp fibers while other groups produce more flexible fibers. Differences in skeletal elements are important for taxonomy of the phylum. The skeletal system can be broken down into two main elements: 1. A protein called sponging forms a strong fibrous network and works with 2. spicules that are constructed from either silica or calcium carbonate. Amoebocytes responsible for maintaining the skeletal system are spongioblasts (secretes spongin fibers) and scleroblasts (secretes spicules) (3 KA). There are three types of structures. Asconoid sponges, are shaped like a simple tube perforated by pores where as Syconoid sponges tend to be larger than asconoids and have a tubular body with a single osculum. Others are part of the Leuconoid type, the largest and characteristic of having no spongocoel, water rather reaching the oscula through excurrent canals (9 J. Stein). **Reproduction** Most sponges are hermaphrodites, meaning they act as both the female and the male in reproduction by producing both sperm and eggs. Cross-fertilization results from neighboring sponges drawing in other sperm. During sexual reproduction among porifera, sperm cells enter the "female" sponge through external pores, oscula, where the sperm will fertilize the egg. The egg turns into little larvae that will eventually attach itself to the ocean floor and grow into an adult sponge. Only adult sponges, those older than 3, can produce another sponge (6 MB). Sponges are also capable or extensive regeneration, the replacement of lost parts. Regeneration is for replacing lost/damaged parts, but also to reproduce asexually from fragments that broke off the parent sponge. Sponges can reproduce asexually in a variety of ways including budding, fragmentation, and gemmulation. (14 VK)


 * Food Intake and Digestion **

Nearly all sponges are suspension-feeders (aka filter feeders), meaning that sponges feed by collecting food particles from water that is filtered through their porous bodies. Sponges utilize specific food-trapping equipment: water is first filtered through the sponge as a result of flagellated choanocytes (collar cells) that generate a water current. Then the membranous collars around the flagellum trap the food particles. Finally, the chaonocytes ingest by phagocytosis. Phagocytosis (aka cellular eating) is the cellular uptake of substances via vesicles. Sponges take in water at the bottom, and then eject it through the //osculum// ("mouth") at the top. (20 DC). Cells known as amoedocytes are found in sponges and perform numerous tasks. One function is taking up food from the water and digesting it. Most sponges are suspension-feeders, however, sponges of the family Cladorhizidae are actually carnivorous. They capture small crustaceans with their spicules (needle-shaped skeletal elements in sponges) and then cells migrate to the prey. Digestion is done extracellularly (CH 7).

Their individual cells have the ability to sense and react to changes in their environment. Sponges have no nervous system, but they can still react to stimuli. Sponges will react by closing oscula. This reaction occurs because of direct physical stimulation, or when the particles are being filter through the sponge, the oscula might close because the particles are too large or highly concentrated. Also, some sponges may respond to electrical impulses. Although they have no formal nervous system, one species of sea sponge, called //Amphimedon queenslandica,// was recently found to synthesize proteins that are prominent in cell-to-cell communication (CS 21). This is a significant discovery because this communication mirrors that in a sophisticated nervous system, giving scientists an idea as to how neurons evolved over time (CS 21).
 * Sensing the Environment **

Sponges are sessile animals, meaning they do not move. However, sponge larvae are free swimming, utilizing flagella to move. Also, nutrients are transported throughout the sponge via amoebocytes, and water currents move substances through the sponge as well.
 * Locomotion **

Oxygen diffuses into the cells from the water that is filtered through the sponge. Carbon dioxide diffuses out of the cells and into the water, leaving the sponge like all wastes do. Sponges are very efficient in respiration, using approximately 75% of the oxygen they take in. (HL 18) In more advanced forms of porifera, there are canals that move the water all throughout the organism (21 SC).
 * Respiration **

Sponges lack digestive or circulatory systems. They rely on the water currents to remove wastes.
 * Metabolic Waste Removal **

Amoebocytes also carry nutrients from cell to cell. Since the sponge lacks a circulatory system, it depends on the water currents similarly as it does for waste removal.
 * Circulation **

The outer cell layer of the sponge consists of tightly packed epidermal cells that function as a protective layer of the sponge. Many species contain toxic substances that ward off predators. Certain other marine animals take advantage of this characteristic of sponges by placing adult sponges on their bodies, where the sponges attach and grow. Sponges release these toxins into the water to ensure themselves space in the ecosystem. These toxins are also used in the case of competition between other sponges and other organisms.
 * Self Protection **

Sponges draw water in through pores into the central cavity known as the spongocoel. Water then exits the sponge via a large opening known as the osculum. Sometimes the cells around the openings contract, closing the openings and regulating the intake of substances.
 * Osmotic Balance **

Sponges can live in a wide variety of temperatures based on the temperature of the surrounding water. They do not regulate their temperature; they adhere to the temperature of their environment.
 * Temperature Balance **

Review Questions: 1. How does the environment of the Phylum Porifera allow it to survive without a circulatory system(JE)? 2. What do sponges have in common with phylum echinodermata? (22 AN) 3. How do sponges protect themselves from predators? (J. Sun 15) 4. If a part of a sponge gets broken off, what happens? Explain how this is possible. (AR 23)

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