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BIOL 2015 – Evolution and Diversity Lab 7: Porifera, Cnidaria and Ctenophora

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BIOL 2015 – Evolution and Diversity Lab 7: Porifera, Cnidaria and Ctenophora BIOL 2015 – Evolution and Diversity Lab 7: Porifera, Cnidaria and Ctenophora IntroduCtion In terms of known living organisms, animals are the most specious group of organisms on Earth. They come in a wide variety of shapes, sizes, and structure. In fact, there are over 35 phyla of animals and it is a continuing challenge for biologists to understand both the phylogenetic relationships among animals and the evolutionary processes that produced the great diversity we see today (a modern phylogenetic tree is shown in Figure 1). There are a few underlying themes in animal evolution which you should keep in mind during the next series of labs. Two related themes are: 1. Evolution of Motility. The ability to find food and escape predation is an important part of life for most animals. Motility was a key innovation and is found in the majority of animals. 2. AdvanCed Heterotrophy. Sophisticated means of prey capture allow animals to consume larger quantities of different kinds of food. Much of the diversity evident in animals comes from different ways animals have evolved to obtain food. Animals are multicellular heterotrophs. That is, they are made of many cells and they don’t make their own food. The simplest animals, the sponges, do not have a gut and cannot move in search of food. They have evolved a pump-like mechanism to move nutrients suspended in water past the individual cells that make up their body. As nutrients pass these cells they are taken up via endocytosis and digested within the each cell. Most animals move about in search of food and have a gut within which digestion can occur. This type of digestion is called extracellular digestion and it enables them to consume larger food items. The ability to move, the ability to capture larger food items, and the ability to avoid being eaten, are all aided by larger body sizes and require an extensive “infrastructure.” For example, sense organs and a nervous system are important for the processing sensory information and the coordination of movement. Other important systems include a system for gaseous exchange (respiratory system), a system for moving materials around the body (circulatory system), and systems for the removal of metabolic wastes (excretory system). All of these systems are required to keep a large, mobile animal functioning properly. In this lab we will introduce several features of body plans that are related to the evolution of locomotor ability, increased body size, and the ability to capture and digest food in animals. In subsequent labs we will address these features of animals in greater detail. Other important themes in the evolution of animals are centered in various developmental processes that lead to the evolution of body plans. Since all of the tissues, organs, and organ systems develop from a single cell, any subsequent change in these from one animal to another will ultimately depend on modifications to the developmental processes from which they form. In this lab we will examine some of the basic features of animal embryology, and explore how major changes in body plans can result from differences in the patterns of early development. 1 Figure 1. Phylogenetic tree based on DNA data. Dunn et al. 2008. 2 Porifera Sponges (Figure 2) are the simplest animals anatomically, lacking many of the features that characterize all other animals. However, close studies of both their embryology and their genomes show that sponges are closely related to other animals, and not to any other kingdom. The apparent simplicity has led biologists to regard sponges as primitive offshoots of the animal lineage, having split off before the evolution of the more complex features that most other animals share. However, some studies have suggested that the ancestors of sponges may actually have been more complex. There are about 5,000 species of sponges worldwide. Figure 2. Example of a marine sponge (Porifera; blue All sponges live in the water and most are marine. color) growing by a coral (Cnidaria; green). We'll observe both groups today. The wall of a sponge contains 3 types of cells. The outer cells are flattened epidermal cells, called PinaCoCytes. The inner cells, called ChoanoCytes, are collar cells with flagella. It is the constant movement of the flagella that produces water currents that flow through the pores into the spongoCoel and out through the upper opening of the body, called the Osculum. AmoeboCytes (or arCheoCytes) are cells with an amoeboid, or irregular, shape. They perform various functions and can differentiate to become other cell types. The amoebocytes are located in the layer called the Mesohyl. This is a gelatinous layer between the outer epidermal pinacocytes and the inner layer of choanocytes. The skeleton of the sponge is composed of tiny needle-like splinters called SpiCules (composed of silica or calcium carbonate), a mesh of protein called Spongin, or a combination of both. Examine the sponges we have on display. Note the following features: Figure 3. Diagram showing sponge wall. • No true tissues. Sponges lack the level of tissue organization seen in other animals. • No symmetry. Sponges have variable and irregular body forms. • IntraCellular digestion. Sponge cells take in small food particles using phagocytosis. • Spicules: Sponges posses hard, crystalline structures (spicules) in mesohyl. 3 Scypha Scypha is a small, tube-shaped sponge. Find the following slides: A slide labeled "Scypha spicule strew" A slide labeled "Scypha Tangential sec." A slide labeled "Scypha c.s. & l.s." (c.s. = cross section. l.s. = longitudinal section.) A slide labeled "Scypha with eggs" A slide labeled "Gemmules" On the "Scypha spicule strew" slide you can see the mineralized skeletal elements of some sponges (spiCules). They occur in a variety of shapes and sizes. Be sure you can identify what these are. Tangential section: This slide will show the porous structure of the sponge body wall. You’ll notice that there are many open spaces. These are either inCurrent canals, through which water enters the sponge, or radial Canals, through which water exits into the spongocoel (Figure 3). Incurrent canals are lined with the pinacocytes, which provide the canals with a smooth surface. The radial canals are lined with choanocytes. You might be able to see some of the flagella, but the collars are poorly preserved in these sections. Cross section and longitudinal seCtion: Scypha uses the flagella on its choanocytes to draw water through its body so it can capture small suspended particles of food. Observe the structure of the animal (Figure 4) with its inCurrent and radial canals, and its spongoCoel (which, by the way, is not comparable to the coelom of other animals, because there are no organs in it; it's just an empty tube through which water is expelled). You should find the Ostia (singular ostium), through which water enters the sponge, and the Apopyles. Apopyles are openings that allow water to enter the spongocoel after moving through the sponge wall. What kind Figure 4. Cross section (top) and longitudinal of canal ends with an apopyle? section (bottom) of Scypha. https://vimeo.com/40240443 – Sponge filter feeding (from Shape of Life series) We have a number of sponge skeletons to look at, so make sure you examine them. Try to identify as many features as possible on these specimens. Can you tell what material each is composed of? Sexual ReproduCtion in Sponges Find a prepared slide labeled "Scypha with eggs". You should be able to see large eggs, which develop directly from amoebocytes. Modified choanocytes produce sperm, and most sponges are sequential hermaphrodites - eggs and sperm mature at different times within any individual. Be able to hypothesize about why this reproductive strategy evolved (rather than a simple form of hermaphrodite)? 4 Asexual ReproduCtion in Sponges (Gemmules) Find a prepared slide labeled "Gemmules". A gemmule (Figure 5) is an asexual reproductive sac produced by freshwater sponges that contain a mass of amoebocytes and are desiccation-resistant. Many freshwater sponges die off in the winter and grow back in the warmer months as gemmules "germinate”. The amoebocytes within the gemmule are released through a micropyle (a pore) and develop into a new sponge. Figure 5. Sponge gemmules. Gastrulation in sponges Gastrulation is the developmental process that defines the basic embryonic tissue layers in animals and it lays the foundation for the later development of complex animal bodies. This will be covered in another lab, later this semester. Many sources state that sponges lack gastrulation, and thus lack defined embryonic tissue layers. However, many sponge specialists believe that gastrulation does occur in sponges, although the process is quite different from that in other animal groups. Cnidaria Cnidaria contains about 11,000 species that include jellyfish, hydrozoans, corals and sea anemones. Most are marine. The Cnidaria are clearly distinct from other animals because they have tentacles with stinging cells called cnidocytes (Figure 3). Each cnidocyte contains a harpoon-like structure called a nematocyst. When the surface of a cnidocyte is triggered by a prey (or predator), the nematocyst is fired, which entangles and poisons the animal. The body of a cnidarian is radially symmetriCal like a barrel or a wheel. Cnidaria are diploblastiC – a major evolutionary innovation - two cell layers, ectoderm and endoderm or gastroderm (so-named because it lines the digestive sac); nerve cells thread through both of these tissue Figure 6. Sea anemone glowing under layers. The mostly non-cellular layer between ectoderm and UV light. endoderm is called mesoglea (middle jelly) and is secreted by the cells. https://www.youtube.com/watch?v=6zJiBc_N1Zk – Nematocyst firing The mouth is central and is surrounded by tentacles. It leads into the gastrovasCular cavity. This two-part word refers to the two chief functions of the cavity, digestion and circulation.
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