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Lab exercise 2: Basal Animal Lineages: Porifera / General Laborarory . Matt Nelson Porifera

SYMMETRY The flatworms represent some of the simplest animals that possess bilateral symmetry. The term bilateral symmetry refers to a shape that has only one axis that divides the body into two halves that are mirror images of one another. Organisms such as cnidarians have radial symmetry, i.e. there are several planes that divide the body into halves that are mirror images of one another. These two types of symmetry, radial and bilateral, are the most common types found in the Animalia. However, there are more specific terms used to describe certain types of radial symmetry (such as pentaradial symmetry) and we will discuss these throughout the semester as they become relevant.

Most of the possess no body symmetry, and are considered to be asymmetrical, although there are some sponges that border on radial symmetry. However, this apparent radial symmetry generally does not apply to the pore system and the interior regions of the body. In the future, when we discuss body symmetry, it will be apparent that sometimes we are referring to the entire body, and sometimes only the external features of the body. In humans, for example, the outside of the body generally possesses bilateral symmetry, but the internal organs are in some cases asymmetrical. Organization Sponges are among the simplest of animals in terms of organization of the body. While they do possess differentiated cells, they do not possess true tissues, or organs, or systems. In many ways, the body of a is not that different from a colonial group of cells. The general body structure is simple. There is an outer layer of cells that surround the body, and a layer of cells that lines the interior surfaces of the body. Sandwiched between these two layers is a gelatinous layer called the mesohyl which may contain skeletal elements (spicules), spongin fibers which act as a flexible skeleton, and amoeboid cells which move about the mesohyl carrying out various functions. Although there are specialized cells in each layer, none of these layers is considered to be a true tissue. The functionally different cell types found in sponges are the key factor in their classification as multicellular organisms. Sponges possess three major cell types:

pinacocytes - These flattened cells cover the outer body of the sponge. In some sponges these cells are contractile and may possess the ability to alter the general shape of the sponge. In the simplest sponges, modified pinacocytes called “porocytes” form the pores that allow water to enter the body. Contraction of porocytes changes the size of the pore, limiting the movement of water through the sponge.

amoebocytes (archaeocytes) - These amoeboid cells migrate throughout the mesohyl and are responsible for carrying out intracellular digestion and transporting the products of digestion throughout the body. Amoebocytes may also be specialized for other jobs, such as the production 1 General Zoology Laboratory. Matthew K Nelson (2011) of spongin fibers or spicules. Like stem cells, they are able to develop into any of the other specialized cell types found in the sponge. As such, they are important in the of some freshwater sponges, which will be discussed later in this exercise.

choanocytes (shown here) - These are flagellated cells similar in form to choanoflagellate algae. They possess a collar which consists of a ring or microvilli (tiny cytoplasmic extensions) that surround the flagellum and are bound together by plasma membrane. The term “choanocyte” means “collar-cell.” Choanocytes allow sponges to feed. These may line one or more internal chambers in the sponge. The flagella of choanocytes beat back and forth creating the flow of water which brings in tiny food particles which stick to the collar and are ingested through phagocytosis.

Most of the diversity in sponges involves the relative complexity of the canal system which carries water through the sponge for suspension feeding. The simplest sponges possess a central chamber called the spongocoel which is lined with choanocytes. Water comes into the spongocoel through pores (ostia) in the body wall, and exits the spongocoel through a larger opening referred to as the osculum. These sponges are considered to be asconoid. Asconoid sponges are usually relatively small sponges, being somewhat constrained by the efficiency of their canal system. Syconoid sponges possess ciliated radial canals contained within the body wall. Water flows into the dermal pores, and then enters the radial canals via small openings called prosopyles. At its interior end, the radial canal opens into the spongocoel. This opening is the apopyle. The largest sponges are leuconoid. Leuconoid sponges are the most common and generally the largest sponges. In leuconoids, there is no spongocoel per se. Dermal pores allow water to flow into incurrent canals which lead to flagellated chambers, where suspension feeding occurs. Excurrent canals carry water from the flagellated chambers to the osculum. apopyle

radial canal

prosopyle

dermal pore

syconoid body wall

asconoid sponge leuconoid sponge

2 General Zoology Laboratory. Matthew K Nelson (2011) Obtain a Scypha sp. (Grantia sp.) whole mount slide. Observe the specimen with the light microscope under low power, then switch to high power. Draw and label the specimen. Classification The body forms of sponges reflected by the levels of complexity in canal systems do not seem to be indicative of relatedness. There are three extant classes of sponges:

Class Calcarea - This group possesses calcareous spicules (CaCO3). The spicules are monaxon, triaxon, or tetraxon (straight, three pointed, or four pointed). These generally smaller marine sponges are most common in the tropics. The Calcarea include asconoid, syconoid and leuconoid forms.

Class Hexactinellida - This class is referred to as the “glass sponges” because of the characteristic siliceous spicules (SiO2) of its members. The name “hexactinellida” refers to the six-pointed (hexaxon) spicules of glass sponges. These are all marine and either syconoid or leuconoid.

Class Demospongiae - Most of the sponges are in this group. These are all leuconoid, but can be marine or freshwater. May have siliceous spicules or spongin, or both. Commercial bath sponges are made from members of this group

Obtain a bath sponge specimen. This is the spongin matrix from an “ex-sponge.” Examine the sample under a dissecting scope.

Freshwater sponges possess a special type of that is used to survive freezing during overwintering. These sponges produce internal buds called gemmules (right), which consist of archaeocytes surrounded by tightly packed spicules. Archaeocytes eventually emerge from the gemmule and develop into new sponges.

Obtain a prepared slide with a gemmule specimen. Observe under low and high power. Note the spicules if visible.

3 General Zoology Laboratory. Matthew K Nelson (2011) phylum Cnidaria

The term Cnidaria ([knide = nettle] + [aria = like]) reflects the specialized stinging cells which cnidarians use for defense and prey capture. This group was once part of a larger group called the coelenterates, encompassing the Cnidaria and the Ctenophora (comb jellies). Since the coelenterates were not monophyletic, they were broken into two separate phyla. The body of a cnidarian is radially symmetrical. As a result, terms such as dorsal and ventral have no meaning for these organisms. Instead, relative anatomical positions are referred to with respect to the position of the mouth. The end of the body that is the location of the mouth is called the oral end, and the opposite end is the aboral. Organization Cnidarians possess incipient tissue-level organization. They are diploblastic, meaning that they possess two embryonic tissue layers. These two layers eventually develop into the two layers of the body: the epidermis and the gastrodermis. Between the epidermis and the gastrodermis is a gelatinous non-cellular layer referred to as the mesoglea. The epidermis comprises the outer layer of the body. Epidermal cells are contractile and can be used to change the shape of the body. are also part of the epidermis. Cnidocytes are stinging cells unique to cnidarians, containing special stinging organelles called nematocysts. The gastrodermis lines the gastrovascular cavity, a chamber connected to the mouth. Cnidarians possess two body forms: polyp and medusa. These two body forms are found within the same . Normally, the life cycle of cnidarians alternates between the polyp and medusa stage. Generally, the main function of the polyp is feeding, while the medusa is mainly concerned with . The polyp is the sessile form. The aboral surface is usually attached to a substrate, while the oral surface is directed upward to allow for efficient feeding. The medusa is motile, swimming with the oral surface downward. In some groups, one or the other of these forms may be reduced or absent. Anemones, for example, do not have a medusa stage. Nematocysts The nematocysts of cnidarians possess a special tactile trigger called a cnidocil. When the cnidocil is engaged, the nematocyst is discharged, extending a long thread that punctures the prey item, injecting it with venom. The nematocysts of some cnidarians can deliver a serious sting to even a large organism such as a nosy human.

Examine the demonstration slide of a discharged nematocyst.

4 General Zoology Laboratory. Matthew K Nelson (2011) Classification There are four classes of cnidarians.

Class - Freshwater or marine. Often colonial. Members usuallly possess both polyp and medusa phases.

Class - Most of the jellies. Generally, scyphozoans are medusae. The polyp is reduced in this group and usually exists as a scyphistoma, that produces medusae through strobilation.

Class Cubozoa - Box jellies. polyp develops directly into medusa without strobilation.

Class - (colonial) and Anemones (solitary). mutualistic associations with single celled algae give these bright coloration. Hydrozoa (Order Hydroidea) The hydrozoan Obelia sp. is an excellent example of a colonial polyp. You should be able to easily distinguish feeding and reproductive polyps in the . Gastrozooids (hydranths) are feeding polyps that capture food items with . Food particles are ingested through the mouth, and passed throughout the colony via a common gastrovascular cavity. Gonozooids (gonangia) are reproductive polyps that produce internal buds through the process of strobilation, releasing medusa into the surrounding water. You should be able to see these chains of medusa buds inside each gonangium. The entire colony is surrounded by a transparent sheath called the perisarc. This chitinous protective covering is produced by all members of the colony.

Obtain a whole mount Obelia colony prepared slide. Observe under the compound microscope. Draw and label: gonangia, hydranths, gastrovascular cavity, tentacles, perisarc.

Obtain a whole mount Obelia medusa prepared slide. Observe under the compound microscope.

Observe the preserved Obelia colony in alcohol. sp. (Order Hydroidea) Hydra sp. are ubiquitous solitary freshwater hydrozoans. These feed mainly on small crustaceans such as and Daphnia that they capture with tentacles. When stimulated, nematocytes in their tentacles discharge nematocysts, which inject their prey with venom, immobilizing and killing it. The tentacles then curl inward, directing the prey into the mouth. Hydra have a very simple sac-like gastrovascular cavity, and can only accommodate one or two prey items at a time. Waste must also be discharged through the mouth, limiting foraging efficiency.

Hydra have no medusa form, and reproduce both sexually and asexually. Commonly, Hydra produce asexual buds that develop into nutritionally independent organisms. This daughter polyp may remain attached for some time, or break off of the parent. Hydra polyps generally attach to a substrate and extend tentacles to feed on passing organisms, but they may also attach to fragments of debris suspended in the water.

The body of a Hydra comprises two layers of cells, each of which is just one cell thick. The epidermis is the outer layer of the body, and the gastrodermis lines the gastrovascular cavity. The epidermis has contractile epitheliomuscular cells that are contractile and allow the hydra to respond to stimuli by changing the overall shape of the body. The body can lengthen, contract, twirl or bend to one side.

Obtain a sample of live Hydra and observe it under the dissecting scope. Note how it moves. After observing the hydra by itself, obtain a sample of Daphnia, and add them to the dish containing your hydra. The hydra should begin initiate feeding behavior almost immediately. The body should extend, and the tentacles should begin slowly moving back and forth. Eventually, a Daphnia will blunder into the hydra and be captured by the tentacles. Observe the hydra feeding. 5 General Zoology Laboratory. Matthew K Nelson (2011) Obtain a whole mount Hydra preserved slide. Observe using the compound microscope. Portuguese man-o-war (Order Siphonophora) Like other siphonophores, the portuguese man-o-war (Physalia sp.) is a colonial hydrozoan polyp that is often mistaken for a medusa. The colony has feeding polyps called gastrozooids, reproductive polyps called gonozooids, and stinging polyps called dactylozooids. The colony is kept afloat by a large modified polyp called a pneumatophore. The pneumatophore is filled with gas, and serves as both a float and a sail.

Observe the preserved Physalia. Fire corals (Order Hydrocorallinae) Fire corals are colonial hydrozoans that secrete a calcareous exoskeleton like true corals. Despite their apparent outward similarity, the fire corals (unlike true corals) can deliver a painful sting to humans unwise enough to touch them. Scyphozoa (order Semostomae) Most of the species that are recognized as “jelly fish” are in the class Scyphozoa. These medusa are dioecious and reproduce sexually. For the most part, they are pelagic and planktonic, traveling mainly through the action of ocean currents. They do swim, but only short distances, and with little directional control.

The largest scyphozoans can reach sizes of up to 40 meters in length. Despite their relatively large sizes, still only consist of two layers of body cells. The outer layer of the body is the epidermis, and the gastrovascular cavity is lined by the gastrodermis. sperm Between these two layers of tissue is a gelatinous mesoglea layer which can be quite large and make up egg most of the body mass.

The polyp phase is reduced in the Scyphozoa. Adult adult medusa medusae are gonochoristic, and produce either eggs or sperm. Fertilization is external, and the fertilized egg develops into a planula . The planula eventually settles to the bottom and develops into the scyphistoma scyphistoma (polyp). The scyphistoma may reproduce asexually to form podocysts which develop into more scyphistomae, or produces ephyra buds through the process of strobilation. These ephyra develop into free- swimming adult medusae. young medusa

Adult medusae are oriented with their aboral surface up, and their tentacles and mouth directed downward. Some scyphozoans possess nematocysts which can strobila deliver a powerful sting to a careless swimmer or beach goer. The venom from a jellyfish sting can result in redness, swelling, extreme pain, cramping, and even death.

6 General Zoology Laboratory. Matthew K Nelson (2011) Observe the preserved Aurelia specimen. note the tetraradial symmetry which is evident in the gastric pouch position of the gastric pouches. Draw and label: gastric pouch, stomach, gonads, oral arms, tentacles, mouth stomach Anthozoa gonads Anthozoans lack the medusa body form and exist as either solitary or colonial polyps. The colonial anthozoans are generally referred to as corals and oral arm the solitary anthozoans are called anemones.

Corals (order ) examine the skeleton examples.

Corals secrete a calcareous exoskeleton which forms the basis for the colony. Each individual polyp in the colony would have been anchored to one of the tiny pits that is evident across the surface of the coral skeleton. A living coral colony is often brightly colored due to the presence of zooxanthellae (mutualistic algae) and protein pigments produced by the coral. This mutualistic association with single-celled algae supplements the nutritional requirements of the colony and is essential for survival of the coral. When corals become stressed by environmental factors, they discharge their zooxanthellae and “bleaching” occurs. is a symptom of an unhealthy reef. Discharge of zooxanthellae can help the colony to survive short-term stresses, but if stressful conditions persist, can result in death of the colony.

Each polyp has a gastrovascular cavity that is branched to increase its surface area. The gastrovascular cavities of polyps near each other are interconnected by gastrovascular canals allowing for exchange of nutrients.

Anemones (order Actinaria) examine the preserved specimen of the anemone Metridium.

Anemones are usually larger solitary anthozoan polyps. Anemones also possess zooxanthellae, and as a result are often brightly colored just like their smaller coral counterparts. However, they lack the calcareous exoskeletons produced by corals, and as such are fairly soft-bodied. The contractile cells of the epidermis work in opposition to the mesoglea sandwiched between the epidermis and the gastrodermis, functioning as a hydrostatic skeleton.

Unlike corals, some anemones can inflict a painful sting in response to human contact. The tentacles of anemones possess many cnidocytes which discharge in response to tactile stimuli. They use their tentacles to catch relatively large prey items such as fish and crustaceans.

The gastrovascular cavity of anemones is highly complex, with a large surface area for absorption of nutrients, and production of enzymes. Just inside the mouth, the pharynx leads downward into the gastrovascular cavity, which is divided into multiple chambers by mesenteries which project inward from the body wall. Despite their

7 General Zoology Laboratory. Matthew K Nelson (2011) large size, the body of the anemone is still only two cell layers thick, but these two cell layers are folded back on themselves many times to create a tough, sturdy body and a subdivided gastrovascular cavity.

Anemones are generally sessile, although they can glide slowly along a substrate. They attach to a substrate using their muscular mucus covered pedal disc. The pedal disc is also involved in asexual reproduction. Lacerations of the pedal disc often result in binary fission of the anemone.

8 General Zoology Laboratory. Matthew K Nelson (2011) NAME: ______SECTION:______LAB EXERCISE 2 questions porifera

1. Which of the body forms found in sponges is present in Scypha?

2. Why do some sponges feel “spongy”?

3. Explain how water passes through a sponge. cnidaria

1. Characterize the two body forms found in cnidarians:

2. What is a nematocyst?

3. The portuguese man-o-war is often mistaken for a jellyfish. How are siphonophores different from jellies?

4. How are corals and anemones different?

9 General Zoology Laboratory. Matthew K Nelson (2011) Drawings

Obelia sp. colony Kingdom: ______Phylum: ______Class: ______Order: ______

10 General Zoology Laboratory. Matthew K Nelson (2011) Drawings

Aurelia sp. Kingdom: ______Phylum: ______Class: ______Order: ______

11 General Zoology Laboratory. Matthew K Nelson (2011)