Guitarfish. Teacher Notes and Answers. with Dr. Michael Heithaus

Transcription

1 Guitarfish with Dr. Michael Heithaus Teacher Notes and Answers BACKGROUND Shark Bay is home to some of the first organisms that appeared Earth. Fossil records date stromatolites back 3.5 billion years. These slimy biscuit-shaped rocks are made up of colonies of ancient cells called cyanobacteria. Shark Bay is also home to another ancient group of organisms called Chondrichthythes. This group appeared hundreds of millions of years ago and came in many weird shapes and sizes. Today, they still encompass an amazing diversity. From tiger and hammerhead sharks to manta and eagle rays, this class of animals is adapted to live in all oceans and is divided into two distinct groups. Rays have flattened bodies and inverted mouths, both of which are features designed for a life on the bottom of the ocean. On the other hand, sharks are torpedo-shaped and have mouths at the front of the body an ideal adaptation for a life of cruising effortlessly in the mid ocean. There is one species that does fit in either mold. Strangely, the guitarfish looks like a cross between a shark and a ray. Its body is long like a shark s; however, its mouth lies on its ventral side and its eyes are on the top of its body like a ray s. In this video, Mike Heithaus and his team will help students classify this unusual animal. They search the shallow waters of Shark Bay and use a net and two very fast boats to capture these elusive fish as well as a plethora of other rays, including eagle rays and whiprays. Then, they head out to deeper waters to catch sharks. The scientists carefully record the captured animals traits, or characters, and collect important scientific data all in an effort to figure out if the guitarfish is a shark, a ray or something else entirely? HMH Environmental Science i

2 Guitarfish Teacher Notes and Answers continued INVESTIGATION 1. Table 1. Condition of traits of five cartilaginous fish species Trait Guitarfish Eagle ray Whipray Lemon shark Tiger shark Skeleton cartilage cartilage cartilage cartilage cartilage Gill slits per side Location of gill slits ventral ventral ventral lateral lateral Spiracles a present present present absent absent Claspers in yes yes yes yes yes males b Pectoral fins fully fused to body Body shape yes yes yes no no streamlined flattened flattened streamlined streamlined Dorsal fin present absent absent present present Mouth modified modified modified normal normal a Spiracles are a pair of openings behind the eye. b Claspers are the modified portions of the pelvic fin used by males for internal fertilization. 2. Table 2a. Similarity matrix for five species of cartilaginous fishes in Species Guitarfish Eagle ray Whipray Lemon shark Tiger shark Guitarfish Eagle ray Whipray Lemon shark 1.0 Tiger shark HMH Environmental Science ii

3 Guitarfish Teacher Notes and Answers continued 3. See the shaded cells in Table 2a in the answer to question Table 2b. Similarity matrix for five species of cartilaginous fishes in Group or species Guitarfish Eagle ray Guitarfish Lemon shark tiger shark group Eagle ray 0.25 Lemon shark tiger shark group 5. See the shaded cell in Table 2b in the answer to question Table 2c. Similarity matrix for five species of cartilaginous fishes in Group Guitarfish eagle ray Lemon shark tiger shark group Guitarfish eagle ray Lemon shark tiger shark group Note. The value in Table 2c is calculated as ( )/2. This is the average distance between the guitarfish and sharks and the rays and sharks. HMH Environmental Science iii

4 Guitarfish Teacher Notes and Answers continued 7. Table 3. Grouping orders and similarity measures for five species of cartilaginous fishes in Group Eagle ray whipray 1.0 Tiger shark lemon shark 1.0 (Eagle ray whipray) guitarfish 0.75 [(Eagle ray whipray) guitarfish] (tiger shark lemon shark) Similarity Guitarfish are rays. The guitarfish is more closely related to whiprays and eagle rays than tiger sharks or lemon sharks. In the cladogram, the guitarfish joins the group that includes the whipray and eagle ray well before these three species group with the two species of sharks. Some of the characters that make a ray a ray are the presence of spiracles, pectoral fins fused to the body, and gill slits underneath the body gill slits, claspers in males, cartilaginous skeleton HMH Environmental Science iv

5 Guitarfish Teacher Notes and Answers continued Both types of data support that guitarfish should be classified as rays rather than sharks. There are slight differences between the two cladograms. Genetic data suggest that whiprays and eagle rays as well as tiger sharks and lemon sharks are more distantly related than the morphological data suggest, but that the other groupings involve closer relationships. In this case, this pattern is mainly because of the small number of characters measured in the morphological dataset. OPTIONAL EXTENSION Have students use the Internet to find more traits that are shared by sharks and rays or that differentiate between them. See if students can remember any other traits from the video (e.g., eyelids, shark-like tail). These traits are not included in the activity to keep the math easy. Advanced students could add traits to Table 1 and create cladograms based on the total number of traits they come up with. Make sure that the traits that they pick are likely to come from shared ancestry (or a lack thereof). Another way to extend the lesson is to have students identify traits that are shared by all rays or shared by all sharks. Once they have completed the activity and know whether a guitarfish is a ray or a shark, have them use the Internet to identify traits not listed in Table 1. HMH Environmental Science v

6 Guitarfish with Dr. Michael Heithaus Problem What exactly is a guitarfish? It looks like a shark that got hit by a steamroller! Should it be classified as a shark or as a ray? To answer this question, we can build an evolutionary tree, called a cladogram. One method of building this cladogram uses morphological data, or data about the physical traits observed and measured in the field. Another method uses genetic data from the fin clips collected in the field. INVESTIGATION To build a cladogram using morphological data, we compare the condition of traits of organisms. Table 1 lists the condition of traits for the species we studied in Shark Bay. 1. Use what you remember from the video to fill in the eight missing values in Table 1. Table 1. Condition of traits of five cartilaginous fish species Trait Guitarfish Eagle ray Whipray Lemon shark Tiger shark Skeleton cartilage cartilage cartilage cartilage cartilage Gill slits per side Location of gill slits ventral ventral ventral lateral Spiracles a present absent absent Claspers in yes yes yes yes males b Pectoral fins fully fused to body yes yes no Body shape flattened flattened streamlined streamlined Dorsal fin present absent absent present present Mouth modified modified normal normal a Spiracles are a pair of openings behind the eye. b Claspers are the modified portions of the pelvic fin used by males for internal fertilization. HMH Environmental Science 1

7 Guitarfish continued The first step toward figuring out how closely related species are is to calculate their similarity. To do this, we count the number of characters that they have that are in the same state and divide this by the total number of characters that were measured. For example, in Table 2a, whiprays and eagle rays have a similarity of 1.0 because all eight characters are in the same state. 2. Data Analysis Complete Table 2a by calculating the four remaining similarity values. Several values have already been filled out for you. Table 2a. Similarity matrix for five species of cartilaginous fishes in Species Guitarfish Eagle ray Whipray Guitarfish Lemon shark Tiger shark Eagle ray Whipray 0.25 Lemon shark Tiger shark Next, we group the most closely related species and figure out how similar they are to the other species, or groups of species. For example, in Table 2b, eagle rays and whiprays have been combined into one group. If any other pairs of species have exactly the same similarity value, they can be grouped together at the same time (for example, if two of the remaining three species have a similarity of 1.0, then group them together). 3. In Table 2a, circle the highest similarity values (this will help you keep track of the values you will need to use later). We then calculate the average similarity between the species that are grouped together and all the other species or groups. For example, to calculate the similarity of the eagle-ray- to guitarfish you would use the formula: HMH Environmental Science 2

8 Guitarfish continued 4. Data Analysis Complete Table 2b by filling in the appropriate third group and calculating similarities for the three empty table cells. Table 2b. Similarity matrix for five species of cartilaginous fishes in Group or species Guitarfish Eagle ray Guitarfish Eagle ray 5. Circle the highest value(s) in Table 2b. (Doing so will help you keep track of the values you will need to use later). 6. Data Analysis Complete Table 2c by creating the two appropriate groups and calculating the similarity value. Table 2c. Similarity matrix for five species of cartilaginous fishes in Group Now that you have calculated all the similarities and average similarities, we need to create a table that will help us determine how to draw our cladogram. HMH Environmental Science 3

9 Guitarfish continued 7. Data Analysis Complete Table 3 by listing the order in which species are grouped together and the similarity or average similarity that groups them. Table 3. Grouping orders and similarity measures for five species of cartilaginous fishes in Group Similarity Eagle ray whipray 1.0 Now we can use the similarity values to create a cladogram. Joining species together at the point of their similarity values creates one type of cladogram. An example of this type of cladogram is shown below. The species being compared are listed on the right side in the order in which they are grouped together. Similarity values are listed at the bottom of the cladogram. The lines drawn in the cladogram show the similarity values between each species or species group. In this cladogram, Species A and B have a similarity of 95 percent. That means that 95 percent of the traits that were surveyed to build the cladogram were the same between species A and B. Species C has 75 percent of its traits in common with species A and B. Species D and E form a second major group. They share 85 percent of their traits in common. The two groups (ABC and DE) only have half (50 percent) of the traits surveyed in common. HMH Environmental Science 4

10 Guitarfish continued 8. Data Analysis Use your data from Table 3 to draw a cladogram in the space below. Start by writing the species names on the right side of the graph in the order they will be joined. Two species have been provided for you. 9. Should the guitarfish be classified as a shark or a ray? Use your data and the relationships shown in the cladogram to justify your answer. Be sure to mention the specific character states that distinguish the sharks from the rays. 10. What traits suggest that rays and sharks descended from a common ancestor? Sometimes it is difficult to get information on enough morphological characters to create a good cladogram, and for some characters it isn t clear whether species share the same traits because they came from a common ancestor. Genetic data can help. By comparing the similarity of genetic codes between species, we can calculate similarity values much as we did for morphological characters. In fact, because we can estimate the rate of mutation for some genes, we can estimate how long ago different groups diverged from one another. Table 4 gives the estimated time that the listed taxa shared a common ancestor based on genetic data (and in some cases, fossil records as well). HMH Environmental Science 5

11 Guitarfish continued Table 4. Grouping orders and similarity measures based on genetic data for five species of cartilaginous fishes in Group Tiger shark lemon shark 50 Eagle ray whipray 125 (Eagle ray whipray) guitarfish 265 [(Eagle ray whipray) guitarfish] (tiger shark lemon shark) Time of divergence (millions of years) Data Analysis Use the data in Table 4 to draw a cladogram in the space provided below. Start by writing the names on the right side of the graph in the order they will be joined. Two species have been provided for you. CONCLUSION 12. Compare the two cladograms you made from the two data sources. Why do you think any differences occur? Do the genetic data support your idea about whether a guitarfish is a shark or a ray? HMH Environmental Science 6