The Activity:
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The Activity: IV. Testing Hypotheses – Using Morphology (physical features) (60 minutes) |
Explain to students that they will investigate three different morphological features to test the hypotheses: gut morphology, muscle morphology, and gill-arch morphology. For each, students will follow the directions on pages 5-7 of the Student Research Notes and on the Morphology Cards to analyze and compare the structures of the three fish groups. They will record their observations and make inferences about the function of the features, which they can then use to determine the impact of the data on each of the hypotheses. Have students analyze each of the morphologies one at a time as outlined below. For more advanced groups, pass the cards out as a set, or have different individuals analyze different morphologies, then share. Be sure to review with students the directions, data tables, and questions on pages 5-7 of the Student Research Notes before they begin their investigations.
Pass out a Gut Morphology Card to each student group. Have students compare the internal features associated with feeding in the three fish groups: stomach, esophagus, liver, and intestine. Students record the presence and absence of each of these features in addition to noting any unusual sizes or shapes of these features in the data table on page 5 of the Student Research Notes. They then make inferences about each group's feeding habits. In addition to having students reference the Fish Reproduction and Life Cycle section of Fish Resource Guide it may be helpful to guide students using the following discussion questions:
Whalefishes: Since Whalefishes have typical guts, what can you infer about their feeding? Their feeding is typical of fishes. Food moves from their mouth, through the branchial basket, into the esophagus, and then into the stomach.
Bignose fishes: If a fish does not have an esophagus or gut, as is the case with the Bignose fishes, how could they get energy to survive? Without a stomach or esophagus, they could not get energy from an external food source. Go back to your earlier external anatomy descriptions. What is unusual about the mouth of Bignose fishes? Bignose fishes have an immobile upper jaw and are thus not very effective at getting food into their mouths. If a fish is not able to feed, how can they get energy resources to sustain them? One way would be to store energy as an immature/larva that then gets used as an adult. Would you expect a fish to live very long if they could not feed? No. Why might the Bignose fish have enlargedtestes, but no ability to feed? Reproduction is more important than feeding and long life span. The Bignose fish also have an enlarged nasal organ thought to aid in finding mates.
Tapetails: Why would a fish, such as a Tapetail, eat more food then it can use? To store energy for later use. How might it store this energy? The liver is an organ that can store such energy.
Students should use their observations and inferences to determine how this line of evidence impacts each hypothesis. They should notice that this evidence best supports the claim that Tapetails are the larval form of Whalefishes and Bignose Fishes and therefore can support the Single Clade Hypothesis.
Pass out the Muscle Morphology Cards to each student group. Students should use the card to make observations about the proportion of red aerobic muscle and white anaerobic muscle present in each fish group. Students record the dominant form in the data table on page 6 of their Student Research Notes, and then make inferences about the swimming ability of each fish group. It may be helpful to guide students using the following discussion questions:
Many organisms, such as yourself have both aerobic (red) and anaerobic (white) muscle fibers. The proportion of each influences an organism's endurance and speed. For example, long distance runners have a higher proportion of red aerobic muscles so lots of oxygen can get to the fibers for sustained effort. Sprinters on the other hand have a higher proportion of anaerobic fibers, which can act quickly but not for very long because of reduced blood flow. Based on your observations, which of the fishes would you infer have better endurance and so are able to swim for sustained periods? Whalefishes and Bignose fishes. Which fishes are less able to swim for long periods of time? Tapetails.
Many types of fish have larvae. Larvae typically get around by drifting in the currents. Would you expect larvae to have lots of aerobic (red) muscle? No. What do you think happens to the proportion of red aerobic muscle as a larva grows into an adult? The amount of red aerobic fibers increases.
Given what you inferred about Bignose fishes from the gut morphology, why would having a large proportion of red aerobic muscle be an advantage? Since Bignose fishes can't eat and have large reproductive organs it was inferred that they focus on finding mates. Having a strong ability to swim could aid in this goal.
Students should explain how this line of evidence impacts each hypothesis. Though this evidence does not rule out the Three Clade Hypothesis, this evidence can support the claim that Tapetails are the larval form of Whalefishes and Bignose fishes, which would support the Single Clade Hypothesis.
Pass out Gill Arch Morphology Card A. Explain to students that the structures of the gills of the different fishes also showed extreme differences. The photos depicted in the Gill Arch Morphology Card are cleared and stained to make viewing the structures easier. As a class, have students compare the labeled structures and record the presence or absence of a tongue; the orientation of bone 1 labeled in the diagram (horizontal, vertical, or the angle from vertical), and the shape of the gill rakers (thick, conical, toothed, or thin, forked, smooth) of each specimen in the table on page 7 of the Student Research Notes. Students will not be able to determine the shape of the gill rakers in Specimen 3. Students should recognize that Tapetails have a tongue, but a tongue is missing in Whalefishes. Bone 1 of the Tapetail is horizontal, whereas the bone of the Whalefish is vertical. The gill rakers of the Tapetail are thin and forked. The gill rakers of the Whalefish are thick and conical.
Discuss the following:
How do your observations impact each of the hypotheses? The extreme difference between the two specimens suggests that they are part of different clades, therefore this data best supports the Three Clade Hypothesis.
Explain to students that Johnson, et al. went back to the collections to search for transitional specimens as a way to test the hypotheses. Transitional specimens are individuals that are in middle stages of development. Ask students:
What features would you expect to find in transitional forms if the Single Clade Hypothesis is supported? Transitional forms would show the loss of a tongue, a movement of bone 1 from horizontal to vertical, and gill rakers that transform from thin, forked, and smooth to thick, conical, and toothed.
Pass out the Gill Arch Morphology Card B that shows the gill region from two small "Whalefish" specimens Johnson, et al. found in the collections that they suspected could be transitional forms. Have students record their observations about the specimen's gill features in the same table on page 7 of the Student Research Notes. For the angle of the bone 1, students will need to record an estimated angle in the degrees from horizontal. Students should look at their data and explain if Specimen 3 and 4 (young Whalefish) could be transitional forms between Specimen 2 (Tapetail) and Specimen 1 (Whalefish) that would support the Single Clade Hypothesis.
Finally, students can revisit the fishes' external morphology using the Deep Sea Fish Specimen Key and the Fish Family Collection Data Table from Part III. They can incorporate their observations from the data in combination with the other lines of evidence and summarize their findings on page 8 of the Student Research Notes. Students can also generate new questions that they would like to explore to further test the hypotheses OR to extend their understanding about some aspect of the fish group(s).
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