II. Classifying Some Unusual Fishes (30 minutes)
Explain that classifying fishes into groups has been a challenge for ichthyologists for centuries.
Pass out a set of Deep Sea Fish Specimen Cards to each group. Explain that these are specimens housed in museum collections, such as those you might find at the National Museum of Natural History in Washington, DC. Challenge students to sort their cards according to which fishes they think are most closely related or are members of the same clade. Students should identify and discuss features that unite members of each clade and that distinguish them from the other clades.
NOTE: Some of the specimens are damaged, a result of being brought in nets to the surface of the ocean from deep depths. Like the scientists who worked with these specimens, the students will need to make their best observations, given the state of the material.
Give students a chance to share their clades and explain the features they used to classify the fish. Then reveal how scientists over the last 100 years have grouped them using the Deep Sea Fish Specimen Key. Students should compare how they sorted the Deep Sea Fish Specimen Cards with the way the scientists did and discuss the names given to each group; Whalefishes, Bignose fishes, and Tapetails.
NOTE: Have students keep the Deep Sea Fish Specimen Key as a reference.
Discuss as a class:
How did your classification scheme compare to that of the science community? Answers will vary.
What features helped you group the Deep Sea Fish Specimen Cards? Responses may include; body shape, number of fins, other. Students could revisit the Evolutionary Classification of Fish section in Fish Resource Guide.
What features presented challenges to classifying the fishes? Some of the fishes may be challenging to classify because they are damaged. There is also no scale bar to show relative sizes of the fishes, which may present a challenge to students.
Students record what they learned and discussed about how these fishes are grouped into three groups on page 2 of their Student Research Notes.
Explain to students that Whalefishes or Cetomimids have been known since the 1890s, and it was not until the 1950s and 1960s that Tapetails and Bignose fishes were discovered. These fishes were initially classified into three families because of the differences in their morphology (what they look like), or where they live. Since then, ichthyologists have had access to a broader range of tools and technologies such as high resolution microscopy, staining techniques, and genetic analysis that allow them to investigate in more detail how different fishes are related, and therefore how they can be grouped into clades based upon their evolutionary relationships.
NOTE: In 1971 it was proposed that these fishes are all part of a single clade and this hypothesis has not been challenged since the 1980s. Johnson et al.'s work looked at different lines of evidence in this context to explain how it is possible that closely related fishes can show such extreme morphological differences. For the purposes of this activity, students are using the same features that Johnson et al. used, but in a slightly different way; to test two hypotheses (that the fishes form three clades or a single clade).
Explain to students that these classification schemes are hypotheses of relationships, and like all hypotheses, can be tested. Grouping these deep sea fishes as three separate but closely related clades will be known as the Three Clade Hypothesis. Grouping them as a single closely related group will be known as the Single Clade Hypothesis.
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