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Are Dolphins Fish? — Reading Comprehension

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This passage addresses the common misconception that dolphins are fish by explaining their biological classification as mammals. The text details key differences in respiration (lungs vs. gills), reproduction (live birth vs. laying eggs), and thermoregulation (warm-blooded vs. cold-blooded). It also highlights unique mammalian adaptations for aquatic life, such as blubber and a vertical tail fluke. The passage is designed for middle school students and aligns with NGSS standards related to the characteristics of living organisms (MS-LS1). It uses clear, concise language to explain complex biological concepts and includes a fun fact and multiple-choice questions to assess student understanding of the key concepts presented.

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The question of whether a dolphin is a fish is a common one, likely because both live in the ocean and share some similar physical features. However, dolphins are not fish; they are mammals. This classification is based on a set of fundamental biological characteristics that distinguish them from fish and place them in the same class as humans, dogs, and whales. Understanding these differences is key to appreciating the unique adaptations of marine life.

One of the most significant differences lies in their respiratory system. Fish use gills to extract oxygen directly from the water. In contrast, dolphins have lungs and must come to the surface of the water to breathe air. A dolphin breathes through a single nostril on top of its head called a blowhole. This is a crucial adaptation for a mammal living a fully aquatic life. Another major distinction is how they reproduce. Fish typically lay eggs, a process known as spawning. Dolphins, like all mammals, give birth to live young and produce milk to feed their calves.

Beyond breathing and reproduction, dolphins possess other mammalian traits. They are warm-blooded, meaning their body can maintain a constant internal temperature regardless of the external water temperature. This is different from fish, which are cold-blooded and whose body temperature fluctuates with their environment. Dolphins also have a layer of fat called blubber under their skin, which provides insulation to help them stay warm. Their tails also move up and down to propel them through the water, while a fish's tail moves side to side. These adaptations highlight their evolutionary path as land-dwelling mammals that returned to the sea.

Dolphins are also highly intelligent and social animals, often living in groups called pods. Their complex social behaviors, communication skills, and use of echolocation—a system of using sound waves to navigate and hunt—demonstrate their advanced cognitive abilities. These behaviors, along with their physical characteristics, solidify their place in the mammalian family tree.

Fun Fact: A dolphin’s tail fin, or fluke, moves vertically (up and down) to propel it through the water, while a fish's tail fin moves horizontally (side to side).

According to the passage, what do dolphins use to breathe?

GillsLungsBlowholesBoth b and c

What is the main purpose of blubber for a dolphin?

To help it swim fasterTo provide insulationTo make it more buoyantTo store excess food

A fish's tail moves from side to side. How does a dolphin's tail move?

From side to sideUp and downIn a circular motionIt does not have a tail

Based on the passage, which characteristic is a dolphin trait?

Laying eggsHaving gillsBeing cold-bloodedGiving birth to live young

How do dolphins navigate and find prey in the water?

By using their sense of smellBy using a system of echolocationBy using their excellent eyesightBy following currents in the water

Why is a dolphin classified as a mammal instead of a fish?

It lives in the oceanIt is a warm-blooded animalIt has fins and a tailIt has a streamlined body

A scientist discovers a new marine animal. It gives birth to live young and uses lungs to breathe. Based on the passage, how should this animal be classified?

As a fish, because it lives in waterAs a mammal, because it has lungsAs a fish, because it has finsAs a mammal, because it is a new species

A student observes a fish and a dolphin swimming. The fish is moving its tail side to side, and the dolphin is moving its tail up and down. What can the student infer about their evolutionary history?

The fish and dolphin are closely relatedBoth animals evolved from the same land mammalThe dolphin evolved from a land animalThe fish evolved from a reptile