To determine which group of organisms is known to be exclusively marine, we will analyze each option provided:
1. **Aquatic Vertebrates**: This group includes all vertebrates that live in water, such as fish, amphibians, and some reptiles. Since they can inhabit both saltwater (marine) and freshwater environments, this group is not exclusively marine.
2. **Tetrapoda**: Tetrapods are a superclass of animals that include amphibians, reptiles, birds, and mammals. They can be found in various habitats, including terrestrial, freshwater, and marine environments. Therefore, this group is also not exclusively marine.
3. **Pisces**: This term refers to fish, which are primarily aquatic animals. However, fish can be found in both saltwater and freshwater environments. Thus, this group is not exclusively marine either.
4. **Protocordates**: Protocordates are a group of animals that are primarily marine. They include organisms like tunicates and lancelets, which are typically found in marine habitats. Therefore, this group is known to be exclusively marine.
Based on this analysis, the correct answer is **Protocordates**.
### Step-by-Step Solution:
1. **Identify the Groups**: Review the options provided: Aquatic Vertebrates, Tetrapoda, Pisces, Protocordates.
2. **Analyze Aquatic Vertebrates**: Understand that this group includes both marine and freshwater species.
3. **Analyze Tetrapoda**: Recognize that tetrapods inhabit various environments, not limited to marine.
4. **Analyze Pisces**: Acknowledge that fish can live in both saltwater and freshwater.
5. **Analyze Protocordates**: Confirm that this group is primarily found in marine environments.
6. **Select the Correct Answer**: Conclude that Protocordates are the only group that is exclusively marine.
