Assertion: Aquatic herbivores are usually more productive as compared to terrestrial herbivores.
Reason: Phytoplanktons achieve faster grwoth rate and more nutritious to heterotrophs than their terrestrial counterparts due to their small size and lack of structural tissues.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Aquatic herbivores are usually more productive as compared to terrestrial herbivores." This suggests that aquatic herbivores convert food into biomass more efficiently than their terrestrial counterparts.

2. **Understanding the Reason**: The reason provided is that "Phytoplanktons achieve faster growth rates and are more nutritious to heterotrophs than their terrestrial counterparts due to their small size and lack of structural tissues." This implies that the primary producers in aquatic environments (phytoplankton) have characteristics that enhance their productivity.

3. **Analyzing Phytoplankton**: Phytoplankton are microscopic plants found in aquatic environments. They have a high surface area to volume ratio, which allows them to absorb nutrients quickly and grow rapidly. Their small size and minimal structural tissues mean that they can be easily consumed and digested by aquatic herbivores.

4. **Comparing with Terrestrial Plants**: In contrast, terrestrial plants have more structural materials (like cellulose and lignin) that are not easily digestible by terrestrial herbivores. This makes it harder for terrestrial herbivores to extract energy from their food, leading to lower productivity.

5. **Growth Rates**: Phytoplankton can reproduce rapidly, leading to high turnover rates. This means that even though their biomass at any given time may be low, their productivity is high. In contrast, terrestrial plants grow and reproduce more slowly, which limits the productivity of terrestrial herbivores.

6. **Conclusion**: Both the assertion and the reason are correct. The reason provided accurately explains why aquatic herbivores are more productive than terrestrial herbivores. Therefore, the correct answer is that both the assertion and reason are true, with the reason being a correct explanation of the assertion.

### Final Answer:
Both the assertion and reason are correct, and the reason is the correct explanation of the assertion.