Why would losing a few Dozen aquatic species lead to the end of life in the ocean as we know it

It would unbalance the ecosystem.

Due to the fact that if some species die off, the other species that rely on those species for food or protection would loose that food/ protection which may lead to their extinction and their extinction would lead to another extinction and so on.

answer; i believe that the best answer is (secondary cardiac arrest due to

1 digestive system

2 stomach

3 bladder

4 carbon deoxide

5 tiger