I'm pretty sure your talking about Vertebrates
Answer:
https://en.wikipedia.org/wiki/Diagram
Explanation:
D) higher density than water
Answer: Almost all the ocean will be in danger
Explanation: Species of fish, whale, squid, and all sorts of animals eat krill! If Krill were to "Vanish", then the bigger creatures that fed upon it would start to vanish also, such as whales and squid
many sea creatures feed upon fish right? if they were gone because of krills non-existants, half of the oceans creatures will be in danger
:D