What's a word that describes many individuals working together to form a whole?

The are both examples of self-organizing systems or of self-organization.

See wikpedia: self-organization

Another term used is swarm intelligence.

See wikipedia: swarm intelligence

I might call them a collective:

collective n

1:  a collective body :  group

2:  a cooperative unit or organization; specifically : collective farm

That may imply some individual agency and intention to contribute to the whole, which wouldn't be quite right for fish or neurons. A more general term would be a system:

system n

: a group of related parts that move or work together

: a body of a person or animal thought of as an entire group of parts that work together

: a group of organs that work together to perform an important function of the body

... and the behavior of the whole an emergent property:

emergent property n

a property which a collection or complex system has, but which the individual members do not have