It is somewhat explained in the tips in the beginning. You can say "Krab jest zwierzęciem" if you think about whole species, thus making English "Crabs are animals" closer in meaning. Of course "The crab in an animal" is also correct.
And I hope you wouldn't mind if I mark the mistakes in your Polish ;) It's really great though!
Dlaczego w sekcji "correct solution" jest także odpowiedź "crabs are animals", jeśli w języku polskim jest liczba pojedyncza?
"rak" is also animal. it lives in clear rivers, streams, creeks, sometimes you can see it also in the spring well. It loves clear water. Rak lives in Czech. http://www.fcps.edu/islandcreekes/ecology/crayfish_%28cambarus%29.htm