Anomopoda
Ceriodaphnia Daphnia
Moina Scapholeberis Simocephalus

Water fleas


argulus

Fish Lice

Not an uncommon parasite of fish this family are thought to be primitive yet there is no evidence of this in the fossil record.


crabs
Geosesarma
Potamon




fairy_shrimp
dendrocephalus
redtail



mutations

Naturally occuring, but bizarre, mutations.


ostracod

Clam Shrimp

Ostracods are "by far the most common arthropods in the fossil record". Found the world over, they look like a tiny clam with a shrimp inside, are free swimming and show up nearly spontaneously in aquaria. Because of their tough shell most fish won't eat them, but fish with poweful jaws (Ie. Bettas, Cichlids) sometimes will.