What biological family does a clam belong too?
If by biological you mean the phylogeny it's phylum mollusca, class bivalvia, subclass heterodonta
Not as long as he is a minor. When 18 and emancipated he can do what he wants. When adopted you legally become a member of the new family and is no longer a member of the biological one. They are not allowed to search for you until you are 18 and you have to follow your new family's rules and if they say no to contact your biological family you have to follow that.
No, they are different species, Goldfish belong to the Cyprinidae family, common Tetras belong to the Characidae family, not even slightly related. If your tetra is small enough, your goldfish can/will eat it. Goldfish can get pretty big whereas common species of tetra usually do not get too large. They are not compatible species at all.
Depends. It's up to the adoptive parents to decide. Your biological parents have given up every right to you. Many feel it would be too hard for a child to bond to the new family if the biological parents are still there. It can also depend on why your biological parents gave you up for adoption. If their rights were taken away due to drugs for instance, it would not be in your best interest…