It's clear that the down side of being dual natured is that such critters can be attacked on the astral plane but the critters stats in the book don't include any Astral Combat skill so can they defend them selves at all?

The obvious thing to do is to let them use their physical combat skills on the astral plane but the rules don't let them do that.

Looks like it. As a GM, I'd just assume that all dual natured critters have astral combat and that the stat entries for those that don't are in error.

I'd rule it that dual natured critters don't have any concept of the seperation between the real and astral worlds as they are permanently dual natured and are that way from birth. Thus, I'd just use their physical combat skills on the astral plane and call it even. Sure, the spirit they're attacking doesn't have a physical form, but that wouldn't really bother me. The extra initiative passes the spirit probably has are more than enough to make the critter pulp, more likely than not.

or rule, that since they are dual natured (and not just astrally projecting) that those CAN default on the astral combat skill.

Makes you wonder a little if the people you see in real life that seem to be fighting thin air, are actually just dual natured and are fending off evil spirits to keep us safe.

And here I though giving dogs orichalcum teeth was just for the bling.

so would a ghoul use unarmed combat to attack an astral form then? Seems logical since they are listed under critters, and are ni many cases dual natured from birth. Would that work differently for a ghoul that became one by infection?

As I've mentioned before.
I believe that Astral Combat is a Combat Skill for an Astral Form, and that if you Are Physical then you use a Physical Combat Skill.
It doesn't matter what you are fighting, it's the skill YOU possess at the craft at hand.