i would add CON score to that.
"--" is also called out as a non-ability in stat rules, such as undead.
i'm pretty sure that wherever you find "--", it is non ability.
Nonabilities only concern ability scores, because Abilities in D&D terms are just the 6 basic attributes.
Nonabilities: Some creatures lack certain ability scores. These creatures do not have an ability score of 0—they lack the ability altogether. The modifier for a nonability is +0. Other effects of nonabilities are detailed below.
assuming that psicrystals having HD grants them feats is just that, an assumption. besides, the information for psicrystals comes from the class feature entry, the monster entry is secondary.
I would tend to disagree that it is an assumption - it is a deduction from the way every other creature is handled in the game. The Psicrystal entry in the Psion description states the following: (And do note that only Psicrystal advancement is a psion feature, actually there is no mention of Psicrystals among the Psion class features.)
Psicrystal Basics: Use the statistics for a psicrystal, but make the following changes.
During the changes it mentions nothing of not giving it certain things that would normally depend on HD.
The "Psicrystal affinity" feat states this:
PSICRYSTAL AFFINITY [PSIONIC]
You have created a psicrystal.
Prerequisites: Manifester level 1st.
Benefit: This feat allows you to gain a psicrystal.
Everything else must be taken from relevant other information - the Psicrystal monster entry, for instance.
again, the fact that they have HD and that the HD change are a function of the fact that a character has a feat.
They gain HD because the monster entry says so. The Familiar entry, for instance, does not, since IIRC familiars don't have a normal monster entry.
being able to gain feats and HD is dependent upon the creature being capable of independent existence and independent gaining of xp. which a psicrystal is not.
No monster is by default capable of gaining XP. Monsters just exist to get killed, they don't have a life of their own. (I do disagree with this, but it's basically how the game is set up.) Monsters gain HD because their governing entity says so (usually the DM, but sometimes others, such as the PC in control of an animal companion, for instance). So the general fact that monsters are able to gain feats can be reliably deduced on them having an Int score that is not a non-ability.
So there are multiple cases within the game of creatures gaining feats from HD, whereas there are few if any cases where creatures gain actual HD but not the accompanying features. There is no specific rule to draw from to handle this any differently.
For comparison: The Riding Dog entry (and I guess most of these animal entries) specifies an Advancement: -. The Druid Animal Companion features gives Bonus HD. And then it also says this:
An animal companion gains additional skill points and feats for bonus HD as normal for advancing a monster’s Hit Dice.
Emphasis mine. Thanks to that "normal" it becomes clear that this is a clarification to use the general case. In the case of Psicrystals there is no such clarification, but it isn't really necessary.
