What is unique or rare about the VM Zodiac signs?

[MarcoP]

I share your analysis to a large extent, but I feel differently about the last bit, at least if I understand correctly what you mean. Do you suggest that, since crayfish drawings are often hit and miss, we can't draw conclusions if artists make the same mistakes as those in the VM? I.e. if enough people copy a crayfish, you're bound to get some of the same mistakes?

I'd say that this is true, but on the other hand there are enough similarities between Prague-VM-Alsace to consider placing them together in one group compared to the other examples.

I too want the Prague-VM-Alsace connection to be true: obviously this bias can be a problem, so I try to be conservative. The fact that Prague-VM-Alsace Cancer images are closer to each other than to the other legs-on-tail crayfish we have found so far is not sufficient evidence to conclude that they all descend from the Prague manuscript.
Voynich researchers tend to easily jump to strong conclusions on the basis of insufficient evidence. "This root looks like a figure on a Babylonian seal, hence the manuscript clearly incorporates Mesopotamian sources". "These two EVA labels look like Finnish words, hence Voynichese must be Finnish". "These plants look more like exotic plants than European plants, hence the manuscript indisputably represents American (or African) plants".  
But visual similarity (even when it is not totally illusory) can be due to several causes, including coincidence. People make errors like these in good faith, because of wishful thinking and the desire to find answers to the numberless questions we face. But it is better to be patient, trying to separate what can be objectively convincing from what could be coincidence.

My proposal is to split the Prague-VM-Alsace problem into two steps:

1. The easy one is checking if the Alsatian ms You are not allowed to view links. Register or Login to view. derives from the Bohemian You are not allowed to view links. Register or Login to view.. I believe that this problem has likely been studied by reliable academic scholars (e.g. Lippincott, Blume or Dekker). Since the two manuscripts provide complete series of the illustrations of the constellations, a detailed image-by-image comparison is possible. Also, the text in the two manuscripts can be compared. If we should find that the two manuscripts are not related, our Voynich crayfish (a single highly anomalous image) will not make much difference: lack off evidence of a Prague-Alsace group would make a convincing argument for a Prague-VM-Alsace group nearly impossible.

2. If the Alsatian and Bohemian manuscripts are related, we can see if the Voynich Cancer (and possibly other zodiac signs) can fit in that tradition. Even if the Prague-Alsace connection is confirmed, linking the VMS will be difficult: the Voynich cycle represents the zodiac signs, not the constellations, and (as far as I know) there are no elements suggesting that it belongs to the Michael Scot tradition.

of course there is not a single crayfish that shares all properties with those in the VM. The VM artist reduced the legs to sticks and the claws to weird attachments without flex. He also over-emphasizes the separation line between head and body.

I agree. This is a very unique image, the most striking anomaly being the presence of two animals connected by a Pisces-like cord.

At the current state of our research, I think that You are not allowed to view links. Register or Login to view. is a more promising line of investigation: the motif is rare, but clearly intentional and it is documented in at least one Gemini image of a close date. Moreover, the similarity with the engraving is total, but for mirroring, costume and quality of execution (all differences that, contrary to the Cancer anomalies, are easily explained).