It's a "well known" and interesting "fact" that the human body is made up of "mostly water". With percentages from 65% to 90% often being repeated as if they were exact and proven amounts.
Obviously the exact numbers will vary from one individual to the next, and from one measurement method to the next, and from one to the other definition of which water is included in the measurement, etc. And it will even vary within the same individual over time depending on water intake and excretion (in all its forms).
Moving on from humans, water is also going to be a significant component of the body mass of animals as well. And the variations in percentages here is likely to vary even more, when you add different species in to the mix of factors to compare. One obvious example is jellyfish, with an extremely high (compared to humans, at least) percentage of their body mass composed of water.
But which animal is at the lower end of the spectrum, with the smallest percentage of their body mass being made up of water?
In my attempt to research this topic, I was unable to find any documented information that was not behind a pay-wall, such as this article.
For clarity and simplicity, please limit answers to multi-cellular animals, assume a normal healthy adult specimen, averages and estimations are acceptable for measurement comparisons, as avoid answers that include grey areas in defined classifications (e.g. are viruses alive), and answers need not be reduced to a specific individual species if a genus or even family is enough to identify the animal to a layman, etc.