Blue vampire crab (Geosesarma dennerle)

That's Geosesarma hagen, not Geosesarma dennerle.

@Sicarius The sign at the enclosure says Geosesarma dennerle. But I had doubts about the exact species for some time. There are also individuals that do contain more purple. On Species360/ZIMS, they are even listed under the genus Geosesarma, with no species.

@MennoPebesma There are lots of Geosesarma species in the trade, and G. dennerle and G. hagen sure are two of the most common ones. They are regularly imported together. I see on another picture from Aquazoo that they also have the real dennerle in the same enclosure. Those are indeed the purple ones with a yellow back. Your specimen here is clearly different. Here's the paper regaring the two species: Species New to Science: [Crustacea • 2015] Geosesarma dennerle & G. hagen • New Species of “Vampire Crabs” (Geosesarma De Man, 1892) from central Java, Indonesia, and The Identity of Sesarma (Geosesarma) nodulifera De Man, 1892 (Crustacea, Brachyura, Thoracotremata, Sesarmidae)

The crab above has its orange back all the way to the front even reaching further than the eyes, which is not typical for G. hagen. They are variable and for now I'd say it remains the closest match, but who knows if they turn out a yet undescribed species. The genus Geosesarma is heavily researched and new species are regularly described.