FISH OF THE WEEK - Vampire Crab (Geosesarma dennerle).

We're back! Today we are looking at the Dennerle Vampire Crab (Geosesarma dennerle). Even though it's a terrestrial crab, I thought it would be good to include it for FOTW anyway due to its popularity in the hobby, and because there is a lot of misinformation floating around. Thanks to VampireCrabGriffin for suggesting the species and Skalle for the research!

Geosesarma are sersarmids with the addition of geo- in the name referring to their very terrestrial lifestyle. The species epithet "dennerle" comes from the German aquarium supply company Dennerle which sponsored one of the authors working on the paper during studies on Jawa, the Indonesian island that the crabs are native to. They inhabit the slopes of a small valley, living under and between rocks or among dense vegetation. They have also been observed excavating burrows in the loose ground at the edge of a creek running through that valley, juveniles may spend more time closer to water. They are omnivores but consume a larger amount of animal matter compared to plant matter. G. dennerle are tiny terrestrial crabs with a carapace width of around .5 inch (15mm) in both sexes. Other sources claim up to 1 inch (25mm) but these may be misidentified or hybrids. Males tend to have bigger chelae (pincers). Their coloration is variable, with the most well-known "form" having a mainly dark purple body, bright purple chelae, yellow eyes and the back-half of the carapace colored a bright light-yellow. There are also individuals with an entirely light-yellow/cream colored carapace as well as individuals that are completely purple. All members of the genus are almost entirely terrestrial, having lost the ability to breathe under water as well as having evolved to skip any aquatic larval stage as is seen in other terrestrial crabs. Instead of aquatic larvae, fully developed and already terrestrial babies hatch from their comparatively large eggs. Their maximum life expectancy is around 2 years. Vampire crabs have been and continue to be caught and sold in the pet trade as a popular pet, G. dennerle being one of the first species in the hobby since at least 2005 and officially described in 2015.

In captivity they need a proper terrarium with only a shallow dish that holds about half a liter of water. Sticks and rocks can be placed in the dish to help the crabs climb out and prevent drowning. Vampire crabs are social species that should be kept in small groups with an excess of females, 3 being a more commonly cited minimum group size, consisting of one male and two females. The minimum recommended terrarium size for an adult trio is around 12x12x12 inches (30x30x30cm). The crabs need climbing opportunities like large pieces of wood and sturdy plants as well as many hiding spots so the crabs feel secure and can avoid each other if needed. Substrate such as organic potting soil that is free of fertilizer works well, and it must be deep enough for them to burrow. They will readily breed and babies can be left in the main terrarium. Plenty of hiding spaces will minimize the chance that the adults prey upon them. Their diet should ideally be mostly carnivorous with a little bit of vegetable matter. Protein rich fish flakes, dried Gammarus, frozen feeders (thawed), and small bits of cooked fish or shrimp can be offered.

Sources and further reading:
• https://www.researchgate.net/publication/275894218_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