Life Cycle and Biology of Geosesarma "Vampire Crabs"

Growth, Molting, and Regeneration: 

Like other incomplete metamorphosis arthropods, young Geosesarma crabs look like their adult counterparts albeit smaller and lacking the adult coloration. Many young Geosesarma are a dark mottled brown color aptly evolved to blend perfectly into the dark brown surrounding soil. To reach adulthood crabs need to complete ecdysis, or the molting process, a number of times as they incrementally get larger and larger each time they shed their old exoskeleton. 

Freshly molted, teneral, Geosesarma crabs as well as their shed exuvium are extremely light in color and lack pigment, the shed exoskeleton remaining that way while the living vampire crab will begin to darken in color as the new exoskeleton begins to harden with the coming days. While Geosesarma are terrestrial in most behavior they do need to return to water to successfully molt. The number of molts Geosesarma takes to reach adulthood may or may not be fixed or variable, and specific ranges has not yet been observed. It appears that most species require reaching full adult size to reproduce while, at least, G. hagen and G. tiomanicum reach sexual maturity at at least 75% of full size and are capable of producing healthy offspring before fully size is reached. 

Perhaps one of the most incredible aspects of crabs' ecdysis process is their ability to regenerate multiple lost legs when molting. Even more incredible is that crabs, including Geosesarma, or vampire crabs, begin to partially regenerate lost legs before they molt. After a leg has been lost the crab will begin to and will form a small leg bud before molting, a leg bud which can be quite sizable, and then will fully reform and regenerate the leg at the next molt. On Geosesarma this is difficult to see due to their small nature. This partially regeneration process preceding a molt is unlike other arthropods commonly kept in captivity which sometimes will only partially reform a leg after molting and can take up to two to three molts to fully regenerate it. 

Ontogeny/ Reproduction: 

Brachyura crabs in captivity are hard to successfully breed because many rely on the need to return to the ocean to breed and or need to feed on zooplankton during the zoea stage. Geosesarma are not this way. 

Geosesarma do not rely on any special water sources such as saltwater or brackish water to reproduce, and the TGD team has found that they do not need any special fresh water either and will breed in non-actively filtered water sources as well. The only necessity is that they have some access to freshwater to molt and for the young to exist in. 

What makes Geosesarma the easiest brachyuran to breed in captivity, apart from water source, is the fact that they go from egg to fully formed immature crab featuring completely abbreviated development skipping the zoea stage. Because of this there is hope for reducing collecting pressures on natural habitats by establishing more captive populations moving forward. To date many Geosesarma have entered the hobby but not all have been reproduced in captivity with regularity. Some species, like Geosesarma sp. "Orchid" may require a fully year after reaching maturity before being able to produce eggs, while some species like Geosesarma tiomanicum and  G. hagen appear to reach sexual maturity at less than fully size (McMonigle, 2018). Other species currently kept by the TGD team such as G. rouxi, G. pontianak, G. notophorum have yet to breed and may simply need more time like G. sp. "Orchid", or perhaps there are seasonal cues not yet recreated in captivity needed to stimulate successful reproduction. The TGD teams hope to unlock the key to all Geosesrama species reproductive needs. 

References: 

1. McMonigle, O. (2018). Breeding the Vampire and Other Crabs (Brachyura and Anomura in Captivity) Husbandry, Reproduction, Biology and Diversity. Coachwhip Publications. 

2. Hohle, M., & Singheiser, M. (2016). Vampire Crabs of the Genus Geosesarma. Chimaira Buchhandelsgesellschaft mbH.