Vampire crabs of the genus Geosesarma – colourful tiny creatures
These small, colorful crabs have been available in the pet market for many years. Nevertheless, knowledge about them remains limited. Many specimens that reach the market have not been scientifically described and are sold under various trade names. One of these names, “vampire crabs,” has now permanently stuck to them. Interestingly, this term is also frequently used in scientific papers as a common name for the genus Geosesarma. One might wonder what crabs and vampires have in common. Some species of these crustaceans sport eyes with yellow or orange coloration, resembling those of vampires to the individual who coined the name.
The perfect crab for beginner terrarists
With their small size, vibrant colors, easy care, and ability to breed in terrariums, vampire crabs of the genus Geosesarma are an ideal choice for beginner terrarists. However, it’s crucial for prospective keepers to understand their requirements and provide them with a well-arranged terrarium. On the other hand, the limited knowledge of their biology means that breeding these crabs can also be a fascinating adventure for experienced hobbyists. Patience, keen observation skills, and the ability to draw conclusions are essential qualities for any caregiver… Before embarking on your adventure with these crustaceans, make sure to learn what is already known about them.
Where do vampire crabs live?
Vampire crabs are found throughout Southeast Asia, inhabiting regions in Malaysia, Indonesia, the Philippines, Thailand, Papua New Guinea, and the Andaman archipelago. However, information from importers indicates that most of the crabs entering the pet market come from Java and Borneo.
The genus includes both aquatic and terrestrial animals, some of which live a considerable distance from bodies of water, where small water-filled depressions are completely sufficient for their needs. Their natural habitats are located in the coastal zones of streams, rivers, reservoirs, as well as in wetlands and swamps. These crabs find shelter in forest litter, climb trees and shrubs, and some species even dwell in caves. It’s worth noting that crabs of the genus Geosesarma are primarily freshwater crabs, with a few exceptions, distinguishing them from their relatives who are more closely associated with brackish or saline waters.
Individual species inhabit small areas, often in close proximity to each other. Many of them are endemics, meaning they are found nowhere else except in their specific, limited habitats.
Systematics
The decapods (Decapoda) are the most diverse and species-rich order of crustaceans, encompassing well-known species such as shrimp, crayfish, lobsters, and crabs. Among these, crabs (Brachyura) are the most numerous, with well over 10,000 described species. This incredible diversity showcases the wide range of adaptations these animals have developed to thrive in various habitats.
Phylum: arthropods (Arthropoda)
Subphylum: crustaceans (Crustacea)
Class: Malacostraca
Superoder: Eucarida
Order: decapods (Decapoda)
Infraorder: crabs (Brachyura)
Family: Sesarmidae
Genus: Geosesarma
The genus Geosesarma, a member of the Sesarmidae family, currently includes nearly 70 scientifically described species. However, the great similarity between species and the variations in color depending on location, age, and sex create a kind of “crab soup” that is challenging to decipher, not only for hobbyists but also for scientists.
How is a crab built?
The genus Geosesarma includes crabs of small size, with abdomens measuring about 1-3 cm in diameter. However, it’s worth noting that among crabs, there are even smaller species, such as Pinnotheres pisum, which grow to only a few millimeters, as well as true giants like the Pacific crab (Macrocheira kaempferi), whose abdomen can reach 40 cm and leg span up to 4 meters. You might now be wondering if this giant would fit in your room!
Crabs have a dorsoventrally flattened body consisting of a cephalothorax and a heavily shortened, curled-up abdomen under the cephalothorax. When observing a crab, you’ll see its large and round or trapezoidal cephalothorax, a pair of eyes on long vertical stalks, and pincers. The pincers are transformed from the first pair of legs, with four more pairs of legs following. Thus, crabs have a total of 10 legs, which is reflected in the name of their order—decapods (Decapoda).
The body of crabs is covered with a tough chitinous cuticle, with the part covering the dorsal cephalothorax known as the carapace. The mouth opening is located on the ventral side, surrounded by maxillae, two pairs of jaws, and three pairs of jawbones, allowing crabs to handle a wide variety of food very efficiently.
On the abdomen, visible when you turn the crab on its back, are the abdominal legs. Females use these legs to support their eggs. In males, two pairs are transformed into copulatory organs known as gonopodia. These structures, along with the gonopores (the outlets of the female genitalia), are typically symmetrical. Thus, the discovery of a new species of crab (Mabui calculus) in Okinawa with asymmetrically developed gonopodia and gonopores was sensational. Despite this, the male and female are a perfect match.
Crabs breathe using gills, which are enclosed in gill cavities under the carapace. It’s interesting to note that the respiratory pigment in crustaceans is hemocyanin, which contains copper. When combined with oxygen, hemocyanin turns blue, while the deoxygenated form is colorless.
Vampire crabs have a lifespan of approximately 3-4 years.
How to distinguish the sexes in crabs?
This is a fairly simple task. All you have to do is roll the crab onto its back. However, this may prove to be the most difficult part of the whole operation, as the crab is likely to resist. Why do we look under the crab? To examine its abdomen. In females, the abdomen is noticeably wider than in males. Below you you will find some useful tips.
Molting
Crabs need to shed their chitinous cuticle in order to grow. This process is called molting. Just before molting, crabs can be observed entering the water, where they stay for a long time. The shed skin, known as exuviae, is colorless. Do not stress your pets during this period. The new skin is light and soft, and will darken as it hardens.
An interesting aspect is the regeneration of lost legs. Crabs, even before molting, produce a kind of knot where the leg was lost. This knot then completely reconstructs the leg during the molting process.
Vampire crab behavior
Although they are sometimes called vampire crabs, they are not aggressive animals. They should be kept in a group, but in small terrariums, it should be a harem consisting of one male and two females. Limited space and a lack of hiding places can lead to fights, which may end in the death of the weaker individual. Crabs are rather secretive animals, so it is important to provide them with places where they will feel safe. Additionally, remember that they are excellent climbers. In the terrarium, they can often be found on higher branches or leaves of plants. They can efficiently hunt small invertebrates, which can be included in their diet.
How to prepare a terrarium for crabs of the genus Geosesarma?
Vampire crabs are native to the rainforests of Asia, so replicating a piece of this ecosystem in a terrarium is essential.
For a harem of three individuals, a terrarium with dimensions of 30 x 20 x 20 cm will be sufficient. The larger the group (always with a preponderance of females), the larger the terrarium, the more hiding places it should have.
In a terrarium, 2/3 of its area should be land, and 1/3 – a body of water. On land, provide as many different hiding places as possible. Coconut shells, ceramic caves, or shelters made from stones are great options. Live plants and properly arranged roots also serve well in this role. Since crabs like to climb, plan the terrarium decor to accommodate this behavior. Expanding the space vertically will significantly increase their living area.
The water section can be a small container filled with about 6 cm of water, with sand spread on the bottom. Ensure that crabs can easily exit this container. Another solution is to shape the bottom of the terrarium with expanded clay or sponges, creating an elevated land zone and a submerged water zone. You can even build a waterfall in the terrarium for added complexity.
Maintain the humidity in the terrarium at 80-90%, and keep the temperature around 25°C.
Terrarium substrate for vampire crabs
The substrate in a terrarium for Geosesarma crabs plays a crucial role. Firstly, it provides a medium for plant growth. Secondly, it retains moisture. Start by placing the substrate on a drainage layer made from sponges or expanded clay. Before spreading the substrate, apply a nonwoven crop cover. This drainage layer ensures excess water drains away, preventing waterlogging. Since crabs dig burrows, the substrate layer should be several centimeters deep.
You can use ready-made substrate for tropical terrariums or prepare your own mixture. Combine soil (without fertilizers), clay, Tropical Bioterra coconut fiber, shredded bark or peat mosses. Additionally, consider adding Tropical Cocochips, crushed charcoal, and perlite. Spread a layer of leaves over the prepared substrate to create a moist mulch in the terrarium. This provides hiding spots for crabs, especially young ones, and promotes the growth of live food like isopods or springtails if you choose to introduce them into the terrarium.
Plants for terrarium with crabs
When selecting plants, consider their requirements. Keep in mind that terrariums with crabs are typically high in humidity, and plants are essentially just an addition. Therefore, opt for species that can tolerate these conditions, and ones you won’t mind if the crabs decide to nibble on them. Suitable choices include Taxiphyllum barbieri, Vescularia dubyana, and from higher plants – different varieties of Epiprenum spp, Pilea spp, Neoregalia spp, Phicus pumila, Pteris spp. You can also incorporate plants in the water zone, such as common duckweed (Lemna minor), which the crabs will gladly include in their diet.
What to feed the vampire crabs?
Vampire crabs are omnivorous. Given their wide range of food sources, it’s important to ensure their diet is as varied as possible in a terrarium setting. You can feed them with frozen foods such as glassworms, bloodworms, Mysidacea, live food (springtails (Collembola spp.)), white isopods, crickets, tropical centipedes (Trichorhina tomentosa), mealworms), freeze-dried (Tropical Red Mico, Tropical FD Brine Shrimp) and granulated (Tropical Insect Menu size S, Tropical Crusta Sticks) foods. You can also supplement natural foods with minerals by using Tropical Vigorept Mineral. Ready-made foods are particularly advisable due to their balanced nutritional content. Crabs also readily consume decaying organic matter, algae, and plants, so you can supplement their diet with vegetables and fruits. By observing your pets, you’ll quickly notice their food preferences.
Crab reproduction
The reproductive process of crabs is fascinating. The male must turn the female onto her back and grasp her with his legs to facilitate copulation. Both gonopodia and gonopores are hidden under the abdomen, so the male and female must position themselves appropriately for mating to occur. During copulation, the male introduces spermatophores (packages of sperm) into the female’s genital tract. The female can store these sperm for up to a year, enabling subsequent reproduction without the male’s involvement.
The loving embrace of a male can last quite a while. In some species fertilization occurs only after molting. Even after fertilization, the male may continue to hold the female, protecting her from predators and rival males. The female then lays several dozen eggs, which she carries under her cephalothorax, holding them with her abdominal legs.
In many species of Geosesarma crabs, we observe ametabolous metamorphosis, meaning that large eggs hatch into miniature versions of their parents. These juveniles typically have dark, camouflaged colors, but it’s important to note that cannibalism can occur among the young.
Despite the ametabolous metamorphosis common in Geosesarma crabs, breeding and keeping young alive in terrariums can still be challenging due to the variety of species and their individual needs. Additionally, some species, such as G. maculatum, G. hednon, G. ternatense undergo hemimetabolous metamorphosis, with larvae hatching from small eggs and undergoing further development in the sea.
dr inż. (Ph.D.Eng) Aleksandra Kwaśniak-Płacheta
Literature
https://wsg.washington.edu/crabteam/about/newsletter/2021-2
Manuel-Santos M., Ng P.K.L., Freitag H. (2016) Two new species of Geosesarma De Man, 1892 (Crustacea: Brachyura: Sesarmidae) from Palawan, the Philippines, Raffles Bulletin of Zoology 64: 335–342.
Naruse T. at all (2021) Remarkable bilaterally asymmetrical gonopores and gonopods in a new genus and species of brachyuran crab from the Ryukyu Islands, Japan (Decapoda: Brachyura: Xanthidae), Journal of Crustacean Biology, Volume 41, Issue 2.
Naruse T., Maenosono T., Ng P.K.L. (2021) Remarkable bilaterally asymmetrical gonopores and gonopods in a new genus and species of brachyuran crab from the Ryukyu Islands, Japan (Decapoda: Brachyura: Xanthidae), Journal of Crustacean Biology, Volume 41, Issue 2, June 2021, ruab022,
Ng P.K.L. (2021) Geosesarma sodalis, a new species of vampire crab (Crustacea, Brachyura, Sesarmidae) from a limestone cave in central Sarawak, Malaysia. ZooKeys 1031: 133–141.
Ng P.K.L., Ridge K., Liu H.-C. (2004) Geosesarma hednon, a new species of terrestrial crab (Crustacea: Decapoda: Brachyura: Sesarmidae) from Taiwan and the Philippines, The Raffles Bulletin of Zoology 52(1): 239–249.
Ng P.K.L., Schubart C.D., Lukhaup Ch. (2015) 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), Raffles Bulletin Of Zoology63: 3-13.
Ng P.K.L., Wowor D. (2019) The vampire crabs of Java, with descriptions of five new species from Mount Halimun Salak National Park, West Java, Indonesia (Crustacea: Brachyura: Sesarmidae: Geosesarma), Raffles Bulletin Of Zoology 67: 217–246.
Ng P.K.L., Yeesin P., Promdam R. (2023) A New Species of Vampire Crab (Crustacea: Brachyura: Sesarmidae: Geosesarma) from a Freshwater Swamp Forest in Narathiwat Province, Southern Thailand, with a Note on Geosesarma serenei Ng, 1986, Tropical Natural History 23: 97–104.
