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Related FAQS: Brackish Water Crabs,

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/The Conscientious Brackish Water Aquarist

 Fresh to Brackish Water Crabs 

Bob Fenner

Cardisoma sp.

To place the "real" crabs and their relatives in taxonomic perspective let's do a/the usual rundown on their systematics starting from the:

Subphylum Crustacea:   

About 42,000 species of some of the most familiar arthropods; crabs, shrimps, lobsters, crayfish, wood-lice (sow-bugs, rolly-pollies, you know, terrestrial isopods, and aquatic, even parasitic marine ones). Many small members in fresh and marine habitats of importance in aquatic food chains. Primarily aquatic, mostly marine.

Some common characteristics of the crustaceans: 1) Their heads are more or less uniform with five pair of appendages: they have two pair of antennae (this feature is distinctive within the phylum); the third pair as opposing, biting, grinding mandibles. Behind the mandibles there are two pair of accessory feeding appendages, the first and second maxillae. 2) Their bodies trunks are composed of distinct segments covered by a chitinous exoskeleton strengthened by deposition of calcium salts. 3) Crustacean appendages are typically biramous (two major elements). 4) They typically have a carapace covering the trunk of their bodies. Enough of this detail. We'll cover this stuff in more general survey pieces of the mega-groups. On toward the lobsters.

A systematic resume of the Crustacea is necessarily large and complex. Allow me to semi-skirt around a full discussion here. The nine Classes that don't include our family of interest enclose the primitive cephalocaridans (C. Cephalocarida), the Class Branchiopoda (fairy shrimps, tadpole shrimps, water fleas (Daphnia); the Class Ostracoda, Class Copepoda (Anchor worm, Lernaea), Classes Mystacocarida, Branchiura, Tantulocarida, Remipedia, Cirripedia (barnacles), whew! & finally, our:

Class Malacostraca

Comprises almost three-fourths of all described species of crustaceans and most of the larger forms, such as crabs, lobsters, and shrimps. Characteristics: Trunks typically composed of 14 segments plus the telson ("tail"); the first 8 segments form the thorax, the last 6 the abdomen; all segments bear appendages. Four Superorders: Syncarida, Hoplocarida, Peracarida, and the one we want to talk about, the Eucarida.

Superorder Eucarida contains many of the large malacostracans. They have highly developed carapaces displaying fusion of all thoracic segments (the cepahalothorax). Eyes are stalked... Two living orders; the Euphausiacea (krill) and the:

Order Decapoda includes the familiar shrimps, crayfish, lobsters and crabs. This is the largest order of crustaceans with @10,000 species. Decapods are distinguishable from euphausiaceans and other malacostracans in that their first three pair of thoracic appendages, The remaining five pairs are legs (Decapoda= "ten feet"). Decapods are further divided into two Suborders, the Dendrobranciata, with "tree-like" branched gills, body laterally compressed..., eggs planktonic, nauplius as the first larval stage (as in Artemia, our brine shrimp), Infraorders, Sections, Superfamilies... See Barnes re their higher taxonomy.

Of animals that are Crabs ("false" and true) there are about eight thousand described species, with about 600 venturing into or living in freshwater. 

Infraorder Anomura, families of Hermit Crabs, Sand or Mole Crabs. Depressed carapaces, third pair of legs never chelate, fifth pair reduced.... 

Superfamily Paguroidea, Hermit Crabs and more. Have oval carapaces, usually asymmetrical. Live either in shells or with abdomen tucked underneath. First pair of legs as chelipeds. Includes the Hermit Crab genera: Pomatocheles, Petrochirus, Clibanarius, Coenobita (land Hermit Crab), Pagurus, Pylopagurus, Birgus (the Coconut Crab), Stone Crabs like Lithodes, Paralithodes (commercial King Crab of the North Pacific).

Infraorder Brachyura, the true Crabs, marine, freshwater and terrestrial. Have broad carapaces which are fused with their epistomes. First legs as heavy chelipeds, third pair never chelate. Have symmetrical abdomens which are tightly held against the cephalothorax (Not able to "flap" quickly). Further divided into five Sections.

Family Ocypodidae; Ghost Crabs, Fiddler Crabs. Common genera: Ocypode, Uca. Former live in moist sand, emerging at night to feed. Ghost crabs have characteristic thickened, elevated eyestalks. Fiddler Crab males sport an oversize claw that they use for signaling. 

Uca crassipes (Adams & White 1848), the Mangrove Fiddler Crab. Tropical Indo-Pacific. Males with one very large claw. The common "Fiddler" used in the freshwater pet trade. Can be kept with fishes, must have a place to get/dry out.  Need clean, consistent high quality water. Temp. 22-30 C. To a little over an inch in diameter.

Family Gecarnidae; Land Crabs.

Cardisoma sp. (likely C. armatum) Southeast Asia, Indonesia Land Crabs. Soap-Box Crabs for how they're individually shipped (in plastic soap-dishes closed with rubber bands) to prevent cannibalism. To eight inches across... Not a community tank item... if you're lucky, yours will crawl out of the tank and leave. 

Sesarma bidens, Red Crab. Southeast Asia. To two inches across. 22-25 C. 



Red Claw Crabs   1/25/12
Most people on the internet seem to think these crabs are mostly aquatic, I saw that you guys say they are mostly land.
<They will'¦>
The problem I have is that my crabs never come out of the water (I've only had them for four days, so I shouldn't say never).
<Quite so.>
I do have the tank set up as aquatic only with a few bits sticking out of the water.  But still, they seem to only like the water.  So, I guess my main question would be, why do you claim they are mostly land crabs
<Because they are -- in the wild at least. They live in mangrove forests feeding on leaf litter and carrion. Do read up on Perisesarma species, which is what these are.
They're shore crabs, If they don't come on land in your aquarium, then something is amiss. They won't come out if there's something scaring them, or if the air is too cold or too dry. They're also EXTREMELY territorial, and if one crab claims the one small bit of land, the others may not be "allowed" out.>
and is there a resource that can give me a step-by-step on how to set up and care for these crabs?
<Not that difficult. Set up something like you'd create for mudskippers, with a couple inches of warm, brackish water (SG 1.005 at 25 C/77 F is fine). An under tank heater would be ideal. The rest of the tank can be sand, rocks, bogwood. Make sure the air inside the aquarium is warm and damp, but a slight flow of air is important to prevent mould. Keep the hood secure because these animals WILL escape if they can. Use filter floss or plastic mesh to plug any large gaps. Crabs are cannibalistic at times, especially when moulting, so provide lots of caves and such hiding places.>
I am also wondering how you know so much about them, and speak with such authority when there seems to be so much miss-information out there?
<Because I'm clever. More seriously, because I'm a marine biologist by training, an aquarist by inclination, and I've spent some 20-odd years reading scientific as well as hobby literature on brackish water animals. Perisesarma spp are classic brackish water, mangrove crabs. There's really nothing at all mysterious about them. Yet unscrupulous retailers continue to sell them as "freshwater crabs" or "aquatic crabs", and willfully ignorant aquarists vigorously resist any attempt by people like me to tell them what these animals need to do well. It's significant that the people who succeed with them, e.g., actually breed them, are those who understand their ecology and provide brackish water for the adults and marine conditions for the larvae. Cheers, Neale.>

Superfamily Paguroidea, have oval carapaces, usually asymmetrical. Live either in shells or with abdomen tucked underneath. First pair of legs as chelipeds. Includes the Hermit Crab genera: Pomatocheles, Petrochirus, Clibanarius, Coenobita (land Hermit Crab), Pagurus, Pylopagurus, Birgus (the Coconut Crab), Stone Crabs like Lithodes, Paralithodes (commercial King Crab of the North Pacific).

Coenobita perlatus, one of several species of Hermits sold as Land Hermit Crabs.  

Bibiography/Further Reading:


Fiddler Crabs:

Mancini, Alessandro. 1992. Fiddling with Fiddler Crabs. TFH 10/92.

Wickstein, Mary K. 1977. Fun with Fiddlers. TFH 12/77. 

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