Phylum — Arthropoda.
Subphylum — Crustacea.
Class — Malacostraca.
Order — Decapoda.
Superfamily — Ocypodoidea
Family — Ocypodidae.
Genus — Uca.
— Uca rapax (S.I. Smith, 1870] — Fiddler crab. (Eng. Fiddler crab). A widely distributed genus across all tropical coasts. They inhabit sandy littoral zones. Size: body length without limbs is 1.5-2.5 cm, with claws 5-10 cm. The body of the fiddler crab varies in color, predominantly dark gray, dark red, and black. In different locations, fiddler crabs have completely different colorations, ranging from nondescript dirty gray to rich orange and bright orange shades, and blue specimens with or without an electric hue have also frequently been observed.
The large claw and limbs feature red-yellow-white and other smoothly transitioning colors. During the breeding season, the brightness of colors in males and females increases significantly, and the large claw of the males becomes particularly noticeable — it acquires a bright orange coloration on its lower half and a bluish or pinkish top. Female fiddler crabs can see such a semaphore from a great distance.
Crabs of this genus are well known for the fact that males have a disproportionately large claw, which they wave while sitting in front of the entrance to their burrow. With its huge hard claw, the fiddler crab “locks” its burrow from the inside so that no predator can get in. Males demonstrate their combat qualities and superiority with its help, and in addition, the huge claw serves to attract the opposite sex. Luring a female, the crab performs special circular motions with its claw, demonstrating that it wouldn’t mind making new acquaintances.
With the smaller claw, it picks up food from the ground and brings it to the mouth, movements reminiscent of moving a violin bow (the large claw). Hence they are called fiddler crabs. A claw that has fallen off for any reason will regenerate. In some cases, the new claw of a fiddler crab grows larger than the previous one. Only male specimens possess the huge claw. In females, both the right and left claws have the same structure.
Very often, fiddler crabs leave their homes and head deep into humid forests, disappearing for a long time near freshwater bodies; this feature allows them to be successfully kept in freshwater aqua-terrariums, but not in aquariums, since the fiddler crab is a terrestrial inhabitant that prefers water only from time to time. They can produce sounds similar to the chirping of a grasshopper. The substrate is best made of coral sand. They dig burrows in the sand 30 cm deep. The water must be brackish (one tablespoon per 10 liters of water).
The aqua-terrarium should be set up with a sandy bank and a water level of no more than 3 cm, otherwise the crabs may drown. They feed mainly on small crustaceans, algae, aquatic plants, and detritus. The water temperature should not exceed 25°C, optimal is 24-25°C, air temperature from 25-29°C. When the temperature drops to 20 degrees, it goes completely into the water and stays there until the heating turns on. Fiddler crabs are dioecious animals. The female crab attaches eggs to her abdominal appendages and carries them there until hatching. They live 2-3 years.
— Uca forcipata [Adams & White, 1849]. Synonym: Uca manii, Gelasimus forcipatus. (Eng. Forceps Fiddler Crab). Southern China, Thailand, Malaysia, and Indonesia, coastal ecosystems, near river estuaries, zones subject to constant tides. Typical oval crab shape. Prominent, not overly enlarged claws. In the male, one of the claws is highly developed and serrated. Eyes protruding on elongated stalks. They reach 4 cm in diameter.
The most wonderful color in this species is blue or purple (claws, legs, carapace), although white and black parts (dorsal posterior area) and a whitish belly are also present. Claws are usually blue with patches of orange or yellowish tones. The upper zone of the carapace is usually dark with yellowish, orange, or white spots. Females are smaller, with symmetrical claws of the same caliber. Her colors are less vibrant than those of the males.
Keep a pair of different sexes in aquariums of about 50 liters. Two males will engage in combat and territorial conflicts. In the aquatic zone, install roots or thin trunks of mangrove trees, as well as a substrate of fine gravel or sand. They tolerate temperatures up to 28°C, but the optimal is 24-25°C. In addition to all this, we must provide them with shelters through rocks, logs, and tangled branches that form hollows.
It is necessary to maintain excellent water quality to house this crab in our aquariums, as it does not tolerate excess pollutants (nitrites, ammonium, heavy metals). On the other hand, it tolerates a wide range of salinity, from freshwater to saltwater. Hardness and pH should be elevated (it does not tolerate acidic or soft waters and requires calcium and iodine), pH 7-8 and dH > 10°.
They are innate escape artists. They will hide between logs or rocks if they feel any threat. They have a fairly peaceful nature, excluding territorial males. They usually do not attack fish or other invertebrates, but attacks can occur under inadequate nutrition. It may occasionally attack small fish or invertebrates, but this is infrequent.
Another habit is digging up plants from the substrate (though they do not eat them). They spend quite a bit of time on the surface if they feel calm and form communities consisting of several females and a male. This is an omnivorous species that will accept any type of food. Reproduction in an aquarium is practically impossible.
Genus — Ocypode.
— Ocypode pallidula [Jacquinot, 1852] — Pallid ghost crab. (Pallid Ghost Crab). Widely distributed across all tropical coasts of the world’s oceans. Usually found on sandy beaches. It digs deep burrows and can run very fast. Easily recognized by the long “eyelashes” on its eyes. The tropical terrestrial ghost crab has the color of sand, making it very difficult to spot on the shore.
However, in the bright sun, its body casts a dark shadow on the light sand, giving away the crab’s presence. That is why it received such a mystical name. Practically unsuitable for captivity, as it requires a large area for normal functioning.
Potamon potamios, Potamonautes lirrangensis, Freshwater crab.
Sesarma bidens, Pachigrapsus marmoratus, Amarinus lacustris, Limnopilos naiyanetri. 
