The Marine Aquarium

Marine Invertebrates

- - Crabs - -

Crabs belong to order Decapoda which refer to a group of animals with ten thoracic appendages. Crabs can be classified into two main groups; brachyuran crabs (infraorder Brachyura) and anomuran crabs (infraorder Anomura). Most species of brachyuran or true crabs have four-pairs of well developed walking legs while anomuran or false crabs have only three pairs of well-developed walking legs. However, this is just a general rule rather than a distinct separating character.

Characters useful for identification are as follows:

1) The teeth of the anterolateral margins of the carapace are also known as the epibranchial teeth.

2) The first anterolateral tooth is often called the "external orbit" or "exo-orbital" angle or tooth and is sometimes counted separately from the following anterolateral teeth. The frontal margin or front becomes elongate and/or spiniform in many crabs such as the homolids and the majids, and is then frequently called a rostrum.

3) The maximum carapace width is used as principal measurement indicating the size of a crab, measured as the greatest distance between the lateral margins of the carapace.

4) The buccal cavern (location of the mouthparts), is bordered on both sides by pterygostomial regions and above by the epistome.

5) The calcareous plate inside the buccal cavern is called the endostome.

6) The outer mouth parts or third maxillipeds.

7) Underneath the third maxillipeds, the second maxillipeds and first maxillipeds are located.

8) Below the three pairs of maxillipeds there are the first maxilla (or maxilla) and the second maxilla (or maxillules).

9) The mouth is bordered by a pair of well calcified, jaw-like, and highly modified appendages called the mandibles.

10) There are five pairs of locomotory appendages which are made up of a pair of powerful chelipeds (legs carrying a chela or pincer) and four pairs of walking legs or ambulatory legs. Chela or claw consists of a palm (or manus) and two fingers, one of which is movable (dactylus or movable finger) whereas the other one(pollex) is fixed.

Example of crabs are Spanner crab (frog crab), Cricifix crab (cross-marked swimming crab), Cannonball sponge crab, Coral crab, Flower moon crab (Beach crab), Sentinel crab (Long-eyed swimming crab), Spectacled box crab (four-spotted box crab) and Lesser red rock crab, ....etc.

- - Spanner crab or frog crab - -

English common name: Spanner crab or frog crab
Scientific name: Ranina ranina (Linnaeus, 1758)
Family: Raninidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

This crab species has a longitudinal ovate carapace in which its dorsal surface is strongly granulose or squmose to smooth. Anterior part of the carapace is much broader than the narrow waist-like posterior part. Front or rostrum is triangular and narrow. Third maxillipeds is very narrow and merus is distinctly triangular in shape. Eyestalks are long and longer than the front. At least one pair of legs has the last two or three segments paddle-like. Thoracic sternum is very narrow, especially sternites 5 to 7. Abdomen is clearly visible from dorsal view. Chelae and legs are laterally flattened and spade-like.

- - Cricifix crab or cross-marked swimming crab - -

English common name: Cricifix crab or cross-marked swimming crab
Scientific name: Charybdis feriatus (Linnaeus, 1978)
Family: Portunidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

This crab species has ovate carapace with five distinct teeth on each anterolateral margin. Palm of the cheliped has four spines on the upper surface. The fouth abdominal segment in male is keeled. First anterolateral tooth is truncate or notched. Merus of cheliped has 3 or 4 spines on its anterior order. Palm has more than two spines on its upper surface. It has distinctive pattern of longitudinal stripes of maroon and white. A distinct white cross is presented on the median part of gastric region. Legs and pincers has numerous scattered white spots.

- - Cannonball sponge crab - -

English common name: Cannonball sponge crab
Scientific name: Lauridromia indica (Gray, 1831) or Dramia orientalis Miers, 1880
Family: Dromiidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

It has round carapace which is wider than long and the surface of carapace is convex and with dense pubescence. Dactylus of the fourth leg with distinct spine on its outer margin. There are six teeth on each anterolateral margin and 4 to 8 spines on the inner margin of dactyli of the first two pairs of legs. The last two pairs of legs which are strongly reduced are inserted obliquely on carapace and directed upwards. Subchelae are modified to carry sponges or tunicates on the back of the carapace. All male abdominal segments are distinct, movable. There is a small platelet-like structure usually intercalated between edges of the sixth abdominal segment and telson. Male first gonopod is stout and simple while second one is long and usually subequal or longer than length of male first gonopod. Male and female genital openings are sternal.

- - Coral crab - -

English common name: Coral crab
Scientific name: Trapezia ferruginea Latreille, 1828 or Trapezia lutea Castro, 1997
Family: Trapezidae or Xanthidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

This minute crab (carapace width is about 1.5 cm wide) is found in between branches of cauliflower coral (Pocillopora damocomis) and staghorn coral (Acropora spp.) It can be found in all reefs in the Gulf of Thailand and the Andaman Sea as well. Lateral margin of carapace bears spines and the area. Carapace is convex and smooth and the area between eyes is divided into 4 lobes. Chelipeds are unequal and laterally flattend and the inner margin of the fourth segment of claw is serrated. Carapace, claws and walking legs are red in color.

- - Flower moon crab or Beach crab - -

English common name: Flower moon crab or Beach crab
Scientific name: Matuta planipes Fabricius, 1798
Family: Calappidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

Carapace is rounded with two long well-developed lateral spines. Anterolateral margins of carapace unevenly serrated. Outer surface of palm with strong oblique ridge. Walking legs are adapted for buring in sand and swimming. Dorsal surface of carapace is mosaic to reticulate network of maroon lines on either a white and yellowish brown background while ventral surface is white.

- - Sentinel crab or Long-eyed swimming crab - -

English common name: Sentinel crab or Long-eyed swimming crab
Scientific name: Podophthalmus vigil (Fabricius, 1798)
Family: Portunidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

Carapace is distinctly (about twice) broader than long, its anterior margin is much broader than the posterior one and posterolateral margin converges strongly towards narrow posterior carapace margin. Eyes with very long eye stalk, reaching to or extending beyond edge of the carapace. The last two segments of the last pair of walking leg is paddle-like. Carapace is greenish blue, chelipeds and parts of legs are violet to maroon in adults.

- - Spectacled box crab or Four-spotted box crab - -

English common name: Spectacled box crab or Four-spotted box crab
Scientific name: Calappa philargius (Linnaeus, 1758)
Family: Calappidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

Carapace is oval in shape and the dorsal side is convex Its width is about 10-12 cm wide. Posterior part and posterolateral side of the carapace have large abtuse spines; three on the posterolateral and seven on posterior parts. All waliking legs are short while and chelipeds are cuved and are closed to the body or be underneath the carapace. Body is light brown and three is one reddish brown blotch on the posterior part of eyes. Ther are two reddish brown blotches, which resemble those behind eyes, on the dorsal side of each claw. This crab prefers sandy to slightly muddy substrate at depth from 10 to 100 m deep.

- - Lesser red rock crab - -

English common name: Lesser red rock crab
Scientific name: Leiomera bella (Dana)??? Liomera cf. cinctimana (White, 1847)
Family: Xanthidae
Order: Decapoda
Class : Crustacea
Phylum : Arthropoda

This small xanthid crab with knobs, ridges or granules on anterior and lateral sides of the carapace. The area between eyes is divided into two lobes and the anterior lateral margin of the carapace is divided into four lobes. Walking legs are rather short and claws are subequal and small. Females posses smaller claws than males. Carapace and claws are orange pink and the short walking legs and claws are marked by darkish red bands. This crab inhabits in the rock crevices along the coral reefs on the Andaman Sea coast.

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	Bangsaen Institute of Marine Science, Burapha University 169 Long-Hard Bangsaen Road, Tambon Saensook, Amphur Muang, Chonburi 20131, Thailand. Tel : 66-3839-1671-3 Fax : 66-3839-1674 contact : tanyong@bims.buu.ac.th