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The Taxonomy of 10 Crab Species Found in Coaco Beach

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10 different species of crabs were collected around the beaches of Davao Region. Some species were similar to the others, some have the same habitat and some have characteristics unique only to them. All these species belong to the Order Malacostraca, Infraorder Brachyura which means true crabs. The specimens collected under the Infraorder Brachyura were from the genus Uca, Thalamita, Sesarma, Xantho and Ocypode with the following species name: Uca vocans, Uca herradurensis, Thalamita coeruleipes, Sesarma cinereum, Ocypode quadrata, Ocypode ceratophthalmus, Ocypode pallidula and Xantho pilipes. Keywords: crab, infraorder, Brachyura, species, order, Malacostraca, genus, Uca, Thalamita, Sesarma, Ocypode

Crabs are decapod crustaceans from the infraorder Brachyura which means very short projecting “tail”. They often reach in large sizes, the largest of which is the spider crab, with long, thin legs spanning up to 3.6 m (up to 12 ft), possessing complicated nervous systems. (Microsoft Encarta Encyclopedia, 2009). Crabs are bilaterally symmetrical (identical halves on each side of an axis) except that many species have one cheliped

larger than the other. Crabs have 5 pairs of legs —one pair of chelipeds (pincers or claws), three pairs of walking legs, and one pair of swimming legs. The claws are used for feeding, excavating burrows, defense (or aggressive behavior), and signaling (a sort of crab language fending off competing crabs for territory, keeping predators at bay, and most importantly, attracting the opposite sex). The crab’s body is protected by a rigid exoskeleton. This is a tough chitinous “skin” that completely covers all parts of the body. As the crab grows, the exoskeleton is periodically shed in a process called molting (ecdysis). The resulting molt looks like a translucent creature without a body. In a few hours, the molted crab absorbs enough water to swell its body by about ten to twenty percent and the exoskeleton hardens. The crab body then grows to fill the new exoskeleton.

Much of the body is protected by the carapace, the covering of the head/thorax, and the crab can pull the legs under the carapace presenting a hard rock-like creature to a predator. Under the front of the carapace two eyes on stalks, two antennae, and a mouth are located. The mouth has several movable parts, and the chelipeds, especially the smaller one, can move food into the mouth at a surprising rate. Most crabs are omnivores (plant- and meat-eaters), some are carnivores (meat-eaters), and a few are herbivores (plant-eaters). Two gill structures are also located laterally in the body cavity under the carapace. As long as these gills can be kept wet, crabs can live out of the water; however, the gills can only process the oxygen as long as they are wet. The abdomen of crabs is curved under the body with its major duty as protection of the reproductive organs. After an adult female molt, the soft shell condition allows her to become impregnated by a male. The male does not nurture the eggs, and the male abdomen is narrowed accordingly. The soft-shelled female and the hard-shelled male sometimes remain together for protection until her carapace begins to harden.

Several days later, the eggs are extruded to be stored until hatching under the widened abdomen. Depending on the species, ovigerous (egg-bearing) females carry a dozen to several hundred eggs. The eggs are kept in constant motion for oxygenation by the swimmerets until they hatch into the surrounding water. The larvae, called a zoea and megalops as they molt, develop as part of the planktonic community. When the larval crab reaches a certain point, it drops to the bottom and starts its life a bottom dweller. They are highly adapted to resist changes in the external environment and they even grow well in rather hostile or unfavorable habitats (Rouse, 2007). The main objective of the study is to be able to describe, identify and give the correct taxonomic name and category for the 10 crab species. Aside from that, this study also aims to distinguish the different species of crabs from the specimens collected. The scope of the study is limited to the description, identification, naming and classification of the organism up to the species level or rank.

All the crab species were analyzed, most by study of actual specimen and some were based on descriptions and figures in current literature. The specimens were collected in the beaches of Coaco, Matina Aplaya, San Juan Maco and Samal Island. Camera was used to provide actual pictures of the specimens. Measurements are in mm. The third maxilliped is abbreviated to MXP3 and the walking legs are indicated as WL1-WL4. The diagnoses and key were based on adult males only.

Ocypode ceratophthalmus
Diagnosis – Horn-eyed ghost crabs can be easily distinguished from other crabs by the horn-stalks on the eyes. Females have smaller shorter horns while the juveniles may not have any. The carapace is box-shaped with deep indentations to allow eyes to be kept. A distinct dark cervical groove (H-shape marking on the back) can also be found on the top of the carapace (Queensland Museum, 2011). Description – Ocypode ceratophthalmus often measures 6-8 cm in carapace width. The eyestalks are extended beyond cornea (stylophthalmous). Exorbital corner are laterally directed in large specimens. In the chela or cheliped, stridulating ridge on inner surface of palm of major chela composed of about 13-16 distant striae in upper half and 20-30 close set ones in lower half. The small chela has both fingers pointed. In pereiopods, the upper half of propodi of second and third pereiopods has velvet. In males, the first pleopod is slender, its distal part is clearly bent laterally, the palp has a long and slender tip, its distance to pleopod tip is slightly more than the palp length. In females, the opercle of female genital duct has depressed operculum; the vulva is opening oblique with respect to median line of sternum and the lateral rims are not elevated (Ng et.al, 2008).

Figure 1. Ocypode ceratophthalmus
A) Anterior View B) External Structure
Actual Specimen C) Carapace D) Anterior

Habitat – Ocypode ceratophthalmus prefer sandy beaches and are commonly found within the intertidal zone, which are covered with their burrows. Crabs tend to select areas with access to water and sand consistency should be suitable for digging.

Distribution – Horn-eyed ghost crabs are found throughout warmer climates i.e. the tropics and are native to regions ranging from East Africa to Australia.

Thalamita coeruleipes
Description – The carapace has 3 pairs of gastric ridges, the epibranchial ridge is interrupted by cervical groove and medially, median cardiac and mesobranchial ridges are absent; the front has 6 rounded lobes; 5 antero-lateral teeth, fourth and fifth shortest, subequal. The basal antennal joint is centrally located with 2-3 spines and a short irregular clump of 4-5 granules. The upper surface of cheliped palm has 4 remarkably sharp spines; inner face is smooth, mesially with a rounded longitudinal carina. The posterior border of propodus of swimming leg has 4 large and 3 small spines. Male’s first pleopod is tapering quite strongly to tip; inner surface has 2 indistinct rows of moderately sized bristles; outer surface has about 15 bristles and 3 large, blunt, rounded tubercles immediately behind tip. (Türkay 1995)

Figure 2. Thalamita coeruleipes

Figure 3. Thalamita coeruleipes
Actual Specimen
Habitat – This species is found on rocky shores and reef flats of broken down corals within the intertidal zones. Distribution – Madagascar , Japan, China – Paracel Islands, Palau , Indonesia – Mamuju, Paleleh, Misool Group, Kepulauan Tanimbar, Kera near Timor, Kupang, Postillon Island, Spermonde
Archipelago near Makassar, Kepulauan Sula, Obi, Morotai, Ambon, Boo Island and Biak Island, Australia – Low Isles, Murray I., Palm I., Port Denison, Hope I., Fairfax I., North West I. and Heron I, Caroline Islands; Kiribati; Fiji; Hawaiian Islands – Oahu, Iles Gambier and intertidal (Sakai, 1976).

Ocypode quadrata
Description – The species is a small straw-colored or grayish-white crab, measuring about 5 cm (2 inches) across the back at maturity (Phillips, 1940). They have a quadrate carapace, large club-shaped eyestalks, unequal chelipeds (claws) and long walking legs. Males are generally larger than females. These are small crabs with carapace having margins that are finely beaded but toothless; the claws are white. The space between the eyes is much shorter than the eyestalks. Their stalked compound eyes can swivel to give them 360° vision. Young crabs are cryptically coloured to blend in with their sandy habitat.

Figure 4. Ocypode quadrata (cryptically coloured to blend with the black to grayish sand of Times Beach, Matina Aplaya) Habitat – The burrows dug by ghost crabs may be up to 1.3 m deep (4 feet). Their habits of periodically closing the burrow opening with sand during the hottest part of the day and of remaining within the burrow through the colder months provide sufficient protection from the climatic extremes that fully aquatic species rarely encounter. These burrows, which take different shapes beneath the sand, are found from near the high tide line to a distance as great as 400 m (0.25 mile) from the ocean. A distributional gradient based on crab age is typical for this species, with younger crabs generally burrowing closer to the shore than older individuals (Williams 1984). Distribution – Ocypode quadrata extends from Block Island, Rhode Island to Santa Catarina, Brazil, including the Gulf of Mexico, the Caribbean and Fernando de Noronha, 200 miles off the Brazilian mainland. It is also found in Bermuda, and the last larval stage has been observed as far north as Woods Hole, Massachusetts; however, adults do not reside at that latitude.

Sesarma cinereum
Description – The squareback marsh crab, Sesarma cinereum, is one a several semi-terrestrial crab species occupying the intertidal habitats. The carapace is brown to olive and the top margin of the upper finger on the claw bears small tubercles or bumps. The last segment of the 4th walking leg is equipped with black spines on the dorsal and ventral surface. A characteristic tooth or spine behind the eye socket, which is found in several similar species, is absent in S. cinereum (Gosner 1978). The squareback marsh crab is sexually dimorphic, with male claws growing relatively larger than those of females (Buck et al. 2003). Habitat – Individuals are commonly seen above the high water line among rocks, on dock pilings, under debris on beaches and in high elevations of salt marshes and mangrove forests. However, this is a highly motile species that has been found up to 100 m inland of the nearest marsh. The squareback marsh crab seems to prefer sandier sediments than similar species, and is a common stowaway on ships. Distribution – The range of S. cinereum extends from Maryland to Florida, Gulf of Mexico, and the Caribbean (Gosner 1978).

Uca vocans
Description – Carapace has orbits moderately oblique; front is narrow, it is narrowest between eyestalk bases, the frontal groove is moderately wide, with sides diverging rapidly posteriorly; antero-lateral margins are short, sometimes absent, converging, exorbital tooth little produced and acute; suborbital crenellations are strong, distinct, there are no tubercles on the floor of the orbit. The palm of the major cheliped has oblique ridge I.e. high, thin and sharp, usually with close set tubercles; the outer face near base of immovable finger has an extremely large low triangular depression, reaching from the middle of the palm to the base of dactyl and ending in a longitudinal furrow on immovable finger; the dactylus are without longitudinal furrows; the gape of female chelae are without enlarged teeth. There is a horny terminal endpiece of the male first pleopod with anterior flange larger than posterior, the inner process is well separated from posterior flange; the palp is of moderate size and subdistal (Türkay 1995). Habitat – Burrows in beaches with mangrove vegetation.

Distribution – Madaum, Mindanao, Philippines, Red Sea, Zanzibar, Tanzania – Dar es Salaam, Lindi, Mozambique – Delagoa Bay, Inhaca Island, South Africa – Durban, mouth of Umgazana River near Port St. Johns, Madagascar – Nosy Faly and Nosy Bé, Nosy Bé, Tuléar, Nosy Bé, Nosy Faly and Tuléar; Mauritius, Rodriguez Island, India – Malabar, Coromandel and Malabar coast, Gulf of Mannar, Bombay; Philippines – Manila, Luzon, Davao Gulf, Zamboanga, Davao Gulf, Sasa, Madaum, Palawan, and Lamao, Luzon, Davao; Sulu Archipelago – Tawitawi and Jolo (Crane, 1975); Borneo – Labuan, Sabah; Indonesia – Java.

Figure 6. Uca vocans Actual Specimen

Ocypode pallidula
Description – A stridulating ridge on palm of major chela composed of 25 to 40 fine, evenly spaced striae. The major and minor chelae are very broad and short, the fingers are pointed, chela and carpus finely and evenly granulated on outer surface, large granules or spikes are absent. The lower orbital margin has lateral notch absent, median notch is weak to absent. The ocular stylet is absent. Anterolateral angles are acute and produced forwards. The anterior upper border of propodus of second and third pereiopod in male has double hairbrushes, in female there is a single hairbrush on second and third pereiopods. The inner dorsal margin of merus of major cheliped has toothed flange and five to six main teeth. Inner margin of carpus of major cheliped has two central teeth, the outer distal margin of carpus is denticulate. Terminal segment of male abdomen is small, equilaterally triangular. Size is moderate, carapace length (c.l.) to 23.1 mm, carapace width (c.w.) to 26.8 mm. Remarks – Compared to Ocypode ceratophthalmus, Ocypode pallidula has tubercles and striae during the juvenile stage while the Ocypode ceratophthalmus only has striae. Distribution — Madagascar; Mauritius; S.E. Sulawesi (Celebes); Wetar (near Timor); New Guinea; eastern Australia; Coral Sea; Norfolk I., Cook Is., Tuamotu Arch. , Hawaiian Is.

Xantho pilipes
Description – The carapace has dorsal surface that is flat and smooth, sometimes sparsely punctate, frontal region slightly produced, sub-median lobes very broad. Antero-lateral margins of carapace with five obtuse to subacute teeth sometimes with margins tuberculate, fourth prominent, at least fourth and fifth setose ventrally. Chelipeds slightly unequal, stout and sparsely punctate, often with short transverse grooves bordered by flat tubercles that are sometimes scale-like; second to fifth pereiopods stout and somewhat compressed, margins of merus-propodus densely setose, setae on fourth and fifth pereiopods long. Male inner proximal margin of first pleopod expanded and with a small lobe. Carapace length is up to 15 (21) mm.

Habitat – Depth range from lower shore to 110 metres.
Distribution – Shetland, Thames, Channel Isles, Plymouth, Scilly Isles, Bristol Channel, Anglesey, Isle of Man, Dublin, Belfast, Clyde & Argyll, Minch, Mayo, Galway Bay, Fastnet, Cork, Nymphe Bank.

The 10 crab species identified were the following with their corresponding taxonomic rank. Uca vocans, Uca herradurensis, Thalamita coeruleipes, Sesarma cinereum, Ocypode quadrata, Ocypode ceratophthalmus and Ocypode pallidula. The Uca species are commonly called fiddler crabs because of theit enlarged cheliped and the other one is small, which makes it looks like a fiddle. They are commonly seen burrowing in the sand. The Uca vocans is also called the orange fiddle crab due to its orange cheliped while the Uca herradurensis is characterized by its spiky cheliped.


Sakai, T., 1976a. Crabs of Japan and the
Adjacent Seas. (In 3 volumes: (1) English text: i-xxix, 1-773, figs 1-379, (2) Plates volume: 1-16, pls 1-251, (3) Japanese text: 1-461, figs 1-2, 3 maps.) Kodansha Ltd, Tokyo.
Ng, Peter K. L. and Daniele Guinot and Peter
J. F. Davie, 2008. Systema Brachyurorum: Part 1. An annotated checklist of extant Brachyuran crabs of the world. The Raffles Bulletin of Zoology. Supplement No. 17, 31 Jan 2008. 286 pp. Williams, A.B. 1984. Shrimps, lobsters, and

crabs of the Atlantic coast of the eastern United States, Maine to Florida. Smithsonian Institution Press, Washington, DC. 550 pp.
Buck, TL, Breed, GA, Pennings, SC, Chase,
ME, Zimmer, M & TH Carefoot. 2003. Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity. J. Exp. Mar. Biol. Ecol. 292: 103-116.
Gosner, KL. 1978. A field guide to the Atlantic
seashore: Invertebrates and seaweeds of the Atlantic coast from the Bay of Fundy to Cape Hatteras. Houghton Mifflin Co. Boston, MA. USA. 329 pp.

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