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Matuta victor
Common moon crab/ Speckled sand crab


Zhiyuan Lin 2018

Summary

Matuta victor, also known as speckled sand crab or yellow moon crab, is widely distributed around Indo-Pacific coast, which is a member of the family Matutidae. It has round body with a pair of strong side spines and flattened walking legs used as paddles for swimming and digging. Yellowish olive body colour with speckled dark red spots provides great cheating colouration to it on the beach environment. Its habitats range from sandy beaches to seagrass beds, also from intertidal zone to about 15 m depth.  It is usually nocturnal and spends the day buried in the substrate just below the surface creating breathing channels to the surface of the sand. As omnivorous crabs, they eat seaweed or carcasses of animals, also they will hunt small shellfish, worms and other animals during the night (Innocenti et al., 2017). They play an important role in the ecology of sandy areas, also they are commercially important and used as food in many tropical and subtropical countries.Due to similar physical appearance and ecological behaviour, it is always easy to mix up M. victor  with another crab species Ashtoret lunaris.

Physical Description

​Matuta victor's subcircular carapace is beige or yellow in its background colour,  and the carapace surface of M.victor is covered by small and minutely scattered red dots. Its carapace is bearing six mid-dorsal obtuse tubercles that are faintly developed (Galil & Mendelson, 2013). Front with slightly rounded lobes laterally and an emarginate rostrum medially; antero-lateral margins are arcuate with six tubercles behind exorbital angle, followed by three crenulate teeth. Its lateral spine is massive and pointed, about 1/4 time carapace width (Figure 1). Orbits communicating with antennular fossa, and suborbital margin laterally interrupted by curved inhalant canal. 

Two chelipeds are massive and always under the front body (Figure 2). Cheliped palm with upper margin are cut into three teeth, two rows of granulate low tubercles and proximal tooth in lower row is largest. Mid-palmar ridge in male rounded and extending to tip of fixed finger, proximally with a granulate tubercle followed by a prominent, acuminate spine. Same region in female with five tubercles, the second being spine-like, and lower proximal angle is with a prominent acute spine. All 4 pairs of walking legs (pereiopods) have changed to fin-shaped swimming legs and shapes are slightly different in each pair (Figure 3,4). The small red spots are also sparsely covered on the surface of the upper and the joint part of the leg. Pereiopods are yolk-yellow with white margins, and the propodi of first pereiopods are distally with prominent purple mark (Figure 4). The paddle fins mainly contribute to swimming and digging for M.victor (Figure 3).

They can grow to 50-70mm in width as males and 40-60mm for females. Although the individuals of both sexes of M. victor appeared quiet similar, several morphological differences are apparent upon detailed examination. The female abdomen is wider than that of the male, with the width becoming more pronounced with increasing size. Also, the first chelae spine on the propodus becomes more prominent with increasing size in females whereas it becomes reduced in males. A ribbing on the dorsal aspect of the dactylus is present in large males but absent in smaller males and all females (Perez, 1985).


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Figure 1
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Figure 2
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Figure 3
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Figure 4

Ecology

Matuta victor is widely distributed in tropical and subtropical waters of the Indo-Pacific Ocean and it is also widely seen in some coasts of temperate areas where the warm current is strong. Kaschner et al (2008) found that  the optimal water temperature for M. victor  is between 22.8°C and 29.3 °C. This crab inhabits from the intertidal zone of the clean, shallow sandy beach to about 15 m in depth (Naderloo, 2017), because beach environments provide it great food sources and refuges for keeping safe from predators. Their body colour look similar with sand colour, which camouflage plays an important role in their life.

They are active omnivorous crabs, and they spend more time hunting at night. As carnivorous and facultative scavengers, their diet mainly composed of crustaceans, mollusks and dead fish. Smaller individuals prefer feeding on smaller, softer-shelled species, when large size M. victor prey on shelled sessile or slow-moving animals such as gastropods, bivalves and anomurans (Perez & Bellwood,1988). To feed, the crabs burrow backward into the sand and face seaward, with only their eyes and first antennae showing. The main predators of M. victor are seabirds and some species of fish. When M. victor feel threatened, they will immediately use paddle fins to dig a hole and then bury themselves in the sand. Or they will get pereiopods and chelipeds under the carapace and appear to apparent death, which may make it difficult for a predator to dismember them. 

As sand-burrowing crabs, M.victor
will form a "exostegal channel" for respiration when they are buried with the sand (Garstang, 1987). They get good ability to bury quickly, which is important in maintaining position in an unstable environments such as that on the surf zone. In addition, rapid burying also prevents desiccation and may provide a means of concealment for animals stranded by a receding tide (Bellwood, 2002).

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Figure 5

Life History and Behaviour

Reproduction

Like other crustaceans, the common moon crabs have a lifecycle defined by growth, they need to increase size by moulting in different life stage. Due to invaluable research reasons, the research and information of M. victor's larval life stage are still lacking. However, some research of their post-larval stage have been done, and it is relatively distinctive in that they spend all their post-larval life in the surf zone (Perez & Bellwood,1989). Perez (1985) found that there seem to be five intermoult stages or instars before  M.victor turn to sexual maturity. These instars are characterised by relatively large moult increment, and the increment between 3rd and 4th instars are higher. In females, the puberty moult is also the terminal moult, and they cannot further moult when in a state of terminal anecdysis. Therefore, female M. victor have a fixed number of instars and sexual maturity, which will be occurred after the terminal moult. However, male M. victor absolute growth likely happens in a variable number of instars with a wide size range, which is characterised by constant by small moult increments.

M. victor
have a year-round production of gametes, but gonad development between individuals is asynchronous. Their mating activity is aseasonal, which means they mate and reproduce all year round. In addition, they have high numbers of eggs per brood. However, Kobayashi (2013) found that the embryonic development of moon crab indicated that eggs were spawned mostly between May and August, then they hatched between July and September in Japan. And females oviposit only once in one reproductive season, and gonads will regress in both sexes after mating. 

There are 3 stages of M. victor reproductive behaviour: precopulatory attendance, copulation and postcopulatory attendance (Perez & Bellwood, 1989). Firstly, in precopulatory attendance, a postpuberty male is going to hold the chela of a prepuberty female. Copulation always happens between a hard-shelled male and a soft, recently-moulted female. After several days of precopulatory grasping, the male swapped its position and hold the female by its chelae from the rear. Then the male released the female but still encircle her with his chelae. After female completed her moult, male will get to the up of female. In this position, the abdomen of the female is extended and overlapped that of male. The male's pleopods are inserted into the female's genital openings (Perez & Bellwood, 1989). After copulation, the female assumed a normal upright position. Lastly, postcopulatory attendance in this position will be maintained for at least 3 hours.

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Figure 6

Feeding behaviour

Matuta victor are voracious and facultative scavengers, their diet mainly composed of crustaceans, mollusks and dead fish. Innocenti et al (2017) found that M. victor are easily and quickly attracted by dead fish on the sandy substrate, and a number of crabs will cluster tightly around their food and compete directly for getting the food source. Larger adults can grab the food firstly, when smaller individuals have to circle the scrum and occasionally join in. They use claws to grab a piece of meat from the dead fish and send it to the mouth. Agonistic behaviour will happen by competing with conspecifics to feed more, each individuals use legs and claws to strike others or try to grip onto the back of other individuals (Innocenti et al., 2017).

M. victor are also carnivorous, their main prey are benthic slow-moving organisms. Young individuals prefer preying on small crustacean such as copepods and sergestidaes, while large ones prey more on anoumurans and gastropods. Mollusca also have high frequency of occurrence in moon crab's natural diet, and the most common taxa is bivalves due to their high distribution around the beach areas. When M. victor find its prey bivalves, it will use its strong pairs of claws to crack the shells of bivalves open and then eat the meat.

Burrowing behaviour

As a member of family Matutidae, M. victor is known for its ability to burrow and bury itself quite quickly into sandy substrata. Matuta use their attened pereiopods for digging to enter the sediment backwards (Bellwood, 2002). So M. victor referred as "back-burrowers", the process of entering the substratum backwards often involves the use of the last pair of pereiopods, especially the last pair - paddle fins, to loosen the substratum and to propel the individual backwards into the substratum. Also, Bellwood (2002) research showed that the Matuta can bury to twice its body depth and can remain completely buried for several hours.

When M. victor find it is in a dangerous position, it will immediately start burrowing and digging backwards to hide in the sandy environments to escape predation. Besides, camouflage of their body colour can easily confuse predators, which is also a kind of anti-predator behaviour. Usually, only the tips or margins of the carapace or the eyestalks can be visible above the sediment.

 The video of Matuta victor burrowing behaviour, filmed in University of Queensland.

Anatomy and Physiology

Sensory system

M. victor have two big compound eyes which are extendable and well raised above the carapace. As one of the most important sensory organs in decapod crustaceans, the eyes help crab observe surroundings when they are buried in the sand. In addition, crabs antennules that grow next to the eyes are used for sensing in crabs, which are referred to as "feelers". Crabs will flick antennules with arrays of olfactory sensilla called aesthetacs through the water to sense odours, which causes water to be flushed within and the held in between aesthetacs to transport odour molecules to olfactory receptors (Waldrop, 2013).

Walking legs and claws of crabs also play an important role in sensing, which help them catch the prey easily and escape from predation (Davie et al., 2015)


 

 The video of Matuta victor using antennules, filmed in University of Queensland.

Respiratory system

M. victor have internal gills that are composed of gill filaments, which help them get enough oxygen in the aquatic environment. An appendage called scaphognathite is a pumping organ to draw the water through the gill cavity, which is located near the base of claws. When they are buried in the sand, they will form a "exostegal channel" for respiration.

The exposure to atmospheric air is the main factor that affecting M. victor oxygen consumption and activity. Santos et al(1987) found that the main two factors to the reduction of oxygen consumption in intertidal crabs are: (1) the gill lamellae would tend to adhere together with a resultant decrease in surface area for respiratory exchange; (2) a decrease of the oxygen tension in the circulating water of the branchial chamber would induce a reduction in oxygen consumption. To breathe in the terrestrial environments, moon crab have to keep their gills moisture.


Biogeographic Distribution

Matuta victor in Australia can be found on the east (QLD, NSW), west (WA) and north coast (NT). This species is widely distributed along the coast of Indian Ocean and the Pacific Ocean from the Red Sea and southeast Asia to New Hebrides and north as far as Japan (Chapgar, 1947). Apart from its natural habitats, M. victor is now introduced to Mediterranean Sea.


The first specimen of M. victor recorded in the Mediterranean was a single specimen which was collected in Haifa Bay, Israel in 2012. Then the population of the Indo-Pacific moon crab, M. victor has increased significantly along the Israeli Littoral and spread to Lebanon and the Mediterranean coast of Turkey (Innocenti et al., 2017).The most likely route of colonisation was from the Red Sea through the Suez Canal and then along the northward current that runs along the Israel-Lebanon-Syria coast towards Haifa Bay. Now large population of M. victor are recorded in Mediterranean Sea, these introduced species may influence native benthic communities (Galil & Mendelson, 2013).

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Figure 7

Evolution and Systematics

Matuta victor is from the crab family Matutidae that contains a distinctive group of predatory crabs with good abilities of digging and swimming. Family Matutidae is known as moon crabs, and there are 15 species in 4 genus from this family (Ng et al., 2008). Members of Matutidae have a distinctive flattened dactyl on each walking leg, which are used for swimming and digging into soft sediments. They differ from the swimming crabs of the family Portunidae in that all five pairs of legs are flattened, rather than just the last pair, as in Portunidae (Tan et al., 2016).

Classification
Kingdom          Animalia
Phylum            Arthropoda
Subphylum      Crustacea
Class                Malacostraca
Subclass          Eumalacostraca
Order               Decaopoda
Suborder         Pleocyemata
Family             Matutidae
Genus              Matuta
Species           Matuta victor

M. victor and Ashtoret lunaris are close relatives, and they are very easy to confuse taxonomically. The way to distinguish them is that while A. lunaris has both the prominent tubercle porteriorly on antero-lateral margin, there is a transversely serrate carina on cheliped dactylus of M. victor.

M. victor has been recorded with 4 other different synonymised names before, but now Matuta victor is the most commonly used name of this crab species. 

Synonymised names:
1. Cancer victor (Fabricius,1781)
2. Matuta lesueurii (Leach, 1817)
3. Matuta peronii (Leach, 1817)
4. Matuta victrix crebripunctata (Miers, 1877)

Conservation and Threats

This species is not protected under appendices I, II and III of CITES 2013. Also, this species is not listed on IUCN Red List till 15/5/2018. There are currently no efforts are contributed to the conservation of this species.

However, there are still some potential threats to this crab species. The main threats to Matuta victor could be increasing coastal construction that lead to more pollution to crab's habitats. Due to the increase of sea level, M. victor's ecological behaviour could be influenced. 

As an invasive species in
Mediterranean Sea, M. victor could be the threats to local environments. With the increasing population and disturbance of M. victor in Mediterranean Sea, M. victor compete more food than native species which may impact the native surf zone community (Innocenti et al., 2017).


References

Bellwood, O. (2002). The occurrence, mechanics and significance of burying behaviour in crabs (Crustacea: Brachyura). Journal of Natural History36(10), 1223-1238.

Chhapgar, B. F. (1947). On the marine crabs (Decapoda: Brachyura) of Bombay State. J. Bombay Natural. History. Society.54, 399-439.

Davie, P. J., Guinot, D. A. N. I. È. L. E., & Ng, P. K. (2015). Anatomy and functional morphology of Brachyura. Treatise on Zoology–Anatomy, Taxonomy, Biology. The Crustacea9, 11-163.

Galil, B. S., & Mendelson, M. (2013). A record of the moon crab Matuta victor (Fabricius, 1781)(Crustacea; Decapoda; Matutidae) from the Mediterranean coast of Israel. BioInvasions Records2(1), 69-71.

Garstang, W. (1897). Contributions to Marine Bionomics: II. The Function of Antero-lateral Denticulations of the Carapace in Sand-burrowing Crabs. Journal of the Marine Biological Association of the United Kingdom4(4), 396-401.

Innocenti, G., Stasolla, G., Mendelson, M., & Galil, B. S. (2017). Aggressive, omnivorous, invasive: the Erythraean moon crab Matuta victor (Fabricius, 1781)(Crustacea: Decapoda: Matutidae) in the eastern Mediterranean sea. Journal of Natural History51(35-36), 2133-2142.

Kaschner, K., Ready, J. S., Agbayani, E., Rius, J., Kesner-Reyes, K., Eastwood, P. D., ... & Watson, R. (2008). AquaMaps: Predicted range maps for aquatic species. World wide web electronic publication, www. aquamaps. org, Version8, 2010.

KOBAYASHI, S. (2013). Reproductive ecology of the Victorious moon crab Matuta victor in a sandy beach of the Genkai-nada Sea. Japanese Journal of Benthology67(2), 56-63.

Naderloo, R. (2017). Atlas of crabs of the Persian Gulf. Springer.

Ng, P. K., Guinot, D., & Davie, P. J. (2008). Systema Brachyurorum: Part I. An annotated checklist of extant brachyuran crabs of the world. The Raffles Bulletin of Zoology17(1), 1-286.

Perez, O. S. (1985). Studies on the biology of the Indo-Pacific sandy shore crab, Matuta lunaris Forskål (Brachyura: Calappidae) (Doctoral dissertation, James Cook University).

Perez, O. S., & Bellwood, D. R. (1988). Ontogenetic changes in the natural diet of the sandy shore crab, Matuta lunaris (Forskål)(Brachyura: Calappidae). Marine and Freshwater Research39(2), 193-199.

Perez, O. S., & Bellwood, D. R. (1989). Observations on the mating behaviour of the indo-pacific sandy shore crab Matuta lunaris (Forskål), with notes on the reproductive behaviour of the matutinae (Decapoda, Brachyura, Calappidae). Crustaceana57(1), 1-8.

Santos, E. A., Baldisseroto, B., Blanchini, A., Colares, E. P., Nery, L. E., & Manzoni, G. C. (1987). Respiratory mechanisms and metabolic adaptations of an intertidal crab, Chasmagnathus granulata (Dana, 1851). Comparative Biochemistry and Physiology Part A: Physiology88(1), 21-25.

Tan, M. H., Gan, H. M., Lee, Y. P., & Austin, C. M. (2016). The complete mitogenome of the moon crab Ashtoret lunaris (Forskal, 1775),(Crustacea; Decapoda; Matutidae). Mitochondrial DNA part a27(2), 1313-1314.

Waldrop, L. D. (2013). Ontogenetic scaling of the olfactory antennae and flicking behavior of the shore crab, Hemigrapsus oregonensis. Chemical senses38(6), 541-550.