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Liomera rubra (A. Milne-Edwards, 1865)

Ian Morrison 2015


Liomera rubra, commonly known as Red Liomera, is a species of Xanthid crab that can commonly be found off the coast of Australia on the Great Barrier Reef. More specifically, L. rubra can be found in the outer reef zone and rubble crest of various coral reefs in the Pacific Ocean.

L. rubra is easily characterized by bright red coloration and unique red patterning on the carapace. This patterning consists of ridges and indents that produce a symmetric patter. Ridged carapaces are characteristic of many species of Xanthid crabs.

Little research has been conducted on Liomera rubra since its discovery by A. Milne-Edwards in 1865, but other species of Xanthid crabs have been the focus of many recent studies due to their toxicity to humans. L. rubra have likely been under-studied as a result of their small size compared to other Xanthid crabs, but their behavior is similar to that of many other well known Xanthid crabs.


Physical Description


Liomera rubra is a small species of Xanthid crab, with both male and female adult crabs reaching a size of no more than 7 mm measured across the width of the carapace. L. rubra claws are typically no more than half the width of the carapace, usually 2-3 mm measured from the tip of the claw to the first joint on the appendage. The pereopods of L. rubra extend out from under the carapace and can measure up to 4 mm in length when fully extended. Figure 1 exhibits the size of the carapace of one specimen of Liomera rubra.

Figure 1


As in other Crustacean species the body plan of Liomera rubra is split in to three main segments, the head, thorax and abdomen. In the case of L. rubra the head and thorax are fused together to form the cephalothorax, which is protected by the crab’s upper shell, or carapace (Ruppert, 2004). These three segments are further divided into smaller segmentations, where the head is composed of 5 segments, the thorax of 8 segments and the abdomen of 6 segments (Ruppert, 2004). The segmentation of Liomera rubra can be seen in Figure 2. 

Crabs are decapods, meaning they have 10 appendages that are used primarily for movement. Liomera rubra have 4 pairs of walking legs or pereopods, along with a pair of front claws called chelipeds. As they live primarily in rocks and coral, there is no need for the swimming appendages that are common amongst decapods. In addition to appendages for movement, L. rubra have two pairs of maxillipeds, or appendages that are adapted to function as mouthparts. These maxillipeds are used in feeding, but can be retracted while not in use. The head also contains a pair of mandibles and two pairs of antennae (Ruppert, 2004). The maxillipeds of Liomera rubra are seen in Figure 3. 

Another major characteristic of Liomera rubra is the raised pattern on that can be seen on the carapace. The pattern can be observed on all Red Liomera, and is the same in all specimens of the species. This pattern is similar to patterns that can be found on many other common species of Xanthid crabs such as Actaeodes tomentosus. Ridged carapaces are a common feature of many species in the Xanthid crab family. Figure 4 shows the textured carapace of Liomera rubra. 

Most specimens of Liomera rubra are a solid red color, though some variations with white splotches on the carapace have also been observed. The red color of Red Liomera can vary from a deep reddish purple to even a bright orange-red, but the reason for the wide variation of color is not known.  

Figure 2
Figure 3
Figure 4


Coral Symbiosis

Minimal research has been conducted on L. rubra, likely due to its minuscule size compared to many other larger Xanthid crab species. Despite this, many behaviors of other species of Xanthid crabs have been heavily studied. More specifically, many species of Trapezia have been observed in order to explore any symbiotic relationships between Xanthid crabs and corals. By using these past studies it is possible to gain insight on the probable behavior of L. rubra. It is known that many species of Trapezia can be found only in specific types of coral, leading researchers to believe there may be some type of symbiotic relationship between the crabs and the coral (Huber, 1983). Huber found that multiple species of Trapezia were found only on specific types of coral, and furthermore, he found that Trapezia would vacate any dead coral.

By studying the food matter inside the stomach of Xanthid crabs, researchers have found that coral mucus is commonly ingested by Trapezia. This supports the notion that Xanthid crabs feed from the surface of coral, and actually gain nutrients by stimulating the coral to produce mucus, which is then ingested (Huber, 1983). Xanthid crabs are also known to graze on detrital particulate matter that builds up on corals, which is beneficial to zooxanthellae that depend on light. Xanthid crabs are known to benefit corals by protecting them from predators, and the removal of detritus matter from coral is known to boost growth rates of some types of coral (Huber, 1983).

Though the behavior Liomera rubra has yet to be described, it is known that L. rubra typically live on coral and therefore likely exhibit similar behaviors to other species of Xanthid crabs that have symbiotic relationships with coral.


No research has been completed to test the toxicity of Liomera rubra, likely because it is far too small for human consumption. Despite this, due to the known toxicity of other Xanthid crab species it is unsafe to ingest L. rubra. Xanthid crabs are not poisonous, rather they accumulate saxitoxin and tetrodotoxin in muscle tissue. This is likely a result of their diet, as not all specimens of Xanthid crabs accumulate these toxins (Hosie, 2012). Both of these toxins are heat stable neurotoxins, so they cannot be “cooked out” of any contaminated organisms.

Larger species of Xanthid crabs such as Lophozozymus pictor, Zosimus aeneus and Atergatis floridus are all known to be toxic to humans, all are native to Australia and are even known to be used for suicide by Pacific Islanders (Hosie, 2012). Many species of Xanthid crabs, including ones previously listed, are brightly colored. This is likely a natural means of warning possible predators of their toxicity and deterring their predation. Since the discovery of the toxicity of larger species of Xanthid crabs humans have been recommended to never ingest Xanthid crabs. Due to its small size Liomera rubra will likely never be consumed by humans, but its bright coloration and close relation to other toxic species of Xanthid crabs suggest that L. rubra may accumulate saxitoxin or tetrodotoxin in its muscle tissue.

Life History and Behaviour

Reproduction and Development

Liomera rubra, like most brachyurans, are gonochoristic, meaning both male and female organisms are present within the species, and gender is usually determined genetically. Xanthid crabs experience ecdysis, otherwise known as molting, as a part of normal growth and development. In order to ensure that reproduction and ecdysis can both occur successfully, it is important that they do not occur at the same time due to the large energetic expenditure that it would require and the vulnerability of the crab while in ecdysis.

It is understood that hormones are used to control the mating cycles of Brachyurans (Ruppert, 2004). These hormones are responsible for the instruction for female gonads to begin egg development in preparation for reproduction. These eggs fill the body cavity of the female crab, which then awaits fertilization by a male crab. During copulation, the male will insert the penes into the chitin-lined vagina of the female in order to pass on spermatozoa, which will then fertilize the eggs produced by the female (Swartz, 1978).

Fertilized eggs are then brooded and carried on the pleuron of Liomera rubra until hatching, though the development time between fertilization and hatching is unknown for the species. The brood size of L. rubra is also unstudied, and it is known that brood sizes for Xanthid crabs are highly variable (Swartz, 1978).

The eggs of Liomera rubra hatch at the zoea stage of development, which is a small free-swimming larval form that feeds on phytoplankton and zooplankton in the water column. The zoea stage of development is the only pelagic stage of development in the lifecycle of Liomera rubra. It is unknown how many molts L. rubra zoea undergo before transitioning to the megalop stage, of development, but it is know that the megalop settles on the surface of the reef to begin feeding (De Rivera, 2007). Once again, the megalop feeds and molts until metamorphosing into a juvenile crab. The juvenile crab is simply a miniature version of a mature L. rubra, and after a series of molts the crab reaches reproductive maturity. The life stages of Liomera rubra can be seen in Figure 5. 

Liomera rubra, like all decapod species is most sensitive to environmental changes while in larval stages. As a result, only a fraction of the brooded eggs will actually mature into adult crabs. This is due in part to predation, but also to changing water quality. As water quality continues to become more acidic, the larval stages of Liomera rubra may experience less successful maturation (Descoteaux, 2014).

Figure 5

Species Interactions

Due to their dependence on coral as a habitat and a food source, many Xanthid crabs have been observed to be highly territorial. Though there has been limited research on the behavior of L. rubra, research has been conducted on the behavior of the larger Xanthid crab genus Trapezia. Both Trapezia and L. rubra exhibit symbiotic relationships with the coral that they inhabit, and both are territorial when protecting the coral that they occupy.
It has been observed that no more than one single heterosexual pair of Trapezia ever inhabits a section of coral at a time (Huber, 1987). Huber also observed that specimens exhibited aggression when approached by other Trapezia as well as other crab species. The aggression between specimens was noted to yield many injuries, and fights were observed at a higher frequency than fights between other types of crabs. The intense aggression that Xanthid crabs exhibit towards each other is likely a direct result of the high degree of dependence that they have for their coral hosts. L. rubra, like Trapezia, depends on a coral host to provide almost all nutrition. As a result, L. rubra and other Xanthid crabs with symbiotic relationships with coral are incredibly territorial.

Anatomy and Physiology

Exoskeleton and Ecdysis: A Closer Look

The exoskeleton and musculature of Liomera rubra play a large role in feeding, provide a solid structure for other organ and transport systems and even provide protection for individuals from predators. Liomera rubra, like all decapods has an exoskeleton that houses all musculature and organ structure. The harmony of the exoskeleton and the musculature of decapods allows for a wide range of movements, most of which are carried out in the jointed appendages of the organism.

Decapods are named for their five pairs of pereopods that are used for movement. In L. rubra four of these pairs are designed for walking along hard surfaces, and the fifth pair is designed for grabbing. The front pair of appendage are claws used for grabbing, called chelipeds. L. rubra use their chelipeds mainly for feeding and intimidation, but unlike some other crab species the chelipeds are not used for hunting. These appendages consist primarily of musculature surrounded by exoskeleton, and multiple joints allow for a wide range of movement in each appendage. The musculature of the chelipeds and pereopods where they connect to the cephalothorax of Liomera rubra can be seen in Figure 6.

In feeding, L. rubra use their chelipeds in order to pick up food particles from the surface of coral.  The food is then passed to a set of maxillipeds, which are appendages that are designed specifically for feeding. The food is then passed to the maxillae, smaller, more delicate feeding appendages, which place the food into the mouth of the crab. The outer maxillipeds are the largest of the mouth appendages, and protect the inner feeding appendages from damage (Ruppert, 2004).

The structure of the Liomera rubra exoskeleton is vital to the health of the organism. There are three main layers of the chitinous exoskeleton of crabs. The outer layer is called the epicuticle, the middle layer is the exocuticle, and the innermost and thickest layer is the endocuticle (Chen, 2008). These three layers make up the exoskeleton, which sits along the epidermis of the crab, protecting it from desiccation and predation. The endocuticle is comprised of the newest layers of the exoskeleton, and the layers of the endocuticle are spaced relatively far apart. New layers of the exoskeleton are produced at the base of the endocuticle. The middle layer, the exocuticle, is comprised of more compact layers than the endocuticle. These layers are older, more compact, and harder than the layers of the endocuticle. It is in these layers that the pigmentation of a crab shell can be found. Lastly, the outer layer of the exoskeleton is the epicuticle, which is a very thin waxy layer meant to waterproof the exoskeleton (Chen, 2008). These layers can be seen in the microscope view of Liomera rubra in Figure 7, and Figure 8 shows a transverse section of the layers of a crab exoskeleton.

As Liomera rubra grows, the exoskeleton must be shed and regrown in order to accommodate the larger organism. This occurs in a process called ecdysis. Ecdysis is triggered by hormonal signals. When it is time for a crab to molt, the Y-organ secretes ecdysone that triggers the epidermis to begin digesting the outer layers of the cuticle (Ruppert, 2004). This digestion occurs in the epidermal cells and in the digestive ceca of the crab. As the cuticle is broken down, nutrients are collected and absorbed in order to be used for the re-production of the new exoskeleton after molting. Once the cuticle of the crab is thoroughly softened, the crab ingests water in order to expand its stomach. The expansion of the stomach causes the softened cuticle to crack, and the crab is then able to systematically crawl out of the old shell (Taylor, 2003).

In order to survive without a hard external shell, crabs utilize changes in hydrostatic pressure in order to allow for muscle function and movement even while molting. Though crabs become less active while undergoing ecdysis, the establishment of a temporary hydrostatic skeleton allows for muscle contraction if necessary (Taylor, 2003). The intake of water during ecdysis provides molting crabs with hydrostatic support, but as the cuticle hardens and begins to bare pressure the crab slowly shifts back to dependence on the hard exoskeleton (Taylor, 2003). The extensive musculature of L. rubra can only be used during molting due to the utilization of a hydrostatic skeleton.

The arthropod body plan is unique in its ability to shift between an external skeletal support system and an internal hydrostatic skeleton many times throughout an organism’s life. This ability allows for metamorphosis among many species including larval crabs, and also allows for growth in juvenile crabs. Liomera rubra utilizes a hydrostatic skeleton during ecdysis in order to allow for predator evasion and other movement while molting. The unique ability to shift between skeletons allows for ample growth in many decapod species from juvenile stages to full maturity.

Figure 6
Figure 7
Figure 8

Respiratory System

Decapod respiration is carried out by pairs of gills that are located in the thoracic appendages of the individual. Different species are known to have varying numbers of pairs of gills, but the exact number in Liomera rubra is unconfirmed. Only one pair of gills was discernible in the specimens available during specimen research. Crabs have thin gill cuticles in order to allow for diffusion of gasses (Ruppert, 2004). The gills are surrounded by branchial chambers, which are extensions of the carapace meant to shield the gills and also produce a current over the gills to allow for respiration (Ruppert, 2004). In Liomera rubra, the current flows from the posterior to the anterior of the crab. A microscope view of the cross section of the gill of Liomera rubra can be seen in Figure 9.

L. rubra, like other decapods, excrete nitrogenous waste across the gill epithelia. Nephridia are present in order to osmoregulate ion concentrations within the organism (Ruppert, 2004).


Figure 9

Nervous System

Brachyuran crabs have highly cephalized nervous system. Due to their reduced abdomens and fused segments, ganglia have fused to form one large thoracic ganglion that runs through the body of the crab (Ruppert, 2004). Brachyurans do not have a ventral nerve cord.

Brachyurans also have highly complex sense organs, which include the eyes and eyestalks, the antennae, and the setae. Brachyuran eyes are compound eyes that are connected by nerves to the brain of the organism via nerve cords in the eyestalks. Though eyestalks are very long in some crab species and allow for ample eye movement, the eyestalks of Liomera rubra are very short. Liomera rubra also utilize antennae near the anterior region around the mouth. These antennae are jointed and are used for detecting movement and orientation of the organism. A structure at the base if the antennae, called the statocyst, senses changes in movement and orientation, then signals the brain of these changes. The antennae of L. rubra are surrounded by small hairs called setae. These setae are highly sensitive to movement, so by measuring the movement of these setae crabs are able to accurately detect water currents and have very acute control of their own body movements (Ruppert, 2004). The setae of Liomera rubra are visible in Figure 10.


Figure 10

Circulatory System

Brachyuran crabs carry out internal transport via a heart, which transports materials throughout the body using a system of arteries. The blood of the crab, or hemolymph, is rich with the respiratory pigment hemocyanin, which transports the majority of the oxygen needed for survival. Hemolymph is delivered directly to many of the organs of the crab via arteries, including the eyestalks, the cardiac stomach, the mandibular muscles and all pereopods (McGaw, 2008).

Evolution and Systematics

The full classification of Liomera rubra can be seen as follows, along with some of the defining features of each of the groups of organisms.


Phylum – Arthropoda

- Presence of an exoskeleton

- Segmented body plan

- Jointed appendages 

- Crustacea

- 2 sets of antennae on head segment and 3-part brain

- Presence of mandibles and maxillae

- Compound eyes

- Gills present


Class - Malacostraca

5 segments on head, 8 in thorax, 6 in abdomen making 19 total segments

- Appendages used as maxillipeds for feeding


Order – Decapoda

- Compound eyes are stalked

- Segments are fused to create the cephalothorax

- 10 pereopods, hence the name “Decapod” or “10 feet”


Suborder - Pleocyemata

- Eggs are carried on the pleuron (Saxena)


Superfamily - Xanthoidea

­Symbiosis with species coral


Family - Xanthidae


Subfamily - Liomerinae


Genus - Liomera


Species - Liomera rubra


Brachyurans are typically easily recognizable as crabs by their wide carapace and 10 appendages. Brachyurans have a highly specialized body form, including fused segments to create the cephalothorax, a flattened wide body shape and their iconic sideways movement. Brachyurans are considered to be the most “successful” of decapods, with more than 4500 species worldwide, most of which are marine (Ruppert, 2004). 

Biogeographic Distribution

Liomera rubra is widely dispersed throughout the reef ecosystems of the Pacific Ocean. L. rubra has commonly been observed in Hawaii, Japan, and along Queensland’s coast on the Great Barrier Reef. A full range of dispersal has not been determined, though L. rubra has been recorded in a variety of reefs in the Pacific Ocean. A map of the known global distribution of L. rubra is seen in Figure 11. L. rubra is always found at relatively shallow depths typically less than 1 meter along the outer reef zone and the rubble crest of many reefs. Liomera rubra can only be found on live coral, and will not inhabit areas affected by coral bleaching events (Tsuchiya, 1992).

A complete list of L. rubra sightings has been compiled by the Marine Species Identification Portal and can be seen below.

Range: Red Sea (Nobili, 1906, Klunzinger, 1913); Zanzibar (Odhner, 1925); Madagascar - Nosy Bé (Serène, 1984); Iles Glorieuses (Serène, 1984); Mauritius (Rathbun, 1906, Odhner, 1925, Serène, 1984); Maldives - S. Nilandu Atoll (Borradaile, 1902b); Japan - Ogasawara-shoto (Odhner, 1925), Yoron-jima, Ishigaki-jima and Taketomi-jima (Sakai, 1976a), Oshima Passage, Amami-Oshima (Takeda, 1989); Taiwan - Su-ao (Lin, 1949); Philippines (Odhner, 1925); Vietnam - Nha Trang (Serène & Luom, 1960); Australia - Cape Jaubert (Rathbun, 1924c); Hawaii - Honolulu (A. Milne Edwards, 1865, Serène, 1984), Oahu (Lenz, 1901), Molokai, Auau Channel, and Penguin Bank (Rathbun, 1906); shallow waters.

Figure 11

Conservation and Threats

The population of Liomera rubra has not been assessed by the IUCN, but like many other invertebrates that thrive in coral reef environments the population of Liomera rubra could suffer as coral bleaching events continue to happen with increasing frequency. Though L. rubra is far too small to every be harvested by humans, it is likely that populations will struggle to thrive during coral bleaching events as ocean acidification continues to occur. With the knowledge of a symbiotic relationship between many Xanthid crabs and corals, it is likely that the destruction of coral would result in decreased food availability for crabs, reducing crab populations (Huber, 1983).

Studies have been completed on other types of crabs to find the impacts of ocean acidification on the development of juvenile crab larvae. In a study of Chionoecetes bairdi, Glebocarcinus oregonensis, and Metacarcinus magister, it was found that as the pH of seawater decreased, larval survival rates also decreased (Descoteaux, 2014). Since brachyurans utilize similar stages of development, it is likely that ocean acidification will have negative impacts on Liomera rubra and other species of Xanthid crabs.

Coral reefs are very economically beneficial to human settlements, so it is in the best economic interest of many countries to improve regulation protecting reef ecosystems. Some of the best methods to protect coral reefs from further damage include education of the public about the benefits of sustainable practices and further management of aquatic ecosystems by government (Goodwin, 2006). Placing stricter regulations on fishing and tourism in reef ecosystems will result in improved coral health, resulting in healthier populations of Liomera rubra and similar species of Xanthid crabs.



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*All photos and images used were taken by or created by Ian Morrison at the University of Queensland in Australia unless otherwise stated.