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Leptodius exaratus

Harrison Riley Johnson 2018



Leptodius exaratus, also known as the stone crab orthe rocky shore crab, is a crustacean that resides in intertidal habitats (Afkhami, et al, 2016) (Naderloo, et al, 2016). Like other crabs in the Leptodius genus, it exhibits a large range of colour patterns and morphological differentiations (Afkhami, et al, 2016). This variability makes it difficult to distinguish between species, hence why this genus is continuing to be studied (Lwin, et al, 2007) (Naderloo, et al, 2016). This genus currently includes 12 species, which are dispersed throughout the warmer regions of the Indo-Pacific, from the Indian Ocean to the Central Pacific Ocean (Al-Aidaroos, Kumar and Al-Haj, 2017). L. exaratus was first studied in 1834 by Milne Edwards, and was inaccurately classified as Chlorodius exaratus (Al-Aidaroos, Kumar and Al-Haj, 2017). The large distribution of L. exaratus has affected the morphometric characteristics between populations (Afkhami, et al, 2016). Populations which are geographically isolated from each other have evolved separately, hence why there may be several differences inappearance between populaces (Afkhami, et al, 2016). This isolation is associated with adaptations to certain regions and the relatively low dispersal rates for larvae (Al-Wazzan, et al, 2015) (Afkhami, et al, 2016). Thus, L. exaratus is known to have a high variation in phenotypic plasticity (Al-Aidaroos, Kumar and Al-Haj, 2017).


Kingdom: Animalia

Subkingdom: Bilateria

Infrakingdom: Protosotmia

Superphylum: Ecdysozoa

Phylum: Arthropoda

Subphylum: Crustacea

Class: Malacostraca

Subclass: Eumalacostraca

Superorder: Eucarida

Order: Decapoda

Suborder: Pleocyemata

Infraorder: Brachyura

Superfamily: Xanthoidea

Family: Xanthidae

Genus: Leptodius

Species: Leptodius exaratus

Physical Description

General description

L. exaratus is a bilaterally symmetrical, dorso-ventrally flattened, coelomate member of the Infraorder Brachyura, which encompasses all true crabs (Al-Aidaroos, Kumar and Al-Haj, 2017). Crustaceans, especially Brachyurans, often display differential size of chelae in mature individuals (Mariappan, Balasundaram and Schmitz, 2000). This commonly occurs for males (see Figure 4), with females usually exhibiting isometric growth of chelae (Mariappan, Balasundaram and Schmitz, 2000). This is evident in L. exaratus, as males possess both crusher and cutter chelae. This characteristic makes L. exaratus sexually dimorphic, as mature males and females can easily be separated from another (Mariappan, Balasundaram and Schmitz, 2000). Chelae are different from chelipeds as they chelae only includes the dactyl and propodus, whereas the cheliped encompasses the entire claw (Wortham and Pascual, 2017). The larger crusher chela is used during aggressive encounters against other males, or to avoid predation (Mariappan, Balasundaram and Schmitz, 2000). The smaller cutter chela is used to capture prey and forself-cleaning. Chelae are the most versatile of all five pairs of periopods, which makes them susceptible to autotomy, which is the process of losing a body part during threatening scenarios (Mariappan, Balasundaram and Schmitz, 2000).

Females can be visually distinguished from males at roughly the fifth instar, as their abdomen is shown to be considerably wider (Lwin, et a.,2007). The morphology of L. exaratus juveniles is also used to distinguish them from other species (Lwin, et al, 2007). Distinct carapace patterns place L. exaratus in the family Xanthidae (Lwin, et al, 2007). However, the motifs in the maxilliped epipod allow distinction between other members in the Xanthidaen family (Lwin, et al, 2007).

The size of L. exaratus can often be used to estimate the age of an individual and therefore the sexual maturity (Afkhami, et al, 2016). As stated, L. exaratus shows clear sexual dimorphism, however unlike many other invertebrates the males are frequently larger than the females (Afkhami, et al, 2016). The females in most invertebrates are usually larger due to the selection of fecundity (Afkhami, et al, 2016). Conversely, the reason for the larger size on males is due to the selection for such individuals. Females become mature when their carapace is approximately 12.5 mm wide, they also begin to produce and carry eggs at this size (ovigerous) (Seyfabadi, et al, 2014) (Afkhami, et al, 2016). On the other hand, males mature when their carapace is approximately 15 mm wide (Seyfabadi, et al, 2014) (Afkhami, et al, 2016). The length of gonopods in males can also be examined to determine maturity (Afkhami, et al, 2016) (Al-Aidaroos, Kumar and Al-Haj, 2017). Additionally, female gonad length and quantity of eggs are also correlated with maturity (Afkhami, et al, 2016) (Al-Aidaroos, Kumar and Al-Haj, 2017). 

Figure 1

Detailed description of specimen

The L. exaratus specimen that was examined in the laboratory was a male with a carapace approximately 17mm wide. Therefore, this was a sexually mature specimen (Seyfabadi, et al, 2014) (Afkhami, et al, 2016). Some distinguishing features of L. exaratus include 4 to 5 ‘teeth’ either side of the exorbital angle on the anterior side of the animal (Lee, et al, 2013). 
The carapace and chelipeds have a smooth, granular surface, this granularity increases in older individuals (Lee, et al, 2013). The carapace also has ridge-like segments that slightly project, combined with regions that have deep indentations (Afkhami, et al, 2016). The indentations are usually present on the most dorsal point of the carapace (Afkhami, et al, 2016). The first pleopod (also known as swimmerets) for the male L. exaratus has six small spines along with an elongated apical lobe
(Lee, et al, 2013).

The abdomen of male L. exaratus is narrow compared to females, with approximately 4 somites fused together. The somites are surrounded by thoracic sternites (see Figure 3).

The majority of the exoskeleton is covered in bristle-like setae, noticeably on the dorsal side of the carapace and ambulatory legs (see Figure 3). The carapace of L. exaratus is approximately 1.5 times as wide as it is long (Lee, et al, 2013). The dorsal surface of the carapace is convex, with a granular surface. The chelipeds are unequal in size, with a larger crusher chela on the right side, while the smaller cutter chela is present on the left side (Lee, et al, 2013). The moveable fingers of the chelipeds show opaque pigmentation, apart from the very tips which are white (Lee, et al, 2013). The dactyl and propal fingers are curved downwards, with irregular dentition on the inside (Lee, et al, 2013). 

The ambulatory legs are slender with red and sand coloured patterns alternating along the dorsal side (see Figure 6). 

Figure 2
Figure 3
Figure 4
Figure 5
Figure 6


L. exaratus inhabits rocky and cobble shorelines and is high in the trophic level (Al-Wazzan, 2017). Consequently, they have a major influence on the structure and biodiversity of intertidal communities (Al-Wazzan, 2017). This species also exhibits seasonal growth and reproduction, these are at their highest between the months of April and September (Al-Wazzan, 2017). The abundance also varies between seasons, with summer having a relatively higher abundance compared to winter (Al-Wazzan, et al, 2015). There is no preference in distribution between males and females, as they have the same spatial distribution across all seasons (Al-Wazzan, et al, 2015). In a dense population of L. exaratus, males will usually be more abundant compared to females (Seyfabadi, et al, 2014). These are shown to have a greater chance of reproduction (Afkhami, et al, 2016). This is evident in populations studied in the Persian and Oman Gulf (Afkhami, et al, 2016).

Life History and Behaviour

L. exaratus are gonochoristic and always breed by internal feritilisation (Tufail and Hashmi, 1964). The changes in male and female gonads show that peak mating season occurs in spring and summer (Watanabe, Yamana and Tomikawa, 1990) (Fahimi, et al, 2017). Furthermore, some females may spawn multiple batches of eggs each year, although this is rare (Fahimi, et al, 2017). The release of larvae by females typically occurs in the night, and is synchronous with the smallest tides, also known as neap tides (Al-Wazzan, et al, 2015). Therefore, there is a significant association with the time of day, level of inundation and the tidal cycles effect on the hatching patterns of L. exaratus (Al-Wazzan, et al, 2015). This synchronous pattern of hatching occurs to decrease the likelihood of larvae being exposed to predation, and harsh environmental conditions (Al-Wazzan, et al, 2015). Thus, the hatching patterns of L. exaratus are influential in determining the dispersal of larvae (Al-Wazzan, et al, 2015).

The development of L. exaratus has five zoeal stages, which is a type of free swimming larvae seen in crustaceans (Al-Aidaroos, Kumar and Al-Haj, 2017). A similar species, L.affinis can be distinguished from L. exaratus by examining zoeal stages, as this species only has four of these stages (Al-Aidaroos, Kumar and Al-Haj, 2017). It should be noted that environmental factors such as temperature variation, changes in salinity, food source and availability can all affect the appearance of zoeal stages (Al-Aidaroos, Kumar and Al-Haj, 2017).

Juvenile L. exaratus in the first instar show some characteristics to that of adults. Such as the carapace and cheliped proportions (Lwin, et al, 2007). However, virtually all other morphological proportions can be distinguished to that of adults (Lwin, et al, 2007). Some juvenile L. exaratus in intertidal habitats are known to show polymorphism, specifically in the colour of their carapace (Todd, Qiu and Yan Chong, 2009). The patterns that are present in young individuals are thought to help camouflage against predators (Todd, Qiu and Yan Chong, 2009). The development of L. exaratus is not fully complete until approximately 13 ecdyses, which they are then classified as adults (Lwin, et al, 2007). After this period, their reproductive organs are fully developed and can therefore breed (Lwin, et al, 2007).

Anatomy and Physiology

Circulatory system

In the past, the circulatory system of crustaceans have been categorised as open (McGaw, 2005). However, recent analysis has revealed that some decapod crustacean circulatory systems are relatively complex and should be re-categorised as “partially closed” (McGaw, 2005). A typical crustacean circulatory system involves hemolymph flowing through a hemocoelic cavity which comprises a single chambered heart connected to seven arteries (McGaw, 2005). Connected to the arteries are numerous arterioles, which mostly separate into a capillary bed of vessels with a complete ending (McGaw, 2005). Nevertheless, some capillary beds drain into a series of sinuses. Due to the combination of capillary beds completely ending and flowing into sinuses, the circulatory system of crustaceans should be defined as ‘partially closed” (McGaw, 2005).

Digestive system

Crustacean digestive systems are described as complex (Ceccaldi, 1989). Initially, food is processed through the mouth and its specialized appendages: the maxillula, maxilla, mandibles and maxillipeds (see Figure 7) (Ceccaldi, 1989). For adult Crustaceans, there is generally three parts to the digestive tract: being the foregut, midgut and hindgut (Ceccaldi, 1989). The foregut is comprised of a relatively short esophagus connected to a dual partitioned stomach, which has specialized grinding appendages for mastication (Ceccaldi, 1989). In the stomach, setae separate food particles and fluid to the irrespective digestive sites (Ceccaldi, 1989). Also present in the stomach are several cells which have specific functions, such as: absorption, secretion and storage (Ceccaldi, 1989).The midgut includes a primitive pancreas that secretes digestive enzymes. Finally, at the hindgut the digestive tract is expanded at the rectum and ceases at the anus (Ceccaldi, 1989).

Figure 7

Endocrine system

The typical crustacean endocrine system consists of glands and endocrine structures (Bliss and Welsh, 1952). This system is associated with many hormones, which are responsible for a process known as ecdysis. This process involves the shedding or molting of the exoskeleton, which is effectively the same for adults and larvae (Bliss and Welsh, 1952). Additionally, some hormones in Crustaceans are dedicated to pigmentation, these are known as Chromatophores (Fingerman, 1997). Most crustaceans, especially Brachyurans, have more than one type of Chromatophore responsible for the colouration of the exoskeleton (Fingerman, 1997). Other hormones in Crustaceans are responsible for the regulation of reproductive activities. For females, the ovaries are the location for the ovarian hormone, which induces reproductive behaviour (Fingerman, 1997). Alternatively, hormones in male Crustaceans testis have not yet been discovered (Fingerman, 1997).

Biogeographic Distribution

L. exaratus is found predominately on shorelines with gravelly and sandy substrates throughout the Indo-Pacific (Hsueh and Hung, 2009) (Afkhami, et al, 2016). This is evident globally (Figure 8) and more locally around Australian coastlines (Figure 9).

Figure 8
Figure 9

Evolution and Systematics

L. exaratus is included in the Arthropoda phylum, which is largest group of metazoans (Stevcic, 1971). Furthermore, L. exaratus is incorporated in the Xandthidae family (see Figure 8), which share several characteristics, including; distinct carapace indentations, compact chelipeds and small abulatory legs (Lai, et al, 2011). Within Arthropoda is the Infraorder Brachyura, which are an incredibly diverse group of crustaceans (Tsang, et al, 2014).Over 7,000 species have been identified and placed in 98 families (Tsang, et al, 2014). These include marine, freshwater and terrestrial crabs. These true crabs, are divided into two major groups, being Eubrachyura and Podotremata, which are thought to be derived and primitive respectively (Ahyoung, et al, 2007). True crabs have been an integral component of marine ecosystems since the Late Jurassic (Fraaije, 2003). Brachyurans are separated from other Infraorders by their increased differentiation of segments and appendages (Stevcic, 1971). The increased differentiation has allowed complex adaptations to occur (Stevcic, 1971). This has allowed Brachyurans to occupy new habitats due to modifications of shape and structure (Stevcic, 1971). The relationships between Eubrachyura are commonly thought to be ambiguous. However, traits such as camouflage a proposed to be phylogenetically significant (Ahyoung, et al, 2007). Interestingly, the absence or loss of camouflage is considered a derived characteristic (Ahyoung, et al, 2007). Although, L. exaratus is often covered in brittle-like setae it is not totally camouflaged, therefore it may be quite a complex Brachyuran. The relationships between families continue to be enigmatic within the scientific community due to the complexity of the taxonomic diversity, therefore extensive taxonomic sampling is needed to resolve this (Ahyoung, et al, 2007) (Tsang, et al, 2014).

Figure 10

Conservation and Threats

L. exaratus is not listed in the IUCN red list of threatened species. Currently, there are no studies that estimate the population size of L. exaratus. There have been studies that look at gene flow between two different populations (Afkhami, et al, 2016). However, this does not involve investigating the population structure across all known habitats in the Indo-Pacific. Perhaps it is possible that speciation could occur in the future for certain populations, as the isolation of populations may continue to cause genetic drift.

Major threats to invertebrates include rapid climate change, which can cause physiological impairment (Hoegh-Guldberg, et al, 2007). Furthermore, ocean acidification drastically reduces the abundance of coral, which plays a vital role in the recruitment of invertebrate larvae such as L. exaratus (Hoegh-Guldberg, et al, 2007).


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