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Morphological description and taxonomic issues of Pilodius pugil


Sebastian Lopez Marcano 2016

Summary

Pilodius pugil is a crab species from the genus Pilodius, Chlorodiellinae subfamily, and Xanthidae family. These small crabs of around 1.4 cm carapace width have a dark purple colouration with white grooves along their entire body (Figure 1). The dark colouration and small size is ideal to blend with the rocky substrates that these crabs use as habitat (Monteforte, 1987). Pilodius pugil, like many other xanthids, bear dark red bands around the cheliped joints (Koyama, 1981). These coloured bands are used to warn predators of its toxic composition (Koyama, 1981; Western Australian Museum, 2012). 

P. pugil is commonly found in reefs and rocky shore ecosystems of the Indo-Pacific region (Monteforte, 1987). Despite its abundance in reef ecosystems, detailed descriptions of this species are lacking. In fact, few descriptions have been made of the genus Pilodius resulting in several taxonomic issues as established by Clark and Galil (1993). Nobili (1906) and Clark and Galil (1993) stated that one of the most representative taxonomic issues is the confusion between P. pugil and Pilodius spinipes. Pilodius pugil is usually mistaken with Pilodius spinipes because of its morphological similarities (Clark and Galil, 1993). This taxonomic issue can only be solved by performing detailed descriptions of these species. 

This species description report will aim to describe the species P. pugil fully and analyse the taxonomic issue between P. pugil and P. spinipes.  
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Figure 1

Physical Description

Malacostracan Morphology
The body of P. pugil, like all malacostracan crustaceans consists of 21 segments and are arranged in the following manner: five head segments, eight thoracic segments and six abdominal segments (Ruppert et al., 2004). In malacostracans, we can observe the fusion (tagmosis) of the five head segments with the three most anterior thoracic segments, creating a cephalothorax (Ruppert et al., 2004). The cephalothorax is usually protected by a broad carapace, and other segments are protected by an exoskeleton that is regularly moulted during the lifetime of the animal (Davie, 2002).  

Most of the 21 segments found in crustaceans bear a pair of appendages that help on feeding, movement or reproduction (Ruppert et al., 2004). Head segments usually contain mandibles, maxillae and two pairs of antennae (Hayward and Isaac, 1995). Thoracic segments include legs that can be divided into two main groups: pereiopods and maxillipeds (Ruppert et al., 2004). Pereiopod is the term used to refer to walking legs and maxillipeds are specialised legs used for feeding (Ruppert et al., 2004). The abdominal segments bear the anus, reproductive structures and pleopods. Pleopods are used by crustaceans for swimming or reproduction (Ruppert et al., 2004).  

Distinctive Characteristics of P. pugil

According to Clark and Galil (1993), the main characteristic that separates P. pugil from the other Pilodius species is the three hook-like dentation observed in the anterolateral margin of the carapace (Figure 2). However, as established by Nobili (1906), the dentation of the carapace is only seen when the animal is fully mature, making difficult to characterise juvenile individuals. Nobili (1906) also argued that other species in the Pilodius genus such as P. spinipes show the same hook dentation, but instead of being three hooks they have four. Dentation of the carapace can also be lost during the lifetime of the individual, which can create problems when trying to characterise this species (Nobili, 1907).
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Figure 2

Size of P. pugil
Pilodius pugil as many other crustaceans show sexual dimorphism. Sexual dimorphism is defined as the differences in size, shape and colour between males and females of the same species (Ridley, 2004). In Pilodius pugil, females (~1.3 cm carapace width) are smaller than males (~2.5 cm carapace width) (Clark and Galil, 1993). It has been hypothesized that such dimorphism in some crustaceans is driven by differences in growth patterns related to the energy investment of females when laying eggs (Bertness, 1981). However, other studies have shown that males are usually larger because it offers an advantage when fighting with other males (Baeza and Hernaez, 2015; Magalhaes et al., 2016). Both findings despite being true in some crustaceans, these have not been yet studied in P. pugil. 

Colouration of P. pugil

The dark purple colouration seen in P. pugil is ideal for camouflaging in the environments that they live in (Stevens and Merilaita, 2011). Such colouration resembles rock rubble and coral structures which make them undetectable when they are inside dark rocky crevices (Parkes et al., 2011; Todd et al., 2006). From past species descriptions, it is known that females show a light carapace colouration (Clark and Galil, 1993). However, this light colouration in females was highly variable and not persistent (Clark and Galil, 1993). In addition to this,  research by Detto et al. (2008) in other crab species determined that when the environmental temperature increased, changes in carapace colouration occurred. The changes in carapace colouration happened as a response to environmental stress and was more evident during moulting (Detto et al., 2008). Several xanthids including Demania macneilli and Eriphia smithi showed this behaviour, but it has not been yet seen in P. pugil (Todd et al., 2006).  

Ecology

Habitat
P. pugil is a common crab found along the neritic zone of tropical reefs and rocky shores (Encyclopedia of Life, 2010). Past descriptions showed that P. pugil lives inside rocky crevices of sandy (inner reef) zones and living coral (outer reef zone) areas with water depths up to six metres (Clark and Galil, 1993; Serene, 1984). However, some specimens were found on rocks at depths of 13 metres (Global Biodiversity Information Facility, 2010). This crab species is highly dependent on corals and rocky areas because they use them for protection from predators and feeding (Vermeiren and Sheaves, 2014).  

Feeding

Crabs are omnivores and opportunistic animals that use the resources that are available for them (Ruppert et al., 2004). Crabs feed mainly on algae but also feed on worms, other crabs, bacteria and detritus (Ruppert et al., 2004). Research by Kennish (1996) showed that crabs maintain a mixed diet because it promotes a faster growth. In the Pilodius genus, most of the species are herbivores and feed mostly on small red algae (Clark and Galil, 1993). When I studied the species, feeding on small red algae by P. pugil occurred regularly. 

Predators
Crabs are predated by many marine and terrestrial animals (Hemmi, 2005). For example, fiddler crabs are mostly predated by shorebirds that forage around the mudflats (Hemmi, 2005). Blue crabs are predated by many species of sea turtles and coastal birds. However,  predation in P. pugil has not been studied yet, but since they are abundant in coral reefs, predation by turtles and fishes must occur (Ruppert et al., 2004).

Life History and Behaviour

Reproductive Behaviour and Growth
 Decapod crabs are mostly gonochoristic, meaning that an individual organism is either a male or a female (Brill, 1979). Males usually have a “V” flap at the bottom of their carapace and females have a rounded flap (Brill, 1979; Ruppert et al., 2004). Females use this rounded flap to mate and to carry the many eggs that these animals lay (Shyamal et al., 2014). Mating between male and females vary depending on seasons (Ruppert et al., 2004). Some crabs mate many times each year, while others mate in specific seasons such as winter or summer(Clark and Backwell, 2015). Most of the crabs located in the Indo-Pacific including P. pugil mate during summer (November to January) (Clark and Galil, 1993).

When mating occurs the female crab stores the male sperm until her eggs are produced and ready to release (Ruppert et al., 2004). When the female releases the eggs, the stored sperm in the abdominal flap covers the recently released eggs becoming fertilised (Ruppert et al., 2004). These fertilised eggs are then stored and held by the female in a porous structure located on the abdominal flap (Ruppert et al., 2004). Pleopods located in the abdominal flap help to maintain the eggs in position by cementing them to the abdomen (Ruppert et al., 2004). When the eggs are ready to hatch, the female releases the eggs into the ocean where they turn into a zoea larvae (Ruppert et al., 2004). The zoea larvae when in the sea becomes part of the plankton and then they progressively grow (metamorphosis) until a juvenile crab is formed (Ruppert et al., 2004). After a serious of moults, the juvenile crabs becomes a mature crab. 

The entire reproductive process of the crabs has been studied by many scientists with a focus on the reproductive behaviour (Kim et al., 2008). Chemical communication, visual displays, population clusters and environmental variables are some of the factors that affect mate preference in crabs (Kim et al., 2008; Tropea et al., 2015; Zeil and Hemmi, 2005). However, from all of these factors, the most relevant are the effects of environmental variables to mate preference. Research has shown that increased CO2 concentrations affect the mate preference in both males and females by reducing the mating frequency (Zeil and Hemmi, 2005, p. 2). Changes in the environment such as increased CO2 levels might pose a threat to the reproductive success of crabs (Doi et al., 2008). However, it has been proven several times that crabs have an outstanding resilience to such environmental changes and can adapt quickly and efficiently to different environments (Bartolini et al., 2009).

Social Behaviour
Social behaviour in crustaceans is highly dependent on the species and the environment that they live in (Bolton et al., 2013). For example, Ylyoplax pusilla, a common intertidal brachyuran crab show two behavioural characters: waving display and barricaded building (Yamada et al., 2009). Waving display is a common social behaviour in crabs, where males wave one of their claws to either attract females or to show superiority to other males (Backwell et al., 2006). This behaviour has been deeply studied in fiddler crabs and other terrestrial crabs but it has not been studied in brachyuran crabs neither in crabs from the subfamily Chlorodiellinae (Backwell et al., 2006). On the other hand, barricaded building has been seen in few crabs in general because it occurs in crabs that burrow and live in highly dense areas (Yamada et al., 2009). Taking into account that P. pugil individuals do not burrow since they live in rock crevices, this behaviour is highly unlikely to be present in this species. However, a social behaviour that might be present in Pilodius pugil is the fighting or aggressive behaviour (Clark and Galil, 1993). Aggressive behaviour is very common in all crustaceans and it is seen between different species or between individuals of the same species (Bolton et al., 2013). Since the habitat that P. pugil lives in is inhabited by many crabs and other invertebrates, competition for resources and mates must be high which increases the aggressive behaviour of the animals (Suzuki et al., 2012). Therefore, aggressive behaviour might be seen in P. pugil, but it must be further studied. 

Defence Strategies
Pilodius pugil employs two different defence strategies to protect themselves from predators. The first strategy is camouflage and it is obtained from the dark purple colouration of the carapace (Serene, 1984). This colour resembles the coral rubble and rocks that these animal use as habitat. When P. pugil feels threatened, the crab stops moving and tries to look like a rocky structure from the substrate that it is standing on (Clark and Galil, 1993; Stevens and Merilaita, 2011). Also, the dark colouration allows P. pugil to be unrecognisable when it is inside crevices of corals and rocks (Cuthill et al., 2005). 

The second and final strategy is toxicity, which is a general characteristic from the xanthids (Koyama, 1981; Llewellyn and Endean, 1988). These toxins are not produced by the crabs but instead are obtained from different organisms (bacteria and diatoms) that produce it (Western Australian Museum, 2012). Pilodius pugil demonstrates its toxicity by showing red coloured bands on its cheliped, as shown in Figure 3(Serene, 1984). The toxins and the concentration of P. pugil are unknown and should be further studied.
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Figure 3

Anatomy and Physiology

External Anatomy of P. pugil

A male P. pugil found in the laboratory was dissected, and a fully external anatomy description was performed. 

Rostrum
Pilodius pugil rostrum had 1 mm of length with a very small mouth field, and the presence of setae was observed. Two very small maxillipeds were seen in the mouth-rostrum region which helps with the feeding process (Colpo and Negreiros-Fransozo, 2013). Pilodius pugil rostrum is composed of many parts that include the eyes, antennae and the antennule. Two round red eyes of 2 mm diameter and black pupils were observed in the specimen (Figure 4). Protrusion of the eyes was very small and only occurred when the crab was outside of the water. When the crab was underwater, the eyes were fixed on small sockets that are located in the rostrum. The specimen’s eye also had a fragile circular layer on the outside which is believed to offer protection to the eyes (Barnes et al., 2002). 

Two pairs of antennae with a length of 3 mm and one pair of antennule was observed in the specimen. Antennules could only be visible when the crab was feeding, as shown in Figure 4. 

Carapace
The dorsal surface of the carapace was shagreened with no visual lumps.  Some excavations were observed near the rostrum, typical of the Pilodius genus, as shown in Figure 5 (Ng and Clark, 2000). Carapace sections were clearly observed, being the gastric region the most evident. Branchial, cardiac and intestinal regions are shown in Figure 6. These structures were not characterised before for P. pugil but are commonly present in most crustaceans (Ruppert et al., 2004).

In terms of shape, size and colour, the carapace was oval shaped with dark purple and white spots colouration. The specimen had a carapace length of 0.8 cm and a width of 1.4 cm in its widest section (Figure 7).  On the anterolateral margin of the individual three acuminate tubercules or three hook-like dentation on each side were observed, characteristic of Pilodius pugil (Clark and Galil, 1993). Scattered setae with acuminate simple form was observed along the carapace, characteristic of the xanthids (Balss, 1984). 

Pereiopods
General characteristics
All the pereiopods of Pilodius pugil were covered with long acuminate simple setae, as shown in Figure 8 and Figure 9.  Superior areas of the carpus, merus and propodus were prominently spinose. 

Walking legs – morphology and function 
Pilodius pugil has four walking legs on each side and lengths were recorded and shown in Table 1. All four pairs of walking legs were similar length and shape. However, third and fourth leg were slightly shorter than anterior legs (Figure 10). 

Table 1: Measurements of the walking legs (pereiopods) of Pilodius pugil.

Side

Leg 1 (2nd pereiopod)

Leg 2 (3rd pereiopod)

Leg 3 (4th pereiopod)

Leg 4 (5th pereiopod)

Left

1.1 cm

1.1 cm

1 cm

0.9 cm

Right

1.1 cm

1.1 cm

1 cm

0.6 cm

As established by De Grave and Goulding (2011) the function and micromorphology of the 5th pereiopod depends on the species. Some crab species use the 5th pereiopod for grooming the gills or grooming intestinal regions (De Grave and Goulding, 2011). Other species as the blue crab replace the 5th pereiopod with a swimming leg, helping them to swim in lakes and rivers (Balcı et al., 2014).  The micromorphology observed in Pilodius pugil might offer stability to the general morphology of the crab but it may also help on the attachment process when it is moving through rocky areas and reef areas (Clark and Galil, 1993; Serene, 1984). However, other functions of this pereiopod need to be further investigated. 

In terms of structure, the walking legs of Pilodius have well defined merus, carpus, propodus and dactyl. A spinous dactyl was observed which may offer good attachment to the substrate (Figure 11). The carpus and propodus promote flexion and extension while movement is occurring. In general, the centre of gravity of crabs is shifted forwards because of the flexion and reduction seen in the abdominal region (Ruppert et al., 2004). This gravity shift is the cause of the sideways movement observed in brachyurans (Brill, 1979). The pereiopod of the leading side will generate motion by flexion whereas the remaining legs will extend and push, thus creating the sideways movement (Ruppert et al., 2004). 

Cheliped
Most of the Brachyurans show sexual dimorphism, where the male will show a bigger cheliped and claw. The analysed specimen had a left enlarged claw of 1.4 cm and a smaller claw of 0.4 cm. The external surface of the cheliped was covered with large and conical tubercule which become smaller on the ventral surface of the propodus (Figure 12). From the literature it is known that claw form plays an important role in determining the biting pressure. For example, species with long thin claws usually have less bite strength than thick muscular claws (Schenki and Wainwright, 2011). In Pilodius pugil we observe a small and thin claw made for grazing algae. The grazing action is performed by the small movement of the dactyl and the cuticle structures inside the movable finger (Figure 13)(Clark and Galil, 1993). According to Huber et al. (2015), claw cuticles are used to scrape algae from rocks in a sweeping motion. Therefore, in Pilodius pugil, we observe an entire cuticle that is used for grazing rather than for nipping predators as observed on other bigger crabs such as the mud crabs (Clark and Galil, 1993; Huber et al., 2015). 
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Figure 4
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Figure 5
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Figure 6
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Figure 7
8
Figure 8
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Figure 9
10
Figure 10
11
Figure 11
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Figure 12
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Figure 13

Internal Anatomy of P. pugil
The internal anatomy of Pilodius pugil and all decapod crustaceans can be divided into six broad systems: the alimentary system, the respiratory system, the excretory- osmoregulatory system, the reproductive system, the circulatory system and the nervous system (Felgenhauer, 1992). The alimentary system of decapods is composed of the oesophagus-foregut, the midgut and the hindgut (Felgenhauer, 1992). These structures have a chitinous texture and are derived from the ectoderm and the endoderm (Ruppert et al., 2004). The oesophagus-foregut occupies a big area of the cephalothorax and is surrounded by a hepatopancreas (Ruppert et al., 2004). The two other structures (midgut and hindgut) are filled with small rounded tubules that help to process food (Felgenhauer, 1992). 

The respiratory system of decapod crustaceans is comprised of one pair of lungs and seven to eight lamellar gills (Felgenhauer, 1992). These lamellar gills are found inside the branchial chamber where gases are circulated (Felgenhauer, 1992). Also, they are thin and allow the diffusion of gases into the bloodstream (Felgenhauer, 1992). Gills also contain marginal canals and nodules which provide rigidity and help airflow in the absence of water (Ruppert et al., 2004). 

The excretory-osmoregulatory system in decapods is composed off a coelomosac and Nephridial canal and a bladder (Felgenhauer, 1992). The coelomosac has smaller podocytes which help in the ultrafiltration process and is involved in the movement of essential proteins and ions that are needed for the animal (Felgenhauer, 1992). These proteins and transport of waste is transported through the nephridial canal and then is released through the bladder (Felgenhauer, 1992). 

The reproductive system in crabs is different in males and in females. Males have two main structures on their reproductive system: the testes and the vas deferencs (Felgenhauer, 1992). The testes lie dorsally on the thoracic cavity and produce the spermatozoa (Ruppert et al., 2004). The spermatazoa is non-motile and aflagellate and is transported through the vas deferens (Ruppert et al., 2004). In females the main structures are the ovaries and the pleopods. The ovaries are located in the cephalothorax and produce the eggs (Felgenhauer, 1992). Pleopods on the other hand help on cementing fertilised eggs on the rounded abdominal flap present in females (Ruppert et al., 2004). 

The circulatory system of decapods is composed of several parts being the most representative ones the bulbous and the dorsal heart (Felgenhauer, 1992). Blood is transported through veins and arteries and reaches the dorsal heart by a series of small ostia (Felgenhauer, 1992). 

The nervous system is mainly composed of the brain and the nerve cord (Ruppert et al., 2004). The regions of the brain are: the protocerebrum, the deuterocerebrum and the tritocerebrum (Felgenhauer, 1992). The nerve cord is the structure that transport most of the signals within the body and is described as a “ladderlike” structure (Ruppert et al., 2004). 

Biogeographic Distribution
Pilodius pugil is a common species found in reefs in the Indo-Pacific. However, specimens have also been found in Comores, Indian Ocean, Madagascar, Mozambique Channels, Red Sea, Republic of Mauritius and in the South Pacific Ocean (Figure 14). As observed in Figure 14, the distribution of this species is mainly tropical which corresponds to the location of most of the world’s coral reef ecosystems (Encyclopedia of Life, 2010). 

Distribution of these animals might also extend from the areas shown in Figure 14 since in South America there is a high diversity of crabs and it has right conditions for these animals to live in (Encyclopedia of Life, 2010; Perez-Losada et al., 2009).  
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Figure 14

Evolution and Systematics

Systematics

Pilodius pugil was first described by Dana (1852) and then by Serene (1984). This crab species has always been named Pilodius pugil, but Pilumnus globosus has also been used before in other studies (Boone, 1934). Clark and Galil (1993) revised the genus and found that Pilodius had 15 species living mostly in tropical coral reefs. From Clark and Galil (1993) we know that Pilodius pugil is a crab species of the Chlorodiellinae subfamily, Xanthidae family, Brachyura infraorder and Decapoda order (see Classification). The Xanthidae family is the biggest family on the brachyurans, and the phylogenetics has been poorly studied. Species in the Xanthidae family are usually black compact cryptic crabs and are highly abundant in the Indo-Pacific. Most of the species in the Xanthidae family are poisonous, especially the genus Zosimus. On the other hand, the subfamily Chlorodiellinae is a group associated with crabs that live in coral reefs and rocky environments. Few descriptions have been made of the Chlorodiellinae, but the two most relevant studies are Serene (1984) where he recognised five genera all in tropical Australia and Lasley et al. (2015) where the evolution and phylogenetic relationships were revised and analysed. 

 Most of the species in the Chlorodiellinae are poorly know due to lack of detailed descriptions and the amount of species that compose this subfamily (Figure 15). The phylogenetic tree from Lasley et al. (2015) displays all the species of the Chlorodiellinae subfamily by using histones and rRNA sequences. The most significant issue with Figure 15 and the study from Lasley et al. (2015) is that Pilodius pugil does not appear on the genus Pilodius, but instead it appears on a new genus called Luniella. The paper written by Lasley et al. (2015) has been the only study that had changed the taxonomy of the Pilodius group since 1993 when Pilodius pugil was fully accepted on the World Register of Marine Species as being part of the Pilodius genus. This new genus proposed by Lasley et al. (2015) includes Pilodius pugil, Pilodius pubescens, Pilodius scabriculus and Pilodius spinipes. These species were included in this new genus because they all show a unique sickle-shaped gonopod (specialised pleopods) tip. Although this new genus is not fully accepted yet, it does separates even further the Chlorodiellinae subfamily by observing particular characteristics that are only present in a group of species. 
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Figure 15

Classification
Kingdom 
Animalia

Phylum 
Arthropoda

Subphylum 
Crustacea

Class
Malacostraca

Order 
Decapoda

Infraorder
Brachyura

Superfamily 
Xanthoidea

Family 
Xanthidae

Subfamily
  Chlorodiinae

Genus 
Pilodius

Species 
Pilodius pugil

Taxonomic confusion between P. pugil and P. spinipes
Two of the most relevant studies performed in the Pilodius genus have reached to the conclusion that there are many taxonomic problems associated with this genus (Clark and Galil, 1993; Serene, 1984). 43 species have been referred to Pilodius but on every new taxonomic revision, species are taken out and allocated on a new taxon (Lasley et al., 2015). According to Clark and Galil (1993), the major problem when identifying species from Pilodius is that only male individuals can be correctly identified and described. Females show different carapace and cheliped colouration making identification barely possible (Clark and Galil, 1993). Other characters such as cheliped morphology and carapace dentation are appropriate morphological characters but are extremely variable and it dependent on the state of maturity of the specimen (Clark and Galil, 1993). These issues when identifying species has caused many taxonomic problems. Several taxonomic problems are revised in Clark and Galil (1993), but the main taxonomic issue is the similarity between Pilodius spinipes and Pilodius pugil. 

Nobili (1906) was the first study to comment on the similarities of Pilodius pugil and Pilodius spinipes. Nobili (1906) saw that the description of Pilodius pugil and Pilodius spinipes from Dana (1852) were the same. After observing such confusion, he examined a considerable amount of specimens of P. spinipes and reached the conclusion that the main difference between P. pugil and P. spinipes was the dentation of the carapace (Clark and Galil, 1993). Pilodius pugil had a three hook-like dentation on the anterolateral margin whereas P. spinipes had four. Then, Serene (1984) confirmed that this was the main difference between the two but suggested that other characteristics to separate them were needed because the dentation of the carapace was not a consistent characteristic of the population she examined. After this study was published, few years later Clark and Galil (1993) created a dichotomous key to the species of Pilodius where P. pugil was separated from the other species by a 3-dentate anterolateral margin (Figure 16). However, this only added more confusion since the 4-dentate carapace that was described as being for P. spinipes is also found in all the other Pilodius species(Clark and Galil, 1993). In their study, they recognise that there is still a considerable confusion on the characters that separate P. pugil from P. spinipes and all the other Pilodius species (Clark and Galil, 1993). Such confusion is still unsolved, as established by Lasley et al. (2015). Lasley et al. (2015) separated some of the Pilodius species into a new genus called Luniella, but P. pugil and P. spinipes are still in the same group. In addition to this, when we observe the phylogeny shown in Figure 15 we can see that spinipes are the sister species of pugil, meaning that they are very closely related and that separating them into different species is difficult. Since there are not enough external characteristics, detailed internal characteristics must be performed in these species to solve the confusion. However, one factor used by Serene (1984) and Clark and Galil (1993) to separate these species is the geographical distribution. 

Pilodius spinipes is mainly restricted to the Red Sea and the Indian Ocean, as established by Serene (1984) and Clark and Galil (1993). Pilodius pugil, on the other hand, has a wider distribution, and it is found in the Red Sea, the Indian Ocean and also in the Indo-Pacific. Therefore, if the species is located in the Indo-Pacific then there are high chances that it is P. pugil, but if described in the Indian Ocean the confusion between the two might be present (Lasley et al., 2015). Nevertheless, maps from Global Diversity Information Facility (2010) and Encyclopedia of Life (2010) showed that some P. spinipes specimens have found and described in the Indo-Pacific. Even though these studies state the presence of this species in the region, this is highly unlikely and contradicts all the studies made with the Pilodius genus. 

In conclusion, the similarities between P. pugil and P. spinipes have been addressed, but with no clear solution to the taxonomic issue. Carapace dentation and phylogenetic relationships have been studied, but it has indicated that these species are very similar, and the possibility of joining into one taxon might be possible (Clark and Galil, 1993; Lasley et al., 2015). On the other hand, the distribution of both species have been recorded, but some inconsistencies are present, which adds more confusion when describing these two crab species. This taxonomic issue may only be solved by observing detailed and concise internal characteristics that are not variable as external characteristics.
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Figure 16

Conservation and Threats
Pilodius pugil as with many crustaceans suffer few direct threats. Due to the small size and toxicity of this species, P. pugil is not consumed by humans anywhere in the world (Clark and Galil, 1993). However, like all species linked with coral reef ecosystems, P. pugil might be threatened by increased ocean temperatures and ocean acidification (Doropoulos et al., 2012). Increased ocean temperatures result in a process called coral bleaching which can reduce crab density and affect the fecundity of these animals (Doropoulos et al., 2012). On the other hand, ocean acidification poses a threat to crustaceans because when there are high volumes of CO2 dissolved in water, the carapace of these animals is dissolved (Kelley et al., 2015). Although these changes in the ecosystem might affect them, crustaceans have the ability to distribute efficiently and adapt to new or stressed habitats (Kelley et al., 2015). It has been hypothesized that movement of crabs out of coral communities might occur if the animals are unable to adapt to such stressed areas (Kelley et al., 2015). 

While certain ecosystems might be under threat because of pollution, coastal development or tourism, Pilodius pugil is still a widespread and commonly found crab species thought its habitat range (Encyclopedia of Life, 2010). However, better and more efficient conservation measurement are needed to protect P. pugil and many other marine species from future human-related impacts. Protection and preservation of marine species can only be achieved by promoting awareness of all the animals that live in coral reefs, not only the charismatic vertebrates but also the small invertebrates that exist in these ecosystems.

References

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