The orange-spined sentinel crab, Macrophthalmus crassipes, is a decapod crustacean belonging to the highly diverse taxa Brachyura or the true crabs. Within the superfamily Ocypodidae, the orange-spined sentinel crab is further classified into the genus Macrophthalmus, which has one of the greatest intrageneric diversities within the Brachyura (Barnes, 2010). The species is found within muddy intertidal zones, often seen amongst seagrass beds. The long eyestalks are an obvious feature of this species, enabling them to maintain vision while submerged within tidal pools. However, the name “orange-spined” is due to the fact that the male chela has a spine near the carpal joint. M. crassipes is a highly understudied species, with little information on it’s biology. The main study focus of M. crassipes was to undergo a field study to investigate the relationship between appendages of the orange-spined sentinel crab using morphometric analysis. 

Due to the limited literature available for M. crassipes, the following will provide descriptions to the many aspects of this species biology and ecology to a level no higher than order and providing information at the species level when possible. All photos and videos on this site are taken by Jacqueline Thomson at the University of Queensland or otherwise noted.

Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Section: Eubrachyura
Subsection: Thoracotremata
Superfamily: Ocypodoidea
Family: Macrophthalmidae
Subfamily: Macrophthalminae
Genus: Macrophthalmus
Subgenus: Macrophthalmus
Subgroup: Brevis
Species: Macrophthalmus crassipes

M. crassipes occurs within the intertidal zones around the world, predominantly within water-logged muddy sand flats (Barnes, 2010) and seagrass beds directly down from sandy shores (Woods & Sheil, 1977) (see Figure 5).  Observations of the species within the field noted M. crassipes to be found mainly taking shelter amongst seagrass beds within the intertidal zone of Morton Bay, Qld (see Figure 6). The species has been noted to have a patchy distribution, forming clusters (Woods & Sheil, 1977). As well, sentinel crabs have been shown to have species-specific zonation patterns along the mudflats (Schuwerack et al., 2006). Within the field, M. crassipes was most abundant on the mid mudflat, where seagrass was also abundant and food concentrations were higher (personal observation). This area of the mudflat remains submerged for longer periods allowing the temperatures to be less extreme. Within the upper mudflats, larger individuals burrowed at lower densities within residual seawater pools (personal observation). There was less food and seagrass within this area as well as light-blue soldier crabs, Mictyris longicarpus, were highly abundant.

Sentinel crabs are deposit feeders, mainly feeding on microphytobenthos and/or detrital material (Barnes, 2010). The video below demonstrates the filtering behaviour  and 'scooping' motion towards the mouthparts by M. crassipes. Note the maxillipeds move rapidly to manipulate the food particles towards the mouth. Some sentinel crabs have also been known to scrape algae off hard strata (Wada & Wower, 1989; Kosuge & Davie, 2001) and/or consume dead or alive animal material (Kituara & Wada, 2005).

M. crassipes is a major food source for many local and migratory shorebirds including the eastern curlew, Numenius madagascarien, who preys on the orange-spined sentinel crab during its non-breeding season (Zharikov & Skilleter, 2004). 

Most decapods are gonochoric with a few being hermaphroditic (Rupert, Fox & Barnes, 2004). Males indirectly transfer spermatophores to the female via gonopods (Rupert, Fox & Barnes, 2004). Olfactory (pheromone) cues are used to attract mates while some males will use courtship displays to attract females, as is shown in the orange-spined sentinel crab (Kitaura & Wada, 2004). The females will brood an egg mass on the pleopods and hatching takes place at the zoea larval stage (Rupert, Fox & Barnes, 2004). 

True crabs (Brachyura) are a highly successful group of crustaceans, which is likely coupled to their life history strategy (Martin, 2014). This evolutionary success includes a pronounced metamorphosis, developing through multiple states of planktonic zoea larva, 1- 5 stages (see Fig. 7). The final zoea larva will eventually moult into a megalopa, which is also a swimming larva. The megalopa larva is both a morphological and ecological transitional phase linking the planktonic zoea and benthic adult form (Martin, 2014). The megalopa larva will settle on suitable substrate and further moult into the first stage adult crab. Within Brachyura there can be many deviations within the described developmental sequence. Some larval stages can be skipped and some hatchlings may develop directly (Martin, 2014).

At present, there is very little information known about the larva of Macrophthalmus species let alone for the orange-spined sentinel crab. However there is some information known about the first stage zoea larva for Macrophthalmus species, which includes Macrophthalmus brevis where the orange-spined sentinel crab sits. Table 1 below shows some characteristics common amongst the zoea for the subgroup Macrophthalmus brevis.

Table 1: Summary table of the first zoea characteristics for the subgroup Macrophthalmus brevis adapted from Kitaura et al. (2010).

Carapace						Antenna	Maxilliped1	Abdomen	Telson
	Lateral Spine (mm)	Dorsal Spine (mm)	Rostral Spine (mm)	Carapace length (mm)	Total length (mm)	Exopod Medial setae	Basal Setation	Lateral Knobs
M. brevis	-	+ (0.105)	+ (0.157)	?	0.78	0	(6)	2-3	Fork< body

+, presence; -, absence; ?, no description available.

Sentinel crabs of the genus Macrophthalmus exhibit diverse and developed social behaviors such as territoriality against individual burrows and feeding grounds, allocleaning, fighting and wave displaying behaviour (Kitaura & Wada, 2004). There are four patterns of wave displaying that have been classified along with two types of fighting behaviors, either grasping or claw-extending fighting. This claw-extending behavior was observed being performed by M. crassipes within the field. However, this behaviour abruptly discontinued once detecting the presence of the observer. The complex fighting behavior, claw-extending, has also been shown to have evolved secondarily in the species of the subgenus Macrophthalmus (Marcrophthalmus) Desmarest, 1823, which includes M. crassipes (Kitaura &Wada, 2004) (see Fig. 8). 

Allocleaning behaviour, where one crab feeds off the carapace or walking legs of another crab, is common amongst Macrophthalmus and is typically observed within sentinel crabs (Kitaura & Wada, 2004).This mutually beneficial interaction provides a food source to the cleaner and the cleanee benefits from the removal of epibionts and sediment, which prevents fouling of the exoskeleton (Bauer, 1981). There are two types of allocleaning behaviour, the first of which is long-duration cleaning associated with foraging. Typically the smaller individual cleans and afterwards may be allowed to forage within the larger individual's feeding grounds after the cleaning event. The second type of allocleaning is short-duration cleaning associated with male courtship (Ueda & Wada, 1996).

A self-cleaning behaviour is commonly developed within decapods due to the negative impacts of harbouring epibionts or mud on the body surface (Bauer, 1981). Another suggested benefit from allocleaning may be the benefit of increased fitness, as the presence of neighbours involved in cleaning, might assist in predator defense (Fujishima & Wada, 2013).

Sentinel crabs live in burrows, typically built at an angle (Schuwerack et al., 2006). Burrowing behavior was observed often by individuals of M. crassipes within the field and within the aquaria. A scooping motion of sediment by the perepods and chela was used to move the body in a sideways manner under the substrate (see video below).

The center of gravity is shifted forward under the thorax and walking legs in Brachyurans. This shift is due to the flexion and reduction of the abdomen, which causes Brachyuran crabs to move sideways, especially when needing to move quickly (see video below). The leading legs pull by flexion and the trailing legs push by extension, to create this movement (Rupert, Fox & Barnes, 2004). The chelipeds are typically not used to create movement when crawling (Ruppert, Fox & Barnes, 2004). Observations within the field, noted the species to be visibly active around low tide within the afternoon and hidden within burrows during early morning low tide.

The long eyestalks of M. crassipes allow them to remain submerged within intertidal pools aiding in predator avoidance yet allowing vision to be maintained (personal observation). Macrophthalmus readily responds to predators by retreating down into their permanent burrows and will return to the surface once the predator leaves the vicinity (Zharikov & Skilleter, 2004). The following videos show individuals of M. crassipes camouflaged amongst the seagrass beds and their ability to use there long eye stalks to maintain vision and remain submerged.

Macrophthalmus typically feed during ebb tides on the same mudflat, spending most of their time feeding and only a short time mating (Schuwerak et al., 2006). Juveniles are noted to feed in the vicinity of their burrows while adults and larger individuals will wander within their zonal band on the mudflats (Schuwerak et al., 2006). The larger chela of males has been shown to reduce the overall rate of food intake and scoop rates compared to the smaller chela of females (Schuwerak et al., 2006). Males will use both chelae to feed, scooping or pinching several times before moving the chela to their mouthparts. This behavior enables them to increase the quantity of food they ingest (Schuwerak et al., 2006). The video below shows an individual of M. crassipes continuously filtering water while moving and burrowing. 

The carapace of M. crassipes covers the cephalothorax and pereon dorsally and laterally. The segments of the thorax can be seen ventrally but not dorsally (see Fig. 10). Decapods have five pereopods (legs), the first are the chelipeds (chela) used in feeding, wave displays, courtship and agonistic behaviour (Kitaura & Wada, 2004). The four other pereopods are used in walking and burrowing (Ruppert, Fox & Barnes, 2004). Each pereopod consists of segments termed as the carpus, merus, propodus and dactylus (see Fig. 9). The abdomen is reduced, inconspicuous and flexed tightly under the pereon. The abdomen is only visible when the individual is upside down (see Fig. 10). Within the sentinel crabs the uropods are lost while the pleopods are retained within the females and are used to hold brooded egg masses under the flexed abdomen (Rupert, Fox & Barnes, 2004). Pleopods in males are lost except for two anterior pairs located under the abdomen and are used for transmission during mating (Rupert, Fox & Barnes, 2004). The maxillipeds, modified legs, are located near the mouth to aid in food manipulation. Sexual dimorphism is apparent in the sentinel crabs (Zucker, 1988). Female's have smaller chelae while males have both medium to large sized chelae (Zucker, 1988).

The crustacean reproductive cycle consists of multiple stages which include the maturation of gonads, reproductive behaviour associated with moulting, males transferring sperm during copulation, ovulation, egg-laying and finally incubation of the eggs by the females (Sastry, 1983). As little information is currently available for reproductive anatomy for the genus Macrothalmus, the following provides a description of the Brachyuran reproductive anatomy for males and females, which is synonymous among many groups.

Male Reproductive Anatomy

The reproductive system is located just beneath the dorsal carapace and is found in the anterolateral part of the cephalothorax (Nicolau et al., 2012). The system is bilaterally symmetrical and H-shaped (see Fig. 11). A pair of tubular testes are joined by ejaculatory ducts and the vas deferens (Nicolau et al., 2012). The gonads are curved (see Fig. 11) and run laterally to the stomach, joining posteriorly to the hepatopancreas.


Female Reproductive Anatomy

The reproductive system is homologous to the male reproductive system, having bilateral symmetry and a similar ‘H’ shape (Nicolau et al., 2012). The ovaries are joined by a transverse bridge and located in the anterior part of the cephalothorax (Nicolau et al., 2012). The bottom region of the ‘H’ shape of the reproductive system is formed by the sprematheca and gonoducts (see Fig. 12), which are positioned posteriorly.

M. crassipes, belongs to the phyla arthropoda, the largest group of metazoans, containing about 80% of all known animal species (Ruppert, Fox & Barnes, 2004). This phyla is characterised by segmentation, grouping of segments (tagmosis), having paired, jointed appendages, having an exoskeleton and cephalization (Ruppert, Fox &Barnes, 2004).

Within arthropoda lies the subphylum crustacean, where its members evolved within the sea and is the only major arthropod taxa to be exclusively aquatic. Crustacean heads typically bare five pairs of appendages: two pairs of antennae, the mandibles and two pairs of maxillae (feeding appendages) (Ruppert, Fox &Barnes, 2004).

Sentinel crabs belong to the class Malacostraca along with other crabs, lobsters, crayfishes and shrimps. Characterised by having eight segments and having abdominal appendages (excluding Remipedia). These crablike decapods are included within the infraorder Brachyura, the true crabs. Brachyura have a highly specialised body form and are the most successful decapods in terms of species richness (Ji et al., 2014). Classification of groups within the Brachyura are based mainly on the position of gonopores (Ji et al., 2014). M. crassipes, belongs within the semi terrestrial superfamily Ocypodoid and within the genus Macrophthalmus of the family Macrophthalmidae. Individuals of  the genus Macrophthalmus are characterised by the spoon shaped tips of the fingers of chelea (Barnes,2010).

Over the past 30 years, a lot has happened with the systematics of the genus Macrophthalmus Desmarest, 1823 (Barnes, 1977). New molecular sequence and allozyme data have challenged the placement and understanding of the relationships within this genus (Barnes, 2010). Macrophthalmus, for many years was believed without question, to be regarded as a subfamily of the Ocypodidae Rafinesque, 1815 (Barnes, 2010). This system was adopted by Martin & Davis (2001) and Sakai (2004) in a recent classification of the genus. Their belief far removed the genus systematically from various genera, which at the time formed the Grapsidae Macleay, 1838 (Barnes, 2010).  Kitaura, Wada & Nishida (2002) however,strongly suggested through molecular sequence data that Macrophthalmus is more closely related to grapsids than to any other crab within the Ocypodidae (Barnes,2010). This conclusion by Kitaura etal. (2002) was confirmed by further sequence data of Schubart et al. (2006). As well Schubart et al. (2006) concluded Macrophthalmus is closely related to mictyrids and gecarcinids.

Ng et al. (2008) place Macrophthalmus as a component genus of the superfamily Ocypodoidea while maintaining the grapsidgenera, associated with Macrophthalmus,within the Grapsoidea (Barnes, 2010). They believe using both molecular and morphological approaches to explain their placement of  where Macrophthalmus sits, will eventually emerge. However, over the last 10 years, there has been a great debate on molecular vs. morphological approaches for the relationship for some arthropod groups (Giribet & Ribera, 2000; Cook et al., 2005). Therefore the family,Macrothalmidae, is beset placed as a separate family outside the ocypodid +grapsoid complex (Kitaura et al. 2002). The family Macrophthalmidae comprises four genera: Enigmaplax Davie, 1993, Lutogemma Davie, 2009, Australopax Barnes, 1966, and finally the genera to which the orange-spined sentinel crab belongs, Macrophthalmus (Barnes 2010).Specifically, the orange-spined sentinel crab belongs to the highly speciose brevis subgroup of the subgenus Macrophthalmus (Macrophthalmus) Desmarest, 1823 (see Fig. 15).

There is much disagreement between the clades identified by molecular sequence and allozyme data and to those identified based on morphological similarities and differences within the subgroups of Macrophthalmus (Barnes, 2010). Kitaura et al. (2006) suggests that a functional convergence of body form between the members of the different Macrophthalmus clades may have occurred, as they have identified a correlation between the type of sediment inhabited and morphology. Barnes(2010) suggests a need for a complete molecular analysis of all species in order to create a clear case for elevating some or all subgroups within Macrophthalmus.