L. variegatus is an opportunistic omnivore (Skilleter & Anderson 1986). Specimens have been observed and filmed grazing on clumps of Corallina spp., Ulva lactuca and various filamentous red algae such as Polysiphonia and Ceramium, scrapping off encrusting algae and demonstrating (Skilleter & Anderson 1986). There is a tendency for larger amounts of coralline turf ingested after ecdysis (McLay 1988). However the crabs show a highly selective diet if the alga Enteromorpha is present, with larger males and females (though to a lesser extent) feeding almost exclusively on it (McLay 1988).

The crabs have also been observed attacking and eating limpets Cellana tramoserica and Siphonaria denticulate, barnacles Tesseropora rosea and Chthamalus antennatus, the small black mussel Xenostrobus as well as eating miscellaneous pieces of fish (Skilleter & Anderson 1986; McLay 1988). Pieces of fish were also accepted when being kept in aquariums and various images can be found on the internet of Leptograpsus eating various smaller crabs and frogs and opportunistic scavenging on dead seabirds. It has been noted in scientific literature that L. variegatus eats carrion when it is available (Skilleter & Anderson 1986) and that sightings of marine crabs eating freshwater frogs are particularly rare (Pyke et al. 2013)

It has been noted by McLay (1988) that diet is affected by the size of the crab, with smaller crabs feeding on littorines and barnacles and larger crabs feeding on barnacles, small black mussels Xenostrobus pulex, chitons and large gastropods.

Greenway et al. (1992) puts Leptograpsus variegatus into Hartnoll’s (1988) Category T3 due to its independence from water and its supratidally residency and that it is active in air.

L. variegatus is often found well above the high-tide mark on algal covered rocky outcrops (Cooper & Morris 1997) and spends most of its time in crevices during the daylight hours and foraging both on the rocks and in water at night time (Forster et al. 1989; Greenaway et al. 1992). It is likely that the crab spends the days in dark crevices in rocks near the splash zone to control its body temperature (Greenaway et al. 1992). Within the endemic group of grapsid crabs, L. variegatus occupies the highest position on the shore and is often the largest invertebrate predator on many of the beaches that it inhabits (Forster et al. 1989). It has the ability to fully take advantage of both terrestrial and aquatic habitats, from supratidal to brackish, to fully marine due to its ability to resemble water-breathing crabs when immersed (Cooper & Morris 1997). It moults in rock pools around the high-tide mark, however does not eat its exoskeleton and proceeds its foraging (Morris & Greenaway 1992). 

L. variegatus can live for over five years and their population is often multimodal with two-five modes (McLay 1988). They are preyed upon by conspecifics, rats, tuataras (McLay 1988).

The abundance of L. variegatus is shown to have a significant effect on both the mussel Xenostrobus and barnacles. The crab appears to have an effect on the formation of mussel mats, often preventing it altogether, and reduces the average mussel and barnacle size in the local area (McLay 1988). As such, the crabs are a major creator of bare space for new settlement (McLay 1988).

Chelipeds were used in alternating cutting-scraping movements in order to finely shred and eat algal clumps before reaching the maxillipeds (Skilleter & Anderson 1986). Mandibular palps (Figure 5) then push the food fragments directly into the mouth. Feeding activity is not restricted by tidal cycles (Forster et al 1989)

When preying on limpets, the crabs used one or both of the chelae to pry up the edge of the limpet before turning it over and exposing the unprotected foot as the limpets moved around the rocks (Skilleter & Anderson 1986). While this method was not successful 100 percent of the time (if the limpets were able to reattach themselves the crab would move on – an interesting note on natural selection), the crab would always carry the limpet (or any other food) above the wave action before feeding on it (Skilleter & Anderson 1986).

Foraging behaviour differs with crab size: larger crabs are shown to migrate down the shore and rocks as far as the sublittoral fringe (McLay 1988). There, there is an abundance of food and prey types and the crabs return to their preferred crevices as the tide comes in (McLay 1988). In contrast, smaller crabs have a much smaller, more restricted area (McLay 1988), perhaps due to competition. Feeding is most intense in the upper eulittoral area where crabs are more abundant and can feed for longer periods due to the longer exposure (McLay 1988).

The tips of both dactyls (fingers) are hardened with the tips slighting indented to essentially form a scoop (Skilleter & Anderson 1986). The main use of the chelae is grazing and transporting food to the mouth – when grazing on filamentous algae or algal tufts the chelae alternately pluck, shred and pass food to the mouth, whereas when feeding on encrusting algae, the hard, hollow tips of the chelae scrape the algae from the rock’s surface (Skilleter & Anderson 1986). However, when feeding on flesh, the chelae shred it into to small pieces and then pass it to the mouthparts (Skilleter & Anderson 1986) possibly due to the likelihood of larger sized fragments. The third maxillipeds are not used in feeding, and are instead held away from the mouth at a 45° angle (Skilleter & Anderson 1986). Similarly, the mandibles are also not primarily used in feeding, as the cutting edges do not overlap and are not used to hold food as the chelae tear it (Skilleter & Anderson 1986). 

Behavioural Response Experiment – Fight or flight response

 Introduction

In the wild, many species compete for food, water, shelter and mates and their survival depends on their response to both intraspecific and interspecific competitors. Grapsid crabs are known to be aggressive and antagonistic to each other when competing, often resulting in injury of the less competitive (Warner 1970).

The point at which Leptograpsus variegatus knows whether to fight or flight is discussed in relation to proximity and relative size. It was observed that when L. variegatus was placed with a conspecific of a slightly smaller size, the larger crab would actively chase and use its chelipeds to attempt to injure the smaller crab. In response, the smaller crab would exhibit bubble blowing behaviour both during and after and attempt to escape the larger crab rather than fight back. At this point the crabs were separated. This action is reflected in the Grapsid crab Aratus pisoni, with the smaller crabs actively avoiding the larger crabs to delay engagement until the crabs are closer than 10cm in which case the smaller is pursued by the larger  (Warner 1970). In Warner’s study (1970) the fighting continued with no injury until one crab eventually broke away in retreat. While this behaviour may occur in the wild, when captive in aquarium and there is limited area to escape, the reduction in sample size across the weeks

Methods

L. variegatus was collected at night during the low tide from the Gold Coast spit. It should be noted that of the initial sample size of seven, by the time of experimentation only one crab remained as four others escaped the lidded aquarium and were found desiccated on the floor, ones body parts were found broken up within the aquarium and one went missing and was thought to be killed by the surviving crab. The remaining crab was male with a carapace of 4.2cm.

The single remaining L. variegatus was placed in an aquarium and submerged in water and allowed 15 minutes to acclimatise to its new environment. No rocks were placed in the aquarium to allow the crab to hide. Before each trial began, a time period of five minutes was allowed for the crab to return to normal, relaxed behaviour. A ruler was placed along the side of the aquarium and, with the crab on one side of the tank (Figure 6), a mirror was placed 30cm, 20cm, 15cm and 10cm away from the crab. As studies have shown that L. variegatus eyes perceive movement (Sandeman 1978), the mirror was gently moved side to side without hitting the bottom of the aquarium or the sides. The crab’s response was timed from time of first reaction until there was no response. When the crab did not respond at all, the mirror was removed from the tank after a period of three minutes. Each distance was replicated five times.

The responses were categorised as such: fight (Figure 7) (in which the crab would show constant aggression towards the reflected image), flight (Figure 8) (where the crab would attempt to escape the image), both (when the crab would initially approach with an aggressive response but then retreat), neither (when the crab would neither show total aggression or total withdrawal but displayed behaviour observed of both characteristics). The behavioural characteristics documented were chelipeds raised, bubble blowing behaviour, mouthparts moving, body position (raised or withdrawn) and movements (approach, retreat, none, both).

Results

 It can be seen from Figure 9 that there were no consistent responses with respect to how far the mirror was placed from the crab, but rather a gradual change in responses. The fight response primarily occurred when the mirror was further away and presenting a smaller crab, whereas the flight response tended to occur when the mirror was closer to the crab and reflecting the image of equal or slightly lesser size, with occasional aggressive behaviour shown.

 Figure 10 shows the percentage in which the crab’s movement occurred in relation to the response. It can be seen that the crab only approached when it was displaying the fight response, whereas it only retreated during the flight response (predictably). Often, when showing the ‘neither’ response, it would approach the mirror slowly showing aggression before calming down and retreating to the side of the aquarium.

 Behavioural responses are shown in Figure 11. Moving mouthparts were common across all responses with varying occurrences. The fight response seemed to elicit the most behaviours, with 100 percent of the responses showing bubble blowing, raised chelipeds, a raised body and moving mouthparts. The body stance otherwise varied for the remaining responses; in one flight response occurrence the crab raised its body to quickly escape the reflection, compared to the relatively slower movement when retreating with a withdrawn body stance.

 While all these figures show gradual trends, the sample size was far too low to accurately demonstrate statistical significance, with an ANOVA between distance of mirror and response being p=0.109. The figures and the statistics are representative of raw data –averages were not taken, hence the lack of error bars. 

Discussion

 While the data above is statistically relevant, it can be seen that some behaviours and movements are linked to certain responses. The raised chelipeds occurred in both the fight and flight response, indicating that even in retreat, the crab was acknowledge that it may have to defend itself from its ‘competitor’.

 Relatedly, while insignificant statistically, it can be seen from the percentage of responses that the crab are more lightly to show the flight response when the mirror is equal to or less that 15cm away, with the majority of flight responses occurring when the mirror was 10cm away.

 The large percentage of ‘no response’ at the 30cm mark is interesting. While there was a 20 percent demonstrated flight response, it remains unclear whether the response is linked to the distance of the reflection (essentially 60cm away) or the seemingly smaller size of the crab reflected. All seven of the initial sample was collected on the two neighbouring algal covered stones that made up a sea wall, well within 60cm of each other. As no fighting was observed in their natural environment, further experimentation on a larger sample size will be required to narrow down what triggers the fight response.

 It is known that L. variegatus are especially active at night (Forster 1989), so it is likely that the noise and lights of the laboratory affected the response of the crab. The ability to observe the crabs in situ to their natural environment would certainly be noteworthy, as with Warner (1969). Additionally, the simulated movements of the crab’s reflection may not have been believed to be natural and it was hypothesised that the constant movement of the mouthparts was to taste the water, as the chemosense of the crab may not have been familiar with the mirror’s materials. These factors are both likely to have affected the response.

 Similarly, if this study were to be repeated it would have been of interest to see whether the gender of the crab had an effect on the response. Should the sample size have been bigger, I would have also attempted to measure the chelipeds of each specimen to see if that had an effect on the response.

 Having the number of specimens to work with constantly changing severely limited the data and required a methodology change, thus the data collected simply represents what there was to work with at the time. Ideally, this experiment would be replicated with live specimens rather than a reflection so that the size ratio and proximity of the conspecifics could be measured to see what affected response the most.

When mating,chance contact may result in a male and female initiating leg contact during which legs are rapidly vibrated (McLay 1988). After this period, either one of the crabs may disengage and wander off, or the male will be allowed to mount the underside of the female (McLay1988). L. variegatus is fast-running and difficult to catch. Each crab seems to have a preferred shelter location to which it returns after a foraging for a period of time; similarly, each crab appears to several preferred sites for foraging (McLay 1988).

Interestingly, the lunar cycle appears to affect crab behaviour however this only occurs during winter (McLay 1988). This behaviour seems to especially affect smaller crabs (McLay 1988). During this time, few crabs are active during the full moon; the peak of activity starts ten days before and ends two days after the new moon (McLay1988). Similarly in winter, L. variegatus can go for several days without getting wet (Forster et al 1989).

Reproduction

L. variegatus mates in October-November, during which time the female has a hardened shell (McLay 1988). Females modify themselves to bear eggs in the summer months, between November to January in Wellington, NZ (Wear 1970), October to February in Chile (Antezanaet al. 1965) and November to February in Leigh, NZ (McLay 1988) . In Wear’s study (1970), all mature females were observed carrying eggs in December. Eggs are then incubated for approximated six weeks, with eggs beginning at a size of 0.37x0.35mm with a dark brown/black colouration and mature eggs increasing in size to about 0.44x0.42mm with a light brown colouration (Wear 1970). The unhatched larvae have large eyes and have, according to Wear (1970) “black to greenish-yellow chromatophores, which in daylight impart a green iridescence to mature eggs”. After some 55,000-144,000 dependent on size (McLay 1988) eggs are released, ovaries did not regenerate for a second spawning (Wear 1970), meaning that reproduction only occurs once a year. Moulting occurs some two months later (Griffin 1966).

Larvae

There is thought to be five zoeal stages of larvae (McLay 1988). Figure 12 shows the lateral and posterior view of the first zoea (Wear 1970). After these five zoeal stages, the larvae moults into a megalopa stage during which they settle outside of the water column and metamorphose into the first juvenile crab stage (Forward et al. 2001). This metamorphosis is triggered by cues, however can be delayed for a short period of time before it occurs naturally – for brachyuran crabs, this is about 20 days (Forward et al. 2001). These cues can shorten that natural time period by 15-25% and include chemical cues and odours from adult substrate, aquatic vegetation, biofilms, conspecifics, estuarine water, humic acids, related crab species and potential prey (Forward et al. 2001). Other cues, such as ammonium, hypoxia, predator odour and extreme temperature and salinity can delay metamorphosis (Forward et al. 2001).  During metamorphosis, the gastric mill is developed (Ceccaldi 1989).


Larval survival depends highly on salinity - the habitat in which the eggs hatch has great salinity variations. As such, grapsid crab larvae had a higher rate of survival and faster development in brackish water (15-25%) than in seawater (35%) (Anger 1996). The capabilities for larvae to gain tolerance to low salinities suggests that larvae gain increasing capability for hyper-osmoregulation (Anger 1996).

L. variegatus can quickly regulate its internal osmolality by actively absorbing ions and optimisation of the Na+/K+ -ATPase pumps using dopamine (Morris & Edwards 1995). This control of the Na+/K+ -ATPase pumps also allows the crab to maximise salt uptake when underwater or reclaim ions from urine entering the gill chambers (Morris & Edwards 1995). Morris and Edwards (1995) also hypothesise that L. variegatus’s mid-gut may be able to reclaim ions from urine entering the digestive tract, making it an important intermediate species. 

Generally, the adult crab’s digestive tube is separated into three main parts, being the fore, mid and hindguts (Ceccaldi 1989). The foregut includes the oesophagus and the part of the stomach where masticating parts are present; the midgut has no chitin but has the hepatopancreas, which secretes digestive enzymes; the hindgut is quite straight and enlarged posteriorly into the rectum and terminates at the anus (Ceccaldi 1989).

The mouth is surrounded by prehensible appendages, being the maxillula, maxilla, mandibles and maxillipeds (Ceccaldi 1989). The oesophagus is usually short and straight in decapods, positioned vertically to join the mouth to the stomach (Ceccaldi 1989). In brachyurans, the oseophagaus enters into the antero ventral wall of the cardiac stomach.

Ceccaldi (1989) summarises the digestive process eloquently: “Food enters the stomach and moves towards the rear along the dorsal wall of the anterior pocket, then they pass through the gastric mill where they are ground before entering the pyloric pocket. The food is continually worked by lateral outgrowths (teeth and ossicles) and return anteriorly to pass through the system again. The liquid phase flows antero-posteriorly along the ventro-lateral sides leading to the principal channels of the pyloric pocket excludes any particles larger than 1nm of the secretory gland… Non digestible particles are rejected into the hindgut” (p248). Ossicles are most complex in Brachyuran crabs, compensating for the less complex mandibular appendages (Ceccaldi 1989).

The digestive gland of decapods is called the hepatopancreas, which is a large, bilobed organ and functions in transport, digestive enzymes, lipid storage, food absorption, glycogen as well as numerous minerals (Felgenhauer 1992). E-cells (‘embryonic cells’) are found at the blind ends of the tubules in the hepatopancreas and give rise to the F-cells (‘fibrillar’ cells) for protein synthesis and storage of minerals, B-cells (‘blister’ cells) that are secretory, and R-cells, which are the most numerous and function in food absorption (Felgenhauer 1992).

 The midgut is endodermally derived and extends from the foregut, through the heptopancreas and into the abdominal somites before joining the hindgut (Felgenhauer 1992). Both the hindgut and the foregut are ectodermically derived and lined with chitin (Felgenhauer 1992).

The heart is bulbous and dorsal and located to the posterior of the cephalothorax (Felgenhauer 1992). It receives blood through numerous ostia and is surrounded by a pericardial sac to which venous blood returns through passageways to the pericardial chamber (Felgenhauer 1992). Figure 16 shows the general layout of the circulatory system in decapods. 

L. variegatus is a bimodially breathing crab (Forster et al. 1989) and can tolerate long periods of immersion due to a slightly elevating its cardiac output without hyperventilating (Cooper & Morris 1997). However after a period of time, the crabs began to show significant metabolic acidosis (Cooper & Morris 1997), meaning that the kidneys do not remove acid from the system fast enough. Haemocyanin is responsible for 92% of oxygen transport within the body and a small amount carried in a simple solution (Greenaway et al. 1992). The crab has a normal resting pH of 7.82 (Greenaway et al. 1992). 

The urinary glads are a pair of excretory organs that are situated at the base of the second antennae and the excretory pore opens on the coxa of the antenna (Felgenhauer 1992). The coelomosac is mesodermally derived and act as a ultrafiltration system (Felgenhauer 1992). The labyrinth is a complex transport system and involves the movement of ions and the reabsorption of proteins (Peterson & Loizzi 1974)  while the nephridial canal (the distial and proximal tubules in the Figure 17) links the labyrinth to the bladder (Felgenhauer 1992).

L. variegatus have 18 gills, nine on each side and occupy about 5.66% of the body volume (Griffin 1966). When underwater the respiratory status of the crab resembles that of water-breathing crabs (Cooper & Morris 1997) despite the species being principally air-breathing and only experiencing brief period of immersion in well aerated water (Greenaway et al. 1992). This is supported by Forster’s (et al. 1989) study suggesting that L. variegatus is a facultative air-breather when given the choice. Predation, desiccation and perhaps hydromineral regulation are factors that cause L. variegatus to breathe water (Morris & Edwards 1995). Greenaway et al. (1992) proposes that the crab’s range and tolerance to aerobic activity suggests the presence of an efficient lung.

Marine crustaceans lose the majority of their body calcium for their exoskeleton during premoult and in the exuviae as their exoskeleton is mostly made up of calcium carbonate (Morris & Greenaway 1992). L. variegatus gains this calcium back primarily through the gut and its diet, which is possibly why the crabs resumed foraging in the wave zone while still very soft and vulnerable as they may attain vast amounts of calcium from the organisms on which they feed (Morris & Greenaway 1992). The gills also absorb a small proportion of the calcium required (Morris & Greenaway 1992). 

Three types of muscle fibers present in the crab’s connective tissue: the innermost are dialation muscle fibers, the middle are circular muscle fibers and the outer are longitudinal muscle fibers (Ceccaldi 1989). The muscles of the legs can be seen in Figure 21.