Soldier Crab Research Articles

Discontinuous Turning Reaction during Escape in Soldier Crabs

ABSTRACT In many animals, the approach of an object is perceived visually and elicits an escape reaction (Dill, 1974). The escape is often aimed directly away from the threat. In some cases, however, the animal heads towards a nearby refuge, or another preferred place. Blue crabs, for instance, escape from the shore into deeper water (Woodbury, 1986). When ghost crabs are threatened while foraging far away from their burrow, they may change their heading erratically during escape in response to the visually perceived movement of the approaching object (Vannini, 1980). Soldier crabs (Mictyris longicarpus Latreille) face a similar problem because they live on estuarine mud flats and have no permanent burrows. They may react to an approaching person at a distance of 29–33 m, i.e. when the person has a vertical angular size of 2.2–3.2° at the eye of the crab. This illustrates the high vertical visual acuity across the acute zone of the ‘flat-world type crab’ Mictyris (Zeil et al. 1986). Usually, they can avoid being caught by ‘screwing’ themselves into the substratum with astonishing speed (Cameron, 1966). On hard substrata, however, they have to escape. Unlike ghost crabs (Burrows and Hoyle, 1973), soldier crabs are slow, with a maximum velocity of about 10 cm s−1. This may be a consequence of their habit - atypical of brachyurans - of walking exclusively forwards (Sleinis and Silvey, 1980). During escape, soldier crabs walk on a straight path away from the approaching object and usually change their direction only in response to changes of the approach direction of the threatening object. I observed that when one walks around a soldier crab on a circular path at some distance, it still walks along straight paths and changes direction by making abrupt turns from time to time.

The morphology and vasculature of the lungs and gills of the soldier crab, Mictyris longicarpus

The five gill pairs of Mictyris longicarpus have the lowest weight specific area reported for any crab. The cuticle of the gill lamellae is lined with epithelial cells which have structural features characteristic of iontransporting cells. Pillar cells are regularly distributed in the epithelium and serve to maintain separation of the two faces of the lamellae. The central hemolymph space is divided into two sheets by a fenestrated septum of connective tissue cells. The dorsal portion of the marginal canal of each lamella receives hemolymph from the afferent branchial vessel and distributes it to the lamella while the ventral portion of the canal collects hemolymph and returns it to the efferent branchial vessel. The lung is formed from the inner lining of the branchiostegite and an outgrowth of this, the epibranchial membrane. Surface area is increased by invagination of the lining which forms branching, blind-ending pores, giving the lung a spongy appearance. The cuticle lining the lung is thin and the underlyng epithelial cells are extremely attenuated, giving a total hemolymph/gas distance of 90-475 nm. Venous hemolymph is directed close to the gas exchange surface by specialised connective tissue cells and by thin strands of connective tissue which run parallel to the cuticle. Air sacs are anchored in position by paired pillar cells filled with microtubules. Afferent hemolymph is supplied from the eye sinus, dorsal sinus, and ventral sinus. Afferent vessels interdigitate closely with efferent vessels just beneath the respiratory membrane. The two systems are connected by a "perpendicular system" which ramifies between the airways and emerges to form a sinus beneath the carapace and then flows back between the air sacs to the efferent vessels. The afferent side of the perpendicular system is the major site of gas exchange. Efferent vessels return via large pulmonary veins to the pericardial cavity. Pa O2 levels were high (95.5 Torr), indicating highly efficient gas exchange.

Experimental studies of food ingestion and assimilation of the soldier crab, Mictyris longicarpus Latreille (Decapoda, Mictyridae)

Mictyris longicarpus Latreille removed significant amounts of organic material during surface-feeding and during hummocking. The percentage of organic material removed averaged 3.7 and 7.7 while feeding on the surface and 7.2 and 5.0 while feeding during hummocking for nitrogen and carbon, respectively. The amount of inorganic material ingested was found to be dependent on the physical nature of the substratum. It was estimated that during surface-feeding, M. longicarpus ingested 0.234 mg for every g of substratum sorted. Bacterial carbon represented 10% of organic carbon in the substratum and 35% of organic carbon in the proventriculus whereas algal carbon represented 10% and 1% of organic carbon in the substratum and proventriculus, respectively. M. longicarpus had a high assimilation efficiency varying from 86.9 to 93.8%.

Visual distance‐keeping in the soldier crab,Mictyris platycheleslatreille (Grapsoidea: Mictyridae). A field study

A visually‐guided reaction for keeping their distance from vertical objects has been discovered in the Australian soldier crab, Mictyris platycheles. Feeding on muddy sand beaches, soldier crabs drop pellets, which soon densely cover the whole feeding area. If any object is placed on the feeding area before feeding commences, a pellet‐free zone is left around the object. Setting up black and white plates of different sizes showed that the crabs’ approach to these objects is related to the contrast between the object and the background and to the ratio of a target's height to its width.

Locomotion in a forward walking crab

1. Mictyris platycheles, the soldier crab, walks predominantly forward (Fig. 2). Two-thirds of the crabs studied used all 10 legs in walking; the remainder held up the chelae and walked with 8 legs. 2. The soldier crab is egg-shaped in contrast to laterally elongated sideways-walking crabs. Its joints which allow anterior-posterior leg movement describe larger angles than in sideways-walking crabs of a similar size, while its joints which permit lateral movement describe smaller angles (Fig. 1). 3. Mictyris walks mainly with two gaits: one overlapping (2534 or equivalents; >50% of the time) and the other metachronal (2345 or equivalents; >30% of the time) (Tables 1 and 2). It uses both at all but the highest stepping frequencies at which the overlapping gait predominates (Fig. 5). 4. Crabs stepped between 2 and 4 steps/s. With increasing rate the durations of both power and return strokes, which are nearly equal in each leg except leg 1, decreased (Fig. 7). 5. Ipsilateral legs 1 and 4 in 10-legged crabs and ipsilateral legs 2 and 5 in all crabs tended to step together (Fig. 8). Adjacent legs on the same side of the body stepped neither in phase nor in antiphase but with a mean phase relationship of 0.33. Contralateral legs of the same segment stepped nearly in opposite phase (Fig. 6 and Table 3). 6. The stepping pattern (Figs. 3, 4 and 9) is generated by alternation at one-sixth of a step cycle between sets of one or two legs on opposite sides of the body. At any given moment during walking crabs support themselves by various numbers of legs. 7. To walk forwardMictyris uses strategies of promotion and remotion similar to those employed by long-bodied decapod crustaceans. It also uses the same gaits and phase relationships on both sides of the body. In these respects the walk ofMictyris more nearly resembles those of other forward-walking animals rather than sideways-walking crabs.

Mechanisms for obtaining water for flotation feeding in the soldier crab, Mictyris longicarpus Latreille, 1806 (decapoda, mictyridae)

During feeding Mictyris longicarpus Latreille collects interstitial and free standing water via setae on the posterior border of the carapace and the proximal edge of the first abdominal segment. Water is lifted to the bases of the scaphognathites by capillary action, travelling from the posterior setae into a posterior chamber, along lateral canals, and through the branchial chambers under the epibranchial membranes. The scaphognathites provide the necessary force to pump water into the buccal cavity. Water which flows out of the buccal cavity is collected between the abdomen and cephalothorax by setae fringing the abdomen, and returned to the posterior chamber by the ventrolateral canals.

Effect of Temperature on the Emergence from Burrows of the Soldier Crab, Mictyris longicarpus (Latreille)

Emergence of M. longicarpus from their burrows was studied in the laboratory under controlled conditions of tide, light and temperature. More crabs emerged at hotter temperatures than at colder temperatures within the range 14.5-29.0�C. However, most crabs emerged within 1 h of the water draining from their tanks regardless of temperature within the same range. These patterns of emergence seem to be important to the crab's feeding behaviour. It is advantageous to emerge at higher temperatures when feeding is more rapid than at lower temperatures. Early emergence enables the crab to undertake long treks to obtain food.