Introduction
Soldier crabs are semiterrestrial, intertidal crustaceans
well known for their tendency to form spectacular wandering droves or swarms of hundreds to thousands of brightly
colored (mainly blue) individual crabs on the tidal flat at
low tide. There are eight recorded species of mictyrid
soldier crabs (Table 1), and their biogeographic range
incorporates coastal areas in the Indo-west Pacific region
from the southern islands of Japan, through the China
Sea and the Southeast Asian region, to Australia
(Figure 1). Climate zones inhabited by the different
species range from tropical humid, to tropical arid, to subtropical and temperate zones. Tidal ranges are also diverse
with soldier crabs occurring in extreme macrotidal
regimes of up to 8 m in tropical areas in northern King
Sound, Western Australia, to microtidal 2 m tides in higher
latitudes such as Hobart, Tasmania. Most soldier crab
habitats occur in areas with semidiurnal or mixed tidal
cycles and less commonly with diurnal tides (Unno and
Semeniuk, 2009).
Soldier crabs are common inhabitants of estuaries, and
an understanding of the habitat requirements, behavior,
and life cycle of the crab combined with the knowledge
of the types of habitats occurring in estuaries explains
why this situation occurs. Soldier crabs are important in
estuaries as diet items for shore birds and fish, and as
burrowing organisms for their major bioturbation effect
on substrates in terms of sediment turnover, benthic
metabolism, and chemical properties (such as oxygenation
and nitrogen fluxes), and sediment structuring.
Habitat requirements of the soldier crab
The highly visible emergent phase of the soldier crab as
swarms or “armies” on the tidal flat surface is only a shortterm expression of what is largely a cryptic, benthic existence for the crab, as infauna within the substrate during
part of the low tidal and all of the high tidal periods. Soldier crabs have an intimate relationship with their habitat
as they reside beneath the surface, living, feeding, and
moving around in their subsurface habitat. Generally, they
do not occur deeper than 30 cm unless pursued by predators. The sediment characteristics of their habitat are
mostly fine to medium sand (quartz or calcareous) with
a small percentage of interstitial mud and organic matter.
Mictyris longicarpus, the largest of the soldier crab
species, has been found in a more diverse range of sediment types from coarse or very coarse sand to muddy
sand. Sediment pellicular water and groundwater salinity
are circa marine salinity, but soldier crabs are
osmoregulators and can tolerate considerable variations
in salinity (Barnes, 1967). With capillary tube lungs as
well as gills (Maitland and Maitland, 1992), soldier crabs
are obligate air-breathers, not subtidal crabs, and therefore
will drown if the frequency of inundation is too high.
However, they do require a moist environment, and
a shallow water table is necessary at low tide to allow for
pellicular water to be present in the substrate via capillary
rise. However, the substrate must be moist enough for the
crabs to be able to pelletize the sand. Similar to other intertidal crabs, such as Dotilla or Scopimera, soldier crabs are
filter feeders, scooping sand into their water-filled buccal
cavities where the organic material is filtered off into the
gut and the heavier sand particles separated and
compacted into a discard pellet (Quinn, 1986).
Unlike many other intertidal crabs such as Uca and
Sesarma, soldier crabs do not have permanent burrows
but create a subsurface circular air cavity by burrowing
into the substrate from the surface in a cork-screw motion,
sealing off the top of the hole with sand pellets in
a characteristic rosette pattern. The crabs move through
the subsurface of the sediment, dragging the air bubble
with them by pushing sand from one side of the bubble
to the other. Crabs will reside below the surface and carry
on feeding in the air bubble during high tide when the substrate is saturated (Unno and Semeniuk, 2008). After surface exposure at low tide, if the crabs have exhausted or
lost their air bubble, they will emerge briefly and then create a reentry rosette while renewing their air cavity. It is the
presence of the mobile air cavity that constrains the soldier
crab to remain relatively close to the substrate surface, and
as a result, the most important habitat requirement for soldier crabs, and the one that determines where many soldier
crab populations reside within estuaries, is that of wave
energy. Waves influence the stability of the substrate, with
high energy waves creating megaripples and massive
shoal movements that would disrupt soldier crab air cavities, eliminating them from the area. Lower wave energy
can result in an increase in the mud content of substrates
to the extent that soldier crabs are eliminated from the
environment as they can no longer feed or create air cavities. The balance for the soldier crab habitat is one
between the appropriate level of wave energy and mud
influx (Unno and Semeniuk, 2009).
Soldier crab behavior and life cycle
Once a soldier crab population has colonized a suitable
habitat, the complexity of soldier crab behavior and the
concomitant organismal structures (ichnos) created by
the crabs become evident. The complexity of behavior
and the resultant ichnological structures increases with
the age of the crab. After a planktonic larval stage, soldier
crabs settle as juvenile recruits into the substrate and their
presence is evinced by small clots of sand on the surface at
low tide. As the crabs molt and grow, juvenile crabs can be
seen emerging after exposure at low tide and creating
small crater-like structures. Adult crabs have the greatest
diversity of ichnological products with their emergent
phase and feeding activities on the surface resulting in
short vertical subsurface shafts, exit holes, scrape marks,
discard feeding pellets (“pseudofecal” pellets), and reentry
rosettes (Figure 2). Discard pellets can densely cover the
tidal flat for tens to hundreds of square meters. Infaunal
crabs that remain in the substrate at low tide but are feeding
close to the surface are indicated by individual circular
SOLDIER CRABS (MICTYRIDAE)
607
Soldier crabs are semiterrestrial, intertidal crustaceans
well known for their tendency to form spectacular wandering droves or swarms of hundreds to thousands of brightly
colored (mainly blue) individual crabs on the tidal flat at
low tide. There are eight recorded species of mictyrid
soldier crabs (Table 1), and their biogeographic range
incorporates coastal areas in the Indo-west Pacific region
from the southern islands of Japan, through the China
Sea and the Southeast Asian region, to Australia
(Figure 1). Climate zones inhabited by the different
species range from tropical humid, to tropical arid, to subtropical and temperate zones. Tidal ranges are also diverse
with soldier crabs occurring in extreme macrotidal
regimes of up to 8 m in tropical areas in northern King
Sound, Western Australia, to microtidal 2 m tides in higher
latitudes such as Hobart, Tasmania. Most soldier crab
habitats occur in areas with semidiurnal or mixed tidal
cycles and less commonly with diurnal tides (Unno and
Semeniuk, 2009).
Soldier crabs are common inhabitants of estuaries, and
an understanding of the habitat requirements, behavior,
and life cycle of the crab combined with the knowledge
of the types of habitats occurring in estuaries explains
why this situation occurs. Soldier crabs are important in
estuaries as diet items for shore birds and fish, and as
burrowing organisms for their major bioturbation effect
on substrates in terms of sediment turnover, benthic
metabolism, and chemical properties (such as oxygenation
and nitrogen fluxes), and sediment structuring.
Habitat requirements of the soldier crab
The highly visible emergent phase of the soldier crab as
swarms or “armies” on the tidal flat surface is only a shortterm expression of what is largely a cryptic, benthic existence for the crab, as infauna within the substrate during
part of the low tidal and all of the high tidal periods. Soldier crabs have an intimate relationship with their habitat
as they reside beneath the surface, living, feeding, and
moving around in their subsurface habitat. Generally, they
do not occur deeper than 30 cm unless pursued by predators. The sediment characteristics of their habitat are
mostly fine to medium sand (quartz or calcareous) with
a small percentage of interstitial mud and organic matter.
Mictyris longicarpus, the largest of the soldier crab
species, has been found in a more diverse range of sediment types from coarse or very coarse sand to muddy
sand. Sediment pellicular water and groundwater salinity
are circa marine salinity, but soldier crabs are
osmoregulators and can tolerate considerable variations
in salinity (Barnes, 1967). With capillary tube lungs as
well as gills (Maitland and Maitland, 1992), soldier crabs
are obligate air-breathers, not subtidal crabs, and therefore
will drown if the frequency of inundation is too high.
However, they do require a moist environment, and
a shallow water table is necessary at low tide to allow for
pellicular water to be present in the substrate via capillary
rise. However, the substrate must be moist enough for the
crabs to be able to pelletize the sand. Similar to other intertidal crabs, such as Dotilla or Scopimera, soldier crabs are
filter feeders, scooping sand into their water-filled buccal
cavities where the organic material is filtered off into the
gut and the heavier sand particles separated and
compacted into a discard pellet (Quinn, 1986).
Unlike many other intertidal crabs such as Uca and
Sesarma, soldier crabs do not have permanent burrows
but create a subsurface circular air cavity by burrowing
into the substrate from the surface in a cork-screw motion,
sealing off the top of the hole with sand pellets in
a characteristic rosette pattern. The crabs move through
the subsurface of the sediment, dragging the air bubble
with them by pushing sand from one side of the bubble
to the other. Crabs will reside below the surface and carry
on feeding in the air bubble during high tide when the substrate is saturated (Unno and Semeniuk, 2008). After surface exposure at low tide, if the crabs have exhausted or
lost their air bubble, they will emerge briefly and then create a reentry rosette while renewing their air cavity. It is the
presence of the mobile air cavity that constrains the soldier
crab to remain relatively close to the substrate surface, and
as a result, the most important habitat requirement for soldier crabs, and the one that determines where many soldier
crab populations reside within estuaries, is that of wave
energy. Waves influence the stability of the substrate, with
high energy waves creating megaripples and massive
shoal movements that would disrupt soldier crab air cavities, eliminating them from the area. Lower wave energy
can result in an increase in the mud content of substrates
to the extent that soldier crabs are eliminated from the
environment as they can no longer feed or create air cavities. The balance for the soldier crab habitat is one
between the appropriate level of wave energy and mud
influx (Unno and Semeniuk, 2009).
Soldier crab behavior and life cycle
Once a soldier crab population has colonized a suitable
habitat, the complexity of soldier crab behavior and the
concomitant organismal structures (ichnos) created by
the crabs become evident. The complexity of behavior
and the resultant ichnological structures increases with
the age of the crab. After a planktonic larval stage, soldier
crabs settle as juvenile recruits into the substrate and their
presence is evinced by small clots of sand on the surface at
low tide. As the crabs molt and grow, juvenile crabs can be
seen emerging after exposure at low tide and creating
small crater-like structures. Adult crabs have the greatest
diversity of ichnological products with their emergent
phase and feeding activities on the surface resulting in
short vertical subsurface shafts, exit holes, scrape marks,
discard feeding pellets (“pseudofecal” pellets), and reentry
rosettes (Figure 2). Discard pellets can densely cover the
tidal flat for tens to hundreds of square meters. Infaunal
crabs that remain in the substrate at low tide but are feeding
close to the surface are indicated by individual circular
SOLDIER CRABS (MICTYRIDAE)
607