The family of grapsid crabs are those that
look very much like 'normal' crabs, usually flat in shape, with long legs
radiating outward, moving sideways, and having strong nippers. This family
is widely spread all over the world and in New Zealand, and contains a
high proportion of endemic species (marked in red).
This family of crabs is interesting because many of its members can readily
be observed in the intertidal.
Sally lightfoot crab Grapsus
grapsus, not found in New Zealand but known from south America
and the Galapagos Islands, resembling our purple rock crab in appearance,
habitat and behaviour.
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Leptograpsus variegatus (Fabricius, 1793),
purple rock crab, large shore crab, papaka nui
(L:
leptos=
fine, small, narrow; variegatus= varied in colour, marked with irregular
patches) The purple rock crab is large, strong, and moves fast. When wedged
into a crevice, it is not easily dislodged. Handling a ferocious crab like
this requires skill, and risks painful nips. The purple rock crab is the
largest native shore crab, exceeded in size only by paddle crabs Ovalipes
catharus (on sand bottom) and the Australian mud crab Scylla serrata
(on mud flats).
It is found between high tide and above it, often occupying moist crevices
up to two metres above high tide. It can remain out of the water for days,
but equally happily can stay submerged. The crab measures up to 5cm across
its back, and its long, flat legs fold neatly together as in an oriental
fan, giving it a very flat profile.
The purple rock crab is active particularly by night, when it leaves its
crevice, scaling the vertical rocks and navigating its way over the rock
flats and into the water, in search of flea mussels, which it cracks without
effort. At night it has no fear and can be approached and photographed
from close up, even in bright lamp light. However, the crabs are inactive
during the full moon phase. Towards the morning, the crab navigates flawlessly
all the way back to its home crevice. Yet, by day, this large crab is extremely
wary, scuttling away noisily, its legs grating the rocks. When a group
of these forays by day, large males can be seen taking guard in a defensive
posture, warning the others at the slightest disturbance. To see several
of these beautiful purple and white crabs feeding off green seaweeds, is
spectacular. This crab is an omnivore, feeding on both animal (flea mussels,
barnacles) and plant matter (seaweeds, plankton slime).
Large crabs migrate all the way down the shore to the sublittoral fringe
where food is more abundant. Small crabs foray more in restricted areas.
This crab runs fast and when cornered, can be very aggressive with
its nippers, which makes it difficult to capture and handle.
Colour: This crab is usually coloured as shown in the paintings
and photographs, but two distinct colour morphs (blue and orange)
have been distinguished in australia.
Life history: the purple rock crab mates in October-November
when the female is hard-shelled. Before mating, the crabs make leg contact,
during which the legs are rapidly vibrated. The male crab may then mount
the underside of the female. The female moults before laying her eggs.
Females with eggs are found from November till January/February (Auckland/Wellington).
The incubation period is about 6 weeks and the crab spawns only once a
year. Newly laid eggs are about 0.36mm in diameter, and develop from a
dark brown colour to light brown just before hatching. The female carries
55,000-144,000 eggs (depending on her size), attached to her pleopods under
her broad abdomen. This crab can become five years old.
Distribution: North Island, Kermadec Islands, Australia and the
islands of the South Pacific. This crab is found in southern warm temperate
(subtropical) Indian and Pacific Oceans from western Australia to western
South America. In NZ from the top of the North Island to Kaikoura or Westport.
Plagusia chabrus, red rock crab,
(Plagusia capensis) (Linnaeus, 1758), papaka ura,
red bait-crab (Australia)
(L:
plagusia=?;
capensis=
of Cape of Good Hope; chabrus=?) The red rock crab lives under water
and seldom or never ventures outside. Small ones are found inside rock
pools, but divers encounter these animals to depths of 25m (the photic
zone where light is sufficient for plant growth). This crab feeds mainly
on plants (brown and red algae, coralline turf), although it will scavenge
as well on limpets, chitons, gastropods, mussels, brittle stars and barnacles.
Small crabs are mainly herbivorous whereas large crabs are more carnivorousIts
carapace can measure up to 7cm across, and its legs can span 20cm. Yet
this animal is more placid than the purple rock crab, and can be handled
with some skill. The painting shows a female, recognisable by her smaller
nippers. Each nipper has three knobbly ridges. The red rock crab's carapace
is covered in fine hairs, which gives it an advantage when attacked by
octopus, their main predator (and shags). Once lodged inside a crevice,
an octopus won't be able to dislodge it. The red rock crab lives mainly
on rocky reefs, but may venture on the sand to forage for food by night.
This crab can run over the rocks, half by swimming with its flattened long
legs. When approached cautiously by divers, it shows no fear.
This crab does not seem to be territorial but may put up a fight when
foraging or courting. It is aggressive to other species of crab, but not
to crayfish with which it often shares a crevice. Studies with tagged crabs
suggest that they spend about 50% of their time foraging, 25% feeding,
20% in agonistic (combative, contestant) encounters and 5% in reproductive
behaviour.
In Australia this crab is favoured by fishermen for use as bait. Groupers
seem to relish it.
Life history: Mating between hardshelled crabs lasts about 7
minutes, with the female on top. Small females carry eggs from September
to February but large females from late October. Eggs are about 0.4mm in
size and change colour from brick red to light green just before hatching.
During the release of the hatched larvae, the female assists departure
by flexing her body up and down, vibrating her abdomen and pulling the
hatching eggs from her pleopods with her nippers. Juveniles and small crabs
moult throughout the year but but large crabs mainly in winter from May
to September, before the breeding season.
Distribution: the southern Indo-Pacific from south Africa to
Chile. Southern Australia, Tasmania, South Africa, Juan Fernandez, Chile,
Tonga. In New Zealand from Parengarenga Hr to Lyttleton Hr.
f003215: A female red rock crab (small nippers) is taking
a break, not far from a wharf pile where she hides inside crevices between
encrusting organisms. Here she is standing tall, not showing any fear for
the photographer. Interestingly, she has perched herself on top of two
cushion stars, one with five legs, which is the norm, the far one with
six legs, which is unusual.
Hemigrapsus edwardsi (Hilgendorf,
1882), (Hemigrapsus sexdentatus), common rock crab,
marbled rock crab, rerere,
(L:
hemi=
half; Edwards= a taxonomist?) A common intertidal crab, occurring
in a variety of habitat: under boulders, on the rocky reef, on sand and
mud flats. They are rarely found outside the intertidal zone. They can
reach 4cm across their backs, and their colour is often a reddish purple,
mottled with dirty white patches. They are usually found sheltering underneath
rocks, extending from the high tide mark down to about mid tide. It is
recognisable by the six teeth, from the eyes half way down on its carapace
(sexdentatus: L: sex=six; dentatus= toothed). It has
a smooth skin. The males have enormous chelae (nippers) with great pads
of muscle bulging at the angles of the nippers. The two nippers are equal
in size and shape.
There are two colour types: light and dark. The lighter crabs have either
a grey or cream-coloured background with markings of light or dark chestnut-red.
Darker crabs are marked with dark purple, sometimes almost purplish black,
and their legs are banded. Usually the front half of the carapace is more
deeply pigmented than the rest. The eyestalks are white, speckled with
dark red. Basal antenna segments arebanded. antennules are pale green with
dark red spots. Belly surface is white.
Hemigrapsus edwardsii is a very agile, rapidly moving crab,
although easy to handle. Males have very large nippers and are able to
crush oyster borers (Lepsiella sp.). The crabs emerge from under
boulders and crevices to forage for mainly plant food at night. This crab
is most active when the tide is in. It lives in the middle region of the
intertidal rocky shore.
Life history: This crab is a late winter breeder, with females
in berry from March to August. Females carry their 26,000 eggs (size 0.3mm)
for about 6 weeks, during which time they change colour from light brown
to transparent.
Distribution: found abundantly throughout the North and South
Islands, endemic in New Zealand. Not in Chatham Islands or southern islands.
This crab is more abundant on southern shores than in the north, where
its intertidal habitat is taken by the blackfinger crab Ozius truncatus.
(L:
hemi=
half; crena= notch; latus=wide) The hairy-handed crab is
often found where the tunnelling mud crab is found, on mud flats and sand
flats, but it may also occur under boulders on the rocky shore intertidal.
It has a dull colour and slightly hairy legs. Its main distinguishing features
are the two hairy pads on the inner/under side of its nippers, but a crab
needs to be handled in order to be able to view this. These crabs are much
less wary than the other mud crabs, often going about their business without
taking much notice of observers.
It is thought that the hairy pads on its nippers allow it to wipe the bottom
and objects such that food particles such as diatoms are caught in it.
With its mouth parts, it can then feed from its hairy hands. These crabs
are quite sturdy and in the aquarium they have been seen hunting and eating
the smaller mud crabs. They are also very effective scavengers.
Males have larger nippers than females, but left and right hand nippers
are similar. The colour of its back is greeny-yellow with white patches,
covered with tiny dark purple or reddish spots. The upper sides of their
forearms (cheliped carpus and prodopus) are marked with deep purple-brown.
The fingers of their nippers are white with small brown tips. Legs are
greeny yellow with dark purple spots.
This crab occurs in a wide variety of habitats, under stones, burrowing
insand and mud, but always in sheltered places.
Life history: Males mature in about 1 year and live for
5 years. Females mature earlier and die earlier. Large males mate with
smaller females. Mating takes about 10 minutes. The females come in berry
in spring, from October to December. They may spawn more than once each
year (June-February). Their 8,000-30,000 eggs (0.27mm) change colour from
brownish-yellow to almost transparent, and hatch after 8-12 weeks. When
releasing her offspring, the female raises hereself on her legs, opening
her tail wide and beating it in rhytmical fashion. The nippers were alternatively
dug into the brood to propel the newly hatched larvae in the surrounding
water.
This crab is a great survivor, being able to live in brackish water.
Its main predators are fish like rig, red cod, sea perch and red gurnard.
When small, more types of fish eat it.
Distribution: A New Zealand species, occurring along the coasts
of North and South Islands and Stewart Island. It is possible that the
Chilean species is identical. It is interesting that two so widely separated
species, look so much alike, but the Chilean species is grey, grey-brown
and black.
f026934: A large hairy-handed crab seen blowing bubbles while
posturing on its patch in the sandy eelgrass habitat. Its hairy patches
on the insides of its nippers are only just visible.
f215306: Hairy-handed crabs do not bother to burrow or run
away in case of danger. They seem to be confident that they are inedible
to birds and other predators. Roaming around over the sand, and following
the tide in and out, they move fast to arrive timely on the scene of death
of other creatures.
f215302: shying away from the photographer, this hairy-handed
crab starts digging itself in, releasing bubbles of methane gas rising
from the putrid sand below. Notice that the orange coloured 'hairs' on
its exoskeleton are actually brown algae. The hairy pads after which it
is named, lie on the insides of its nippers.
f215328: Two hairy-handed crabs have arrived on the scene
of a casualty, but long after whelks beat them to it. With their strong
nippers, these crabs are able to tear the flesh apart. The whelks however,
can drill through skeletons and reach the smallest corners inside, by means
of their flexible trunks (probosces).
(Gk:
elisso=
to revolve; L:
Helix/Helice= a spiral curve;
crassus= dull)
The tunnelling mud crab is found in large numbers on healthy mud flats,
where they burrow. These small crabs, measuring up to 4cm across their
backs, are very wary, scuttling and scurrying in hordes at the slightest
sign of movement, making the mud flats seem to move for a fraction of a
second. They take time to dig elaborate tunnels in the mud, often with
more than one entrance. Active by day, the tunnelling mud crabs sleep by
night, often closing their burrows with a plug of mud.
These mud crabs are actively preyed upon by birds and many other animals.
Their survival depends on how quickly they can withdraw into their burrows,
and how capricious and unpredictable such tunnels are formed. It will occupy
its tunnel for a long period of time and will defend it against intruders.
Competitors are warned off with a typical threatening posture.
The
tunnelling mud crab has a squarish back and two short stalked eyes, and
is brownish in colour. But older specimens are beautifully coloured greenish
orange. It cannot be confused with the stalk-eyed mud crab, because this
one has an elongated carapace, long stalked eyes, and beautiful colours.
The tunnelling mud crabs live from the mud, which is rich in diatoms, algae
and bacteria. Even the spoils of their burrows are spread carefully for
optimal productivity (see photo; how many crabs can you see?). With their
flat, spade-like nippers, they till the mud, like farmers do. It takes
up a scoop of mud on its nippers, and then sorts it out with its mouth
parts. In this rich, sun-lit environment, small organisms like diatoms
and bacteria grow profusely, particularly when the mud warms up during
low tide in mid day. Only a very small area, the size of a hand, is sufficient
to feed a mud crab for life. Mudcrabs grow at least 5-6 years old, but
some may survive a decade or longer. The large ones are rare, but beautifully
coloured. One finds these in the cleaner reaches of an estuary.
The colour of these crabs is tawny brown in juveniles, becoming green
to yellow in large adults. Small patches of bright orange can be seen in
the joints. Its antennae are brown, antennules light purple, eyestalks
pale green dorsally (on top) but white ventrally (below). Legs are dark
green with pale yellow margins. Males grow to 21mm measured over the width
of their carapace (CW), and females are slightly smaller. These crabs are
larger in the South Island.
The tunnelling mudcrab lives in sheltered places where it can burrow
in the sand or mud, or well-drained sediment exposed to the air for more
than six hours on each tidal cycle (upper intertidal region). In places,
up to 500-1800 crabs can be counted per square metre! H crassa must
return frequently to its burrow to wet its gills.
Tunnelling mudcrabs become suddenly active as soon as the tide moves
out around them. They start cleaning the area around their burrow, and
also themselves. When feeding on mud, the crab walks slowly forward with
the nippers held in front, while probing the surface. Small pinches of
surface sediment (algae and detritus) are transferred to the mouth parts.
Wastes (sand, silt) accumulate at the bottom of the mouth frame, and these
are wiped off by the nippers. These wastes (pseudo-faeces) form small pellets.
H. crassa seldom forages more than 200mm from its burrow, often defending
its territory against other mudcrabs. H.crassa will scavenge when
the opportunity arises.
In mangrove swamps, where mangrove trees shade the mud underneath,
resulting in lower algal productivity, crabs may climb up the trees to
a height of 1m, feeding from the mud on branches and leaves.
Their burrows have complicated shapes, often joining up with those
of other mud crabs. The end of their burrow slopes down, holding 20-50mm
of water at low tide. When the tide moves in again, the crabs plug up the
entrances to their burrows.
H crassa has many enemies, from wading shore birds to fish (parore,
cod, flounder, yellow-eyed mullet), including eels.
Life history: Female H crassa mate in hard-shell condition.
Mating occurs August to May, with peak periods in October and May. Copulation
occurs on the surface of the ground, or still partly covered by water,
with the male flat on his back and the female uppermost. Males usually
mate with smaller females. The female carries her eggs in early spring
and summer (August to March). Females mature at CW of 10mm. A 16mm female
will carry about 16,000 eggs of 0.3mm, which change colour from brownish-yellow
to transparent green during 42 days of incubation. Females may reproduce
more than once per season, and do not require to mate for this. H. crassa
appear to reach maturity after 1 year, and they can live a 5-6 year life.
Distribution: this is a New Zealand species, widely distributed
over the North and South Islands and Stewart Island.
f992724: above, a tunnelling mud crab in its burrow.
This is a large and old one, beautifully coloured.
f992725: on left the burrows of a community of old and
large mud crabs, who survived to an old age, perhaps because the sand here
is consolidated and hardened by iron leachates.
f215226: A tunnelling mudcrab postures near its burrow. Notice
its wide but flat nippers, which are eminently suitable for moving soil,
like a spade. This is a large specimen, perhaps over ten years old.
f215237: As this mudcrab is cautiously leaving its burrow,
it affords us a view of its back, characterised by its square shape and
three spines along the edge.
f215229: a mudcrab is disappearing sideways into the
mud. This is how it starts a new tunnel.
f215231:mud crab disappearing in the sandy mud.
f215227: frontal view of a large specimen of Helice
crassa. When mudcrabs grow old, their skins also become more colourful.
f215227: closeup of Helice's face. Its mouth parts are shaped
for sieving mud and sand.
Cyclograpsus
insularum (Campbell and Griffin, 1966), northern smooth
shore crab
This species is easily confused with the smooth shore crab Cyclograpsus
lavauxi, which has a wider range in New Zealand, but similar habitat
and eating habits.In first instance, this crab is distinguished by its
smoothly graded colour from brown/purple in front, to paler brown towards
the back, whereas C lavauxi is distinctly speckled. The northern
smooth shore crab is also smaller than C lavauxi (CW 22mm versus
28mm). Its legs are not pointed and sharp but end in a kind of nipple.
Its nippers are smooth, and so are their cutting edges. The righthand claw
shows a distinctive gape between fingers.
This crab is found in the upper reaches of the intertidal on exposed
shores in the presence of boulders, just lower than high tide level, and
below C lavauxi when the two are found together. It can be found
in densities of over 30/m2. For further specifics, see those of the smooth
shore crab.
Life history: This crab breeds in winter from June to August,
in a very short season of 9 weeks. They produce only one brood. Eggs are
relatively large, measuring 0.45mm across and they are dark purple. They
hatch after 7 weeks.
Distribution: Lord Howe Island, Norfolk Island, Kermadec Islands
and in New Zealand from North Cape to East Cape.
(L:
cyclo=
circle; Lavaux= name of an explorer) The smooth shore crab is a
New Zealand species, common on rocky shores all around new Zealand. It
is also found in estuaries and mud flats. Usually hiding at high intertidal
levels, under boulders, it looks at first sight like a young (15mm) common
rock crab, but its carapace is smoother (missing the six teeth) and elegantly
curved. The eye orbits (sockets) are curved. A rare species exists, Cyclograpsus
whitei, which looks the same, but does not have the carved eye sockets.
The colour of the smooth shore crab is distinctly speckled in dark reddish-brown
on a background varying through slate blue, bluish grey, fawn to yellowish
brown. Its underside is pale. Its nippers are white. This is a small crab,
measuring only 28mm CW in adult males.
This crab is claimed the most terrestrial of the common crabs, living
in the upper intertidal area around the high tide level. It spends a great
deal of time out of the water. It usually outnumbers the hairy handed crab
H
edwardsi which lives lower down the shore. It normally rests under
stones, becoming active only once daily during the night high tide.
The smooth shore crab is usually found under stones, and may be heard
making clicking sounds. It has the curious habit of grasping stones with
its rear legs, presumably to stabilise itself in the presence of wave action.
When prodded, this crab takes a defensive posture, which is often explained
as aggressive.
This crab normally dines on tough seaweeds, but it will also scavenge
and predate on worms.
Life history: Females start breeding at around 11mm CW. Their
egg-carying period runs from September to January. Their eggs are 0.32mm
across, coloured dark purple, becoming paler as development proceeds. Incubation
time is about 8 weeks. Some females can produce two clutches of eggs in
a season. Most crabs die after 2-3 years but some may live up to 5 years.
Distribution: This crab is found mainly in two places, very far
apart: New Zealand and Juan Fernandez in Chile. It may well be that the
one in Juan Fernandez is a different species, not capable of interbreeding
with its NZ relative. For this reason we consider C lavauxi an endemic
species (found in NZ only).
Related species: Cyclograpsus granulosus is found in Tasmania,
C
punctatus in south Africa and C integer in Jamaica.
Planes cyaneus,
Blue Pacific weed crab
(Gk: planes/planetes= wanderer; cyaneus= blue; blue wandering
crab) The Pacific weed crab is a small crab (12mm across its back, 25mm
length), living in the open ocean, often clinging to seaweed and flotsam.
In this way it is a frequent visitor to New Zealand waters, where it may
get beached with its raft of flotsam, debris, fishing float, pumice, dead
shell or rams horn shell. It is sometimes seen attached to sea turtles.
This crab has a variable and protective colouration ranging from blue,
bluish brown to yellowish, clouded with brown. It is likely that this crab
is able to change its colours when moulting, in a similar manner as its
relative P minutus. With its chromatophores, this crab can respond
to white, black, blue, yellow and green backgrounds.
Life history: Not much is known about this crab. Females start
to breed at about 8mm CL, and like P minutus, may breed all year
round. Crabs in berry have not been recorded from NZ.
Distribution: Pacific Ocean.
A related species is the Gulf-weed crab Planes minutus, which
is found in the open Atlantic ocean, associated with the seaweed Sargassum
(Spanish: sargasso= seaweed).
Planes
marinus (Rathburn, 1914), Brown Pacific weed crab
(L: planes= wanderer; marinus= of the sea; sea wanderer)
The brown Pacific weed crab is an oceanic species, found attached to seaweed
and flotsam, often between goose barnacles. It is a small crab (20mm CW)
with a squarish back and dark reddish-brown colour. Very little is known
about it. Ovigerous (in berry) females have been found in NZ.
Distribution: Indo-Pacific, Japan, west coast of N America, Australia,
Kermadec islands, in New Zealand from the Bay of Plenty to Chatham Islands.
Grapsus grapsus, Sally
Lightfoot crab
The Sally Lightfoot crab is found in south America and features on many
shore photographs of the Galapagos Islands. It is not found in New Zealand.
It resembles the purple rock crab in behaviour, feeding and habitat preference.
Plagusia
depressa tuberculata (Lamarck, 1818)
The depressed red rock crab is similar to the red rock crab Plagusia
chabris and occupies a similar niche in the tropics. It is sometimes encountered
in New Zealand, clinging to floating objects like driftwood. This crab
is recognisable by its flat shape and slightly hollow (convex) back, which
is covered with flattened tubercles. Its colour is reddish with darker
blood-red spots and speckles.
(L:
leptos=
fine, small, narrow; variegatus= varied in colour, marked with irregular
patches) The purple rock crab is large, strong, and moves fast. When wedged
into a crevice, it is not easily dislodged. Handling a ferocious crab like
this requires skill, and risks painful nips. The purple rock crab is the
largest native shore crab, exceeded in size only by paddle crabs Ovalipes
catharus (on sand bottom) and the Australian mud crab Scylla serrata
(on mud flats).
(L:
plagusia=?;
capensis=
of Cape of Good Hope; chabrus=?) The red rock crab lives under water
and seldom or never ventures outside. Small ones are found inside rock
pools, but divers encounter these animals to depths of 25m (the photic
zone where light is sufficient for plant growth). This crab feeds mainly
on plants (brown and red algae, coralline turf), although it will scavenge
as well on limpets, chitons, gastropods, mussels, brittle stars and barnacles.
Small crabs are mainly herbivorous whereas large crabs are more carnivorousIts
carapace can measure up to 7cm across, and its legs can span 20cm. Yet
this animal is more placid than the purple rock crab, and can be handled
with some skill. The painting shows a female, recognisable by her smaller
nippers. Each nipper has three knobbly ridges. The red rock crab's carapace
is covered in fine hairs, which gives it an advantage when attacked by
octopus, their main predator (and shags). Once lodged inside a crevice,
an octopus won't be able to dislodge it. The red rock crab lives mainly
on rocky reefs, but may venture on the sand to forage for food by night.
(L:
hemi=
half; Edwards= a taxonomist?) A common intertidal crab, occurring
in a variety of habitat: under boulders, on the rocky reef, on sand and
mud flats. They are rarely found outside the intertidal zone. They can
reach 4cm across their backs, and their colour is often a reddish purple,
mottled with dirty white patches. They are usually found sheltering underneath
rocks, extending from the high tide mark down to about mid tide. It is
recognisable by the six teeth, from the eyes half way down on its carapace
(sexdentatus: L: sex=six; dentatus= toothed). It has
a smooth skin. The males have enormous chelae (nippers) with great pads
of muscle bulging at the angles of the nippers. The two nippers are equal
in size and shape.
(L:
hemi=
half; crena= notch; latus=wide) The hairy-handed crab is
often found where the tunnelling mud crab is found, on mud flats and sand
flats, but it may also occur under boulders on the rocky shore intertidal.
It has a dull colour and slightly hairy legs. Its main distinguishing features
are the two hairy pads on the inner/under side of its nippers, but a crab
needs to be handled in order to be able to view this. These crabs are much
less wary than the other mud crabs, often going about their business without
taking much notice of observers.
(Gk:
elisso=
to revolve; L:
Helix/Helice= a spiral curve;
crassus= dull)
The tunnelling mud crab is found in large numbers on healthy mud flats,
where they burrow. These small crabs, measuring up to 4cm across their
backs, are very wary, scuttling and scurrying in hordes at the slightest
sign of movement, making the mud flats seem to move for a fraction of a
second. They take time to dig elaborate tunnels in the mud, often with
more than one entrance. Active by day, the tunnelling mud crabs sleep by
night, often closing their burrows with a plug of mud.
The
tunnelling mud crab has a squarish back and two short stalked eyes, and
is brownish in colour. But older specimens are beautifully coloured greenish
orange. It cannot be confused with the stalk-eyed mud crab, because this
one has an elongated carapace, long stalked eyes, and beautiful colours.
The tunnelling mud crabs live from the mud, which is rich in diatoms, algae
and bacteria. Even the spoils of their burrows are spread carefully for
optimal productivity (see photo; how many crabs can you see?). With their
flat, spade-like nippers, they till the mud, like farmers do. It takes
up a scoop of mud on its nippers, and then sorts it out with its mouth
parts. In this rich, sun-lit environment, small organisms like diatoms
and bacteria grow profusely, particularly when the mud warms up during
low tide in mid day. Only a very small area, the size of a hand, is sufficient
to feed a mud crab for life. Mudcrabs grow at least 5-6 years old, but
some may survive a decade or longer. The large ones are rare, but beautifully
coloured. One finds these in the cleaner reaches of an estuary.
(L:
cyclo=
circle; Lavaux= name of an explorer) The smooth shore crab is a
New Zealand species, common on rocky shores all around new Zealand. It
is also found in estuaries and mud flats. Usually hiding at high intertidal
levels, under boulders, it looks at first sight like a young (15mm) common
rock crab, but its carapace is smoother (missing the six teeth) and elegantly
curved. The eye orbits (sockets) are curved. A rare species exists, Cyclograpsus
whitei, which looks the same, but does not have the carved eye sockets.
(Gk: planes/planetes= wanderer; cyaneus= blue; blue wandering
crab) The Pacific weed crab is a small crab (12mm across its back, 25mm
length), living in the open ocean, often clinging to seaweed and flotsam.
In this way it is a frequent visitor to New Zealand waters, where it may
get beached with its raft of flotsam, debris, fishing float, pumice, dead
shell or rams horn shell. It is sometimes seen attached to sea turtles.
