Saturday, July 06, 2013

Porcelain Crabs & Hermit Crabs (Phylum Arthropoda: Infraorder Anomura) of Singapore

Anomurans (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, suborder Pleocyemata, infraorder Anomura) include the hermit crabs, porcelain crabs, king crabs and their allies. They are often crab-like, but have long antennae and three or less pairs of obvious walking legs. The number of walking legs may not always be a useful distinguishing feature though, as some true crabs have some of their legs modified for functions other than walking, and hence may have fewer than four pairs of walking legs.

Anomurans are decapods, and like other members of the order Decapoda, they have five pairs of legs ("deca" means "ten", while "poda" means "feet"). And as with other crustaceans from the class Malacostraca, their body comprises three main parts - a head with five segments, a thorax with eight segments, and an abdomen with six segments. The head is fused with the thorax to form a cephalothorax.

As with other members of the suborder Pleocyemata, they have leaf-like gills. In addition, they have separate sexes, and reproduce sexually. The females brood their eggs under their abdomen until they hatch.

A) Porcelain Crabs (Family Porcellanidae)

Porcelain crabs (family Porcellanidae) superficially resemble true crabs, with a pair of large pincers (or chelipeds) and a reduced abdomen being held under the thorax. However, instead of having four obvious pairs of walking legs like the true crabs, they only use three pairs of legs for walking, while the fourth pair are very much reduced and held on the back of the carapace. In addition, they have a pair of long antennae (true crabs have short antennae). Porcelain crabs usually appear flattened, and most species are found hiding under rocks or living as commensals on cnidarians and sponges. They got their common name from the fact that they readily lose their limbs to distract predators when disturbed.

Red Porcelain Crab (Petrolisthes cf. haswelli)
The Red Porcelain Crab (Petrolisthes cf. haswelli) is possibly one of the largest porcelain crabs found in Singapore - the carapace is about 2cm wide. They can be either all red, or reddish brown above and red below. It is found living in the cracks and crevices of huge rocks along the shore.

Brown Porcelain Crab (Petrolisthes cf. moluccensis)
The Brown Porcelain Crab (Petrolisthes cf. moluccensis) is smaller, with the carapace growing to about 1cm wide, but is still considerably larger than most other porcelain crabs found in Singapore. The irregular lines on the exoskeleton give it a scaly appearance. It is usually found under rocks.

Porcelain Crab (Family Porcellanidae)
Many tiny porcelain crabs can be found under rocks or coral rubble on the shore. This one has a small orange marking on each claw. The carapace is just a few millimetres wide.

Porcelain Crab (Family Porcellanidae)
Here is another tiny unidentified porcelain crab found under a rock.

Sea Pen Porcelain Crab (Porcellanella picta)
The Sea Pen Porcelain Crab (Porcellanella picta) is an example of a commensal porcelain crab that lives in a cnidarian - a sea pen in this case. It is white or whitish in colour, with several black-bordered grey spots on its carapace and claws.

The above unidentified porcelain crab was found on a sandy lagoon. The carapace was a bluish grey, while the legs have alternating dark and pale bands. It has unfortunately lost many of its appendages, and hence hard to identify to the genus.

B) Diogenid Hermit Crabs (Family Diogenidae)

Diogenid hermit crabs (family Diogenidae) are sometimes called "left-handed hermit crabs", as many species have a bigger left claw. Much of my knowledge of hermit crabs came from Dr Dwi Listyo Rahayu, who kindly helped conducted a hermit crab work shop when I was working with the Raffles Museum of Biodiversity Research.

Unlike true crabs, most hermit crabs have a soft (and coiled) abdomen, and making them very vulnerable to attacks from predators. As a result, they usually live in gastropod shells for protection, though some are known to live in sponges or loose rocks with holes. As the hermit crabs grow, they will need to acquire bigger shelters. When forced to live in a smaller shell, the growth rate of the hermit crab may be reversed. While the shell provides some protection, it can still be broken by predators with strong claws/jaws/arms, such as crabs, parrotfishes and octopuses.

Hermit crabs only use two pairs of legs for walking, while the third pair (often much reduced) are often used for holding the shell, and the last pair are very much reduced and have numerous hair-like filaments for cleaning the body and the shell. On the abdomen, there are usually some small appendages on the left side that help the hermit crab to cling on to the shell, and in the case of females, for holding on to the eggs.

Hermit crabs can be carnivores, herbivores or omnivores. Some species mainly scavenge, while others may even feed entirely on detritus (i.e. tiny decaying organic materials).

Diogenid hermit crabs are mostly intertidal or shallow water species, while the other hermit crab families (except the terrestrial family Coenobita) comprise deep water species. As such, apart from the land hermit crabs, the hermit crabs seen on our shores are mostly from this family. The left claw (or cheliped) of diogenid hermit crabs is usually much larger than the right claw, though there are some cases whereby the right claw may be slightly bigger. The carapace is cylindrical or somewhat cylindrical, and often longer than wide (there are exceptions). The rostrum (nose-like structure on the forehead) is triangular or straight.

Blue Antennae Hermit Crab (Pseudopaguristes monoporus)
The above features a preserved hermit crab, Pseudopaguristes monoporus, without the shell.

Blue Antennae Hermit Crab (Pseudopaguristes monoporus)
The Blue Antennae Hermit Crab (Pseudopaguristes monoporus) can be recognised by the bluish coloration of the antennules (small tentacle). The legs may be marked with black or dark brown horizontal bands, and sometimes fine black vertical lines in some specimens. This small hermit crab is sometimes seen clinging onto sea fans or seaweed in the intertidal area. The carapace is just a few millimetres wide.

Diogenes Hermit Crab (Diogenes sp.)
The Diogenes Hermit Crabs (Diogenes spp.) are the commonest hermit crabs on our shores, and huge populations can be seen in tidal pools some times. There are several species recorded from Singapore, but while they are hard to tell apart in the field, they can be distinguished from the previous species in local waters by their plain coloration of white, black, grey and sometimes, brown. The Diogenes species seen in Singapore generally lack bright colours. They are very small, and the carapace is often no more than 1-2 millimetres wide. The genus was named after a Greek philosopher, Diogenes of Sinope, who lived in a gigantic jar/barrel for storing wine.

Bluespot Hermit Crab (Paguristes longirostris)
The Bluespot Hermit Crab (Paguristes longirostris) can be recognised by having a blue spot on the inner side of the first segment (or merus) of each claws. The body is usually light brown or pinkish brown, and the carapace gets to about 0.5cm wide.

Orange Striped Hermit Crab (Clibanarius infraspinatus)
The Orange Striped Hermit Crab (Clibanarius infraspinatus) is commonly seen on intertidal areas, especially in seagrass meadows. This fairly large hermit crab can be recognised by the orange stripes on the legs. Its carapace can get to about 1cm wide.

Blue Striped Hermit Crab (Clibanarius longitarsus)
The Blue Striped Hermit Crab (Clibanarius longitarsus) is very common in our mangroves, but sometimes they also venture to nearby shores. It can be recognised by the long blue stripes running down the sides of each leg. Its carapace gets to about 0.5cm wide.

Mergui Hermit Crab (Clibanarius merguiensis)
The Mergui Hermit Crab (Clibanarius merguiensis) is named after the Mergui Archipelago in Myanmar. It is occasionally seen in sheltered intertidal areas, either near the edge of mangroves or in lagoons. It can be recognised by the blue legs with brownish joints. This hermit is smaller than the previous ones, and the carapace is usually just a few millimetres wide.

Gold-spotted Hermit Crab (Clibanarius cruentatus)
The Gold-spotted Hermit Crab (Clibanarius cruentatus) is occasionally seen on our shores, and can be recognised by the gold-coloured spots on the black body. The carapace gets to about 0.5cm wide.

Pale Anemone Hermit Crab (Dardanus deformis)
The Pale Anemone Hermit Crab (Dardanus deformis) is an interesting hermit crab which attaches Calliactis Sea Anemones (Calliactis spp.) on its shell for protection. Research suggests that the stinging tentacles of the sea anemone deter predators from feeding on the hermit crabs. Meanwhile, the sea anemone gets to feed on the leftover scraps of food from the hermit crab. Sometimes, this hermit crab may be seen without any sea anemones attached to it. It can be recognised, however, by the thin pale stripe which runs along the sides of each leg. The carapace grows to about 0.5cm wide, and this species is often seen in seagrass meadows or nearby sand bars.

Hairy Reef Hermit Crab (Dardanus lagopodes)
The Hairy Reef Hermit Crab (Dardanus lagopodes) is sometimes seen in coral reefs or nearby reef flats. It can be recognised by the dark patches on its legs, and the maroon coloration. The carapace gets to about 1cm wide.

White-spotted Hermit Crab (Dardanus megistos)
The White-spotted Hermit Crab (Dardanus megistos) is usually seen in coral reefs or nearby reef flats. This rather large hermit crab can be recognised by its orange body covered in white spots. The biggest I have seen had a carapace of about 2cm wide.

C) Terrestrial Hermit Crabs (Family Coenobitidae)

Terrestrial hermit crabs (family Coenobitidae) are typical found in terrestrial environment near the seashore, or at the upper shore. As a result, they are often dry, unlike the marine species which keep themselves wet in their natural environment. Despite their largely terrestrial habits, the females still need to migrate to the sea to release their larvae during the breeding season. Apart from their habitat, they can also be recognised in the field by the large, blockish left claw (or cheliped) which is somewhat triangular in cross-section. Also, there first pair of shorter antennae is relatively longer compared to the diogenid hermit crabs, and the tip is somewhat club-like in shape.

Land Hermit Crab (Coenobita sp.)
Several Land Hermit Crabs (Coenobita spp.) are believed to occur in Singapore. The colour ranges from brown to purple, and the biggest I have seen had a carapace of about 1cm wide.

Land Hermit Crab (Coenobita sp.)
Occasionally, Land Hermit Crabs without a shell can be seen running along the upper shore.

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