Wednesday, June 12, 2013

Mantis Shrimps (Phylum Arthropoda: Order Stomatopoda) of Singapore

Mantis shrimps (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Stomatopoda) are crustaceans with a pair of enlarged appendages that they carry folded beneath the head, somewhat similar to the forelimbs of a praying mantis. The way they strike the prey is different though - the mantis strikes its prey overhand, while the mantis shrimp strikes its prey underhand, thus hitting the fish or other smaller crustaceans from below where they are more vulnerable. The strike is very fast, taking as little as three milliseconds, making it hard to avoid.

As with other crustaceans from the class Malacostraca, their body typically 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. They have a tough exoskeleton strengthened with calcium carbonate, and the carapace covers the gills but not the abdomen. The abdomen is elongate, and ends off with a tail-like structure called a telson, and and a flattened tail fan is usually present at the tip.

Mantis shrimps can generally be divided into two main groups - the smashers and the spearers - based on the appearance of their striking appendages and how they are used. The smashers have a heavily calcified knob at the tip of the appendages, which they use to punch their prey, including crabs and snails, breaking theirs shells before they are consumed. The punch of these smashers is so strong, that they cannot be kept in normal fish tanks because they can easily break the glass. There are also many stories of mantis shrimps being unknowingly introduced aquariums, killing the other inhabitants and smashing the corals and other sessile animals into pieces.

The spearers lack the knob at the tip of the hunting appendages, but instead the last segment is armed with numerous sharp spines. They often prey on fishes and other soft-bodied animals, striking them at high speed and impaling them on the sharp spines.

Mantis shrimps have a pair of compound eyes, and each compound eye has been modified to be able to accurately judge distance (unlike human and many animals, which need both eyes to do so). This allows the animal to effectively hunt even in confined spaces.

Many spearers are known to ambush prey near their burrows or come out to hunt actively at night. As a result, their eyes have developed such that they can see well in the dark, and only possess single visual pigment which does not allow them to distinguish colours. This basically means that they will not be distracted by the different colours, and have an easier task of watching out for movements of prey and predators. Smashers, on the other hand, hunt in the day, and studies have shown that many of them have at least 10 visual pigments and several colour filters. It is impossible for humans to imagine the colourful world viewed by a smasher shrimp, when we only have three visual pigments.

Mantis shrimps have separate sexes, and reproduce sexually. Depending on the species, the female may lay the eggs in a burrow, or carry them with her.

Here are the mantis shrimps that I have photographed in Singapore so far:

A) Smashers

The smashers I have seen in Singapore are from the family Gonodactylidae, and can be easily recognised by the heavily calcified knob at the tip of their hunting appendages.

Green Smasher (Gonodactylus sp.)
This Green Smasher (Gonodactylus sp.) is sometimes found under rocks or foraging among the seagrass. The ones I have seen are usually not more than 7-8cm long.

B) Spearers

The spearers I have photographed in Singapore are from the family Squillidae, though members of the families Lysiosquillidae and Harpiosquillidae have also been recorded. They are often collected by commercial trawls as bycatch, but the numbers are significant enough for a steady supply to some of the regional markets.

Squillids can be distinguished lysiosquillids by having an obvious "ridge line" in the middle of its tail, and from harpiosquillids by the lack of an angular corner on the sides of the carapace.

Spotted Squillid Mantis Shrimp (Chloridopsis scorpio)
The Spotted Squillid Mantis Shrimp (Chloridopsis scorpio) is easily recognised by having a black patch on the forward-curving spine at its collar. It is usually seen among the seagrass on sandy bottoms, and can grow to about 10cm.

Oratosquilla sp.
This Oratosquilla sp. can be recognised by having an obvious and unbroken Y-shaped ridge on the back of its carapace in the middle (see earlier picture on squillid mantis shrimps for a close-up of the carapace). It is a rather big mantis shrimp, and the biggest I have seen is probably about 15cm long.

This unknown squillid mantis shrimp with a vague "ridge-line" in the middle of its carapace is seldom seen, and I have not been able to identify it yet. It is only about 10cm long.

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