Wednesday, June 12, 2013

Prawns (Phylum Arthropoda: Suborder Dendrobranchiata) of Singapore

Prawns (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, suborder Dendrobranchiata) can be distinguished from other similar looking decapods by having gills that are branch-like, instead of leaf-like. They also do not hold their eggs until they hatch, but release the fertilised eggs directly into the water column. While they are commonly called prawns in the region, they may be referred to as shrimps (not to be confused with the caridean shrimps) in other countries. Unlike the penaeid prawns, caridean shrimps does not have pincers at the tip of the third pair of walking legs, and also, the "shell" of the second segment of the abdomen is enlarged, overlapping and covering part of the first segment and the third segment.

Their body typically comprises three main parts - a head with five segments, a thorax with eight segments, and an abdomen with six segments. They have two pairs of antennae on their head - the first pair is usually very reduced, while the second pair can be much longer than the body. They have a pair of compound eyes on stalks. As with most other decapods, they have five pairs of walking legs or pereiopods ("deca" means "ten", while "poda" means "feet") attached to the thorax. The first three pairs of legs have small pincers at the tip to assist with feeding. The abdomen is elongate, bearing five pairs of swimming appendages (or pleopods) and ends off with a tail fan comprising a pointed, tail-like structure (or telson) in the middle and a pair of flattened appendages (or uropods) by the sides. The pleopods are used for for swimming, while the uropods are used for steering while swimming.

Prawns are very important in commercial fishing, as many species are consumed in the region.

A) Sergestid Prawns (Family Sergestidae)

The sergestid prawns (family Sergestidae) seen in Singapore usually swarm in the hundreds and thousands, feeding on algae and plankton. Some species are often collected to make prawn paste (belachan), and hence locally they are commonly called Belachan Prawns. Generally, their carapace has poorly developed crests and grooves, and the rostrum (pointed, "nose-like" structure on the forehead) is shorter than the eye-stalk. The first pair of walking legs may or may not have pincers, but the second and third pairs definitely come with mall pincers. Meanwhile, the fourth and fifth pairs of walking legs are shorter than the rest or even absent.

Belachan Prawns (Acetes sp.)
The Belachan Prawns (Acetes spp.) can be differentiated from other sergestid prawns by the lacking of the fourth and fifth pairs of walking legs altogether. They occur in huge swarms, and sometimes several thousands can be seen in shallow waters or in tidal pools. While they are quite small sized, being not more than 4cm long, they are collected by the locals to make into prawn paste, or dried under the sun and cooked with other ingredients. Since they occur in huge numbers, they are an important source of protein for fishing villagers.

Belachan Prawns (Acetes sp.)
The above picture features a small part of a huge swarm of Belachan Prawns in a tidal pool.

B) Penaeid Prawns (Family Penaeidae)

Penaeid prawns (family Penaeidae) are typically much larger than those of the previous family. They have a very well-developed and toothed rostrum (pointed, "nose-like" structure on the forehead) which usually extends beyond the eyes. The fourth and fifth pairs of walking legs are very well developed, and are about the same size as the others. Penaeid prawns are very important commercially, being widely consumed in the region, and some species are thus farmed in prawn ponds.

The Western King Prawn (Melicertus latisulcatus) is very commonly seen in Singapore, especially in sandy lagoons where they are often seen scampering around or partially buried under the sand, revealing only the eyes. It can be recognised by the bluish tail, and obvious black spots on the sides of its abdomen. Those without the black spots are deemed to be a different species. This prawn can get to about 20cm long.

The Longarm Prawn (Heteropenaeus longimanus) is usually seen in coral reefs, and hence rarely seen in the intertidal area. It does not get longer than 10cm, and is less often consumed due to the smaller size compared to the other prawn species.

This Penaeus sp. with red appendages is often seen in seagrass meadows. The biggest Penaeus species can get to over 30cm long, though I am not sure the exact species of the one featured in the above photo and hence cannot provide the length. Some Penaeus species are reared in prawn ponds in the region, and the creation of these ponds has resulted in the destruction of many mangrove forests.

Penaeus sp.
This Penaeus sp. which is all greenish in colour is a juvenile. The colour helps it to blend into the surrounding seaweed and seagass.

Penaeid Prawn
This unidentified penaeid prawn was seen once swimming near the surface of the water. It is a rather big prawn, about 20cm long.

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