Food and feeding:
Almost all crustaceans have the same basic mouthparts although the diversity in form and function is enormous. The main part of the prawn diet is made up of plant material, microorganisms, small shellfish and worms.
In aquaculture, prawns are fed artificial diets. Most digestion is rapid and completed in about six hours.
Prawns search the bottom for food with their pereopods which have chemo-sensory hairs attached. Any food that is found is held by those head appendages known as maxillipeds and the teeth or mandibles are used to bite or tear pieces off.
The maxillipeds can be used to push large pieces of food away as the mandibles grasp it, thus tearing the food into manageable portions.
Reproduction:
The prawn species which are commercially fished are bisexual, that is, the genders are separate, with the mature female being larger than the male. For example, a mature female eastern king prawn could be several times heavier than a mature male.
Male prawns are readily identified by the large copulatory organ, known as the petasma, found between the foremost pair of swimming legs (see Figure 2). Corresponding structures are not present in the female and the foremost pair of pleopods is identical with the other four pairs.
Females possess a structure known as the thelycum between the last pair of pereopods which provides an anchor site for the male sperm packet received during copulation (see Figure 2). Males and females can thus be readily identified on the presence or absence of a petasma or a thelycum.
Ovaries containing eggs can often be seen through the shell in the head and tail of mature female prawns. The colour of ovaries changes from pale yellow to olive or orange-brown as they mature. The mature female needs to be in the soft-shelled or newly moulted stage just prior to copulation while the male must be in the hard-shelled condition.
Fertilisation of eggs is internal and spawning is thought to occur soon afterwards. Females are capable of spawning more than once in a season. The number of eggs carried by a female varies with size and between species. The seasonality of spawning varies between species and between stocks of the same species. For instance, eastern king prawns spawn year around in waters off Queensland, whereas they do not spawn in winter off NSW.
Life cycles and age:
Prawn life cycles in NSW waters can be considered to be of three types:
A. Estuarine – the entire life cycle is completed in waters of less than sea water salinity. An example of this type would be the inshore greasyback prawn;
B. Marine – the entire life cycle is completed in oceanic waters. An example of this type would be the royal red prawn
C. Mixed – postlarval and juvenile stages of the prawn’s life cycle occur in estuarine waters of less than sea water salinity whilst the adolescent, adult, egg and larval stages occur offshore in oceanic waters. Examples of this most common type include eastern king and school prawns.
The ‘mixed’ life cycle is diagrammatically represented in Figure 3. Fertilized eggs are shed onto the bottom in oceanic waters where they remain for a short period until they hatch. The first larval stage called a nauplius emerges from the spherical egg and starts swimming up towards the surface. This naupliar stage goes through several moults, getting larger with each moult, and ultimately changes to a protozoea.
The prawn similarly moults and grows through several protozoea then mysis stages before developing into a postlarva. The mysis stage is the first stage that somewhat resembles the appearance of the adult prawn.
The development time from spawning to postlarva for prawn species with ‘estuarine’ or ‘mixed’ life cycles is between two and three weeks. Hatching success and larval survival are affected by water temperature and salinity and are greatest in waters of the same temperature and salinity as that where spawning took place.
Up to the mysis stage the larvae are free swimming and form part of the zooplankton found in the open sea. Zooplankton together with phytoplankton is eaten by a multitude of other animals, including larval prawns, and is a vital part of the ocean’s food system.
After the free swimming, planktonic mysis stage, postlarva adopt a bottom existence, reach the shore and enter the rivers and coastal lakes In the estuaries and lakes, the juvenile prawns have to adapt to wide fluctuations in, among other things, salinity and temperature. School prawns appear to be more tolerant of these fluctuations than eastern king prawns.
After over-wintering in the estuaries and lakes the juvenile prawns grow rapidly and upon some cue, start to migrate back to the ocean. These migrations rise to a peak in the ‘dark’ period of the summer, lunar cycles and are fished enthusiastically by both amateur and professional fishermen.
The physiology of prawns with a ‘mixed’ life cycle is such that the prawns require the conditions found in the deeper ocean waters to reach maturity. Once they reach oceanic waters, the prawns rapidly mature, mate and spawn – thus completing the life cycle.
The life cycle of prawns is rather short with species such as school prawns living, for the most part, a little over a year while larger species such as eastern king prawns are probably up to two years old, with some individuals perhaps entering a third year of life.
Prawns have specific habitat preferences which differ between species and between life stages for the same species. For instance, eastern king prawns are less tolerant of low salinity than school or inshore greasyback prawns and so whilst in estuaries will be found in the ‘marine dominated’ area of the estuary. Generally, prawns prefer a substrate with high mud content.
School prawns though are found in muddy substrate in estuaries but in ocean waters prefer fine sand. Eastern king, school and inshore greasyback prawns prefer a vegetated to an unvegetated substrate.
In terms of movements during their life times, inshore greasyback prawns would be confined to their ‘home’ estuary or coastal lake. Tagged school prawns on the other hand have been reported to have moved up to 120 km from their ‘home’ estuary, whilst tagged eastern king prawns have been recorded as moving northwards over distances of up to 1333 km (Figure 4).
Total prawn catch in any one season therefore depends upon a complicated interaction of many factors including the success of spawning, the effects of environmental fluctuations, the success of larval stages in finding suitable habitat and, the effects of predators and diseases on all stages of prawn’s life cycle.
Diseases and parasites:
As is common with all other animals including humans, prawns may suffer from a range of diseases and parasites. Some of these kill the prawn whilst others result in changes in appearance and behaviour.
One common symptom of stress in a prawn is opaque and white segments in the tail. This can be caused by viruses, bacteria, protozoans or physical trauma. The incidence of disease or parasitic infection is low in the wild but increases under aquaculture conditions when the prawns are in high densities and transmission is thereby enhanced.
Viral diseases including taura syndrome, ‘white-spot’ disease, and ‘yellow-head’ disease are major health problems with prawn farming worldwide, but Australia remains free of these exotic diseases. However, infections caused by indigenous prawn viruses such as gill-associated virus do affect the industry.
Species descriptions:
What follows is a guide to identifying the main prawn species contributing to production in NSW.
Eastern king prawn – Melicertus plebejus – Body is cream to yellow in colour. The rostrum is straight, extends to the back edge of the carapace and has one ‘tooth’ on the under side. The tips of the uropods are blue.
School prawn – Metapenaeus macleayi – Body is translucent with tinges of brown to green. The rostrum is curved upwards, does not extend to the back of the carapace and does not have any ‘teeth’ on the under side. The tips of the uropods may be blue.
Inshore greasyback prawn – Metapenaeus bennettae
– Body is translucent with speckles of brown. There are fine hairs in patches over the body that give the animal a ‘rough’ or ‘greasy’ feel. The rostrum is straight, does not extend to the back of the carapace and does not have any ‘teeth’ on the under side. The tips of the uropods may be brown.
Royal red prawn – Haliporoides sibogae – Body is uniformly red to pink in colour. The rostrum is short and at the front edge of the carapace.
Black tiger prawn – Penaeus monodon – Body is pigmented dark brown to blue-black and has distinct white saddles. The rostrum extends to the back of the carapace and has two or three teeth on its under side.
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