Biology And Life Cycles Of Prawns

Food and feedingAlmost all crustaceans have the same basicmouthparts although the diversity in form andfunction is enormous. The main part of the prawn dietis made up of plant material, microorganisms, smallshellfish and worms. In aquaculture, prawns are fedartificial diets. Most digestion is rapid and completedin about six hours.Prawns search the bottom for food with theirpereopods which have chemo-sensory hairsattached. Any food that is found is held by thosehead appendages known as maxillipeds and theteeth or mandibles are used to bite or tear pieces off.The maxillipeds can be used to push large pieces offood away as the mandibles grasp it, thus tearing thefood into manageable portions.ReproductionThe prawn species which are commercially fishedare bisexual, that is, the genders are separate, withthe mature female being larger than the male. Forexample, a mature female eastern king prawn couldbe several times heavier than a mature male.Male prawns are readily identified by the largecopulatory organ, known as the petasma, foundbetween the foremost pair of swimming legs (seeFigure 2). Corresponding structures are not presentin the female and the foremost pair of pleopods isidentical with the other four pairs. Females possessa structure known as the thelycum between the lastpair of pereopods which provides an anchor site forthe male sperm packet received during copulation(see Figure 2). Males and females can thus bereadily identified on the presence or absence of apetasma or a thelycum.Ovaries containing eggs can often be seen throughthe shell in the head and tail of mature femaleprawns. The colour of ovaries changes from paleyellow to olive or orange-brown as they mature. Themature female needs to be in the soft-shelled ornewly moulted stage just prior to copulation while themale must be in the hard-shelled condition.Fertilisation of eggs is internal and spawning isthought to occur soon afterwards. Females arecapable of spawning more than once in a season. Thenumber of eggs carried by a female varies with sizeand between species. The seasonality of spawningvaries between species and between stocks of thesame species. For instance, eastern king prawnsspawn year around in waters off Queensland, whereasthey do not spawn in winter off NSW.Life cycles and agePrawn life cycles in NSW waters can be consideredto be of three types:A. Estuarine – the entire life cycle is completed inwaters of less than sea water salinity. An example ofthis type would be the inshore greasyback prawn;B. Marine – the entire life cycle is completed inoceanic waters. An example of this type would be theroyal red prawn6  Primefact 268 – Biology and life cycles of prawnsC. Mixed – postlarval and juvenile stages of theprawn’s life cycle occur in estuarine waters of lessthan sea water salinity whilst the adolescent, adult,egg and larval stages occur offshore in oceanicwaters. Examples of this most common type includeeastern king and school prawns.The ‘mixed’ life cycle is diagrammatically representedin Figure 3. Fertilized eggs are shed onto thebottom in oceanic waters where they remain for ashort period until they hatch. The first larval stagecalled a nauplius emerges from the spherical eggand starts swimming up towards the surface. Thisnaupliar stage goes through several moults, gettinglarger with each moult, and ultimately changes to aprotozoea. The prawn similarly moults and growsthrough several protozoea then mysis stages beforedeveloping into a postlarva. The mysis stage is thefirst stage that somewhat resembles the appearanceof the adult prawn.The development time from spawning to postlarva forprawn species with ‘estuarine’ or ‘mixed’ life cyclesis between two and three weeks. Hatching successand larval survival are affected by water temperatureand salinity and are greatest in waters of the sametemperature and salinity as that where spawning tookplace.Up to the mysis stage the larvae are free swimmingand form part of the zooplankton found in the opensea. Zooplankton together with phytoplankton iseaten by a multitude of other animals, includinglarval prawns, and is a vital part of the ocean’s foodsystem. After the free swimming, planktonic mysisstage, postlarva adopt a bottom existence, reachthe shore and enter the rivers and coastal lakes Inthe estuaries and lakes, the juvenile prawns haveto adapt to wide fluctuations in, among other things,salinity and temperature. School prawns appear to bemore tolerant of these fluctuations than eastern kingprawns.After over-wintering in the estuaries and lakes thejuvenile prawns grow rapidly and upon some cue,start to migrate back to the ocean. These migrationsrise to a peak in the ‘dark’ period of the summer,lunar cycles and are fished enthusiastically by bothamateur and professional fishermen. The physiologyof prawns with a ‘mixed’ life cycle is such that theprawns require the conditions found in the deeperocean waters to reach maturity. Once they reachoceanic waters, the prawns rapidly mature, mate andspawn – thus completing the life cycle.The life cycle of prawns is rather short with speciessuch as school prawns living, for the most part, alittle over a year while larger species such as easternking prawns are probably up to two years old, withsome individuals perhaps entering a third year of life.Prawns have specific habitat preferences whichdiffer between species and between life stagesfor the same species. For instance, eastern kingprawns are less tolerant of low salinity than schoolor inshore greasyback prawns and so whilst inestuaries will be found in the ‘marine dominated’area of the estuary. Generally, prawns prefer asubstrate with high mud content. School prawnsthough are found in muddy substrate in estuaries

but in ocean waters prefer fine sand. Eastern king,school and inshore greasyback prawns prefer avegetated to an unvegetated substrate.Species descriptionsIn terms of movements during their life times, inshoregreasyback prawns would be confined to their ‘home’estuary or coastal lake. Tagged school prawns onthe other hand have been reported to have movedup to 120 km from their ‘home’ estuary, whilst taggedeastern king prawns have been recorded as movingnorthwards over distances of up to 1333 km (Figure 4).Eastern king prawn – Melicertus plebejus – Body iscream to yellow in colour. The rostrum is straight,extends to the back edge of the carapace and hasone ‘tooth’ on the under side. The tips of the uropodsare blue.Total prawn catch in any one season thereforedepends upon a complicated interaction of manyfactors including the success of spawning, the effectsof environmental fluctuations, the success of larvalstages in finding suitable habitat and, the effects ofpredators and diseases on all stages of prawn’slife cycle.Diseases and parasitesAs is common with all other animals includinghumans, prawns may suffer from a range of diseasesand parasites. Some of these kill the prawn whilstothers result in changes in appearance and behaviour.One common symptom of stress in a prawn is opaqueand white segments in the tail. This can be caused byviruses, bacteria, protozoans or physical trauma. Theincidence of disease or parasitic infection is low in thewild but increases under aquaculture conditions whenthe prawns are in high densities and transmissionis thereby enhanced. Viral diseases including taurasyndrome, ‘white-spot’ disease, and ‘yellow-head’disease are major health problems with prawn farmingworldwide, but Australia remains free of these exoticdiseases. However, infections caused by indigenousprawn viruses such as gill-associated virus do affectthe industry.ESTUARINEWhat follows is a guide to identifying the main prawnspecies contributing to production in NSW.School prawn – Metapenaeus macleayi – Body istranslucent with tinges of brown to green. The rostrumis curved upwards, does not extend to the back of thecarapace and does not have any ‘teeth’ on the underside. The tips of the uropods may be blue.Inshore greasyback prawn – Metapenaeus bennettae– Body is translucent with speckles of brown. Thereare fine hairs in patches over the body that givethe animal a ‘rough’ or ‘greasy’ feel. The rostrum isstraight, does not extend to the back of the carapaceand does not have any ‘teeth’ on the under side. Thetips of the uropods may be brown.Royal red prawn – Haliporoides sibogae – Body isuniformly red to pink in colour. The rostrum is shortand at the front edge of the carapace.Black tiger prawn – Penaeus monodon – Body ispigmented dark brown to blue-black and has distinctwhite saddles. The rostrum extends to the back of thecarapace and has two or three teeth on its under side.ResearchThe NSW Government has a history dating backto around 1926 in providing funding to explore forprawn resources, develop fisheries for these, andthen to monitor and assess the impact of this fishing.Current prawn research by the NSW Government isdirected towards doing research either through its ownscientific resources or by contractual arrangementswith external service providers to collect theinformation necessary to satisfy the requirements ultmatingspawningFigure 3. Generalised ‘mixed’ life cycle of a prawnPrimefact 268 – Biology and life cycles of prawns  7

LadyElliotIsland34BRISBANE BRISBANE BRISBANE 2Tweed Heads2Ballina245Coffs Harbour5Port StephensSYDNEY SouthWestRocks24SYDNEY SYDNEY Port Jackson CANBERRA CANBERRA CANBERRAGippsland LakesScale in kmFigure 4. Inferred movements of tagged eastern king prawns. Each map is labelled with the point of release. Where more than onerecapture occurred at a location, the appropriate number is recorded next to the pathway (after Montgomery 1990).the fisheries management strategies for the prawnfisheries. This includes (i) providing the information toassess the status of prawn stocks that are importantto the commercial and recreational fishers of NSW,(ii) minimizing the catch of unwanted animals(bycatch) in the fishing gear and maximizing thesurvival of discards, and (iii) minimizing the impact offishing gear upon aquatic habitat.Further, the NSW government was amongst thepioneers of prawn farming research in Australia anddeveloped many of the techniques used by industrytoday. More recently, research on prawn aquaculturehas focused upon prawn farm health managementand the use of inland, saline water to culture prawns.Details of the research and management of theprawn resources of NSW can be found on theIndustry & Investment NSW website atwww.industry.nsw.gov.auManagement State of New South Wales through Department of Industry andInvestment (Industry & Investment NSW) 2010. You may copy,distribute and otherwise freely deal with this publication for anypurpose, provided that you attribute Industry & Investment NSWas the owner.The fisheries of NSW are managed under theFisheries Management Act (1994). Fisheriesmanagement strategies and environmentalassessments have been completed for theestuary general, estuary prawn trawl and oceantrawl fisheries in accordance with NSW andCommonwealth law. The management strategiescontain goals and objectives for each fishery anda detailed description of the fishing arrangementsand new environmental requirements to ensureconservation of biodiversity and sustainable fishing.Like wise the aquaculture industry must follow strictdevelopment guidelines and is subject to the processof environmental assessment.8  Primefact 268 – Biology and life cycles of prawnsAcknowledgementsThis is an updated version of the documentpublished as NSW State Fisheries Leaflet No. 8.The update was done by Dr S. Montgomery. DrG. Allan and Mr D. Hale provided information aboutthe aquaculture industry and commercial fisheriesmanagement, respectively.ISSN 1832-6668Check for updates of this information sheet at:www.industry.nsw.gov.au/publicationsDisclaimer: The information contained in this publication isbased on knowledge and understanding at the time of writing(October 2010). However, because of advances in knowledge,users are reminded of the need to ensure that informationupon which they rely is up to date and to check currency of theinformation with the appropriate officer of Industry & InvestmentNSW or the user’s independent adviser.Job Number 7006

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 .