the effects of salinity on the survival, growth and...

2006) 151–162www.elsevier.com/locate/aqua-online

Aquaculture 260 (

The effects of salinity on the survival, growth and haemolymphosmolality of early juvenile blue swimmer crabs, Portunus pelagicus

Nicholas Romano ⁎, Chaoshu Zeng

Tropical Crustacean Aquaculture Research Group, School of Marine Biology and Aquaculture, James Cook University,Townsville, Qld 4811, Australia

Received 4 April 2006

Abstract

The blue swimmer crab, Portunus pelagicus, is an emerging aquaculture species in the Indo-Pacific. Two experiments wereperformed to determine the effects of salinity on survival, growth and haemolymph osmolality of early juvenile P. pelagicus crabs.The salinities tested for the first experiment were 10, 15, 25 and 40 ppt, and for the second experiment 5, 20, 30, 35 and 45 ppt.Each salinity experiment was triplicated, with each replicate consisting of 10 stage 4 juveniles. Each experiment lasted 45 days.Mortalities and incidence of “molt death syndrome” were recorded daily, while the intermolt period, carapace length, carapacewidth and wet weight were measured at each molt. At the end of the experiments the haemolymph osmolality and dry weights weremeasured.

Results demonstrate that salinity significantly affects both the survival and growth of early P. pelagicus juveniles. Mortality wassignificantly higher (p<0.01) for juveniles cultured at salinities ≤15 ppt and at 45 ppt. At a salinity of 5 ppt a complete mortalityoccurred on day 20. In all salinity treatments, the majority of mortalities were due to “molt death syndrome”. In experiment 1,immediate effects of salinity on growth and development were seen at 10 ppt as the intermolt period was significantly longer(p<0.01) and the mean carapace size increase was significantly less (p<0.01) at the first molt compared to the other treatments.Meanwhile, the specific growth rates (carapace length, width and wet weight) were significantly lower (p<0.05) at high salinities(≥40 ppt) due to longer intermolt periods and significantly lower (p<0.05) carapace size or wet weight increases.

The haemolymph osmolality exhibited a positive linear relationship with the culture medium with an isosmotic point of1106 mOsm/kg, equal to a salinity of approximately 38 ppt. Based on the osmolality graph, high metabolic cost for osmoregulationdue to increased hyper- and hypo-osmotic stress appeared to cause lower survival and specific growth rates of the crabs. The resultsdemonstrate that a salinity range of 20–35 ppt is suitable for the culture of early juvenile P. pelagicus.© 2006 Elsevier B.V. All rights reserved.

Keywords: Salinity; Survival; Growth; Osmolality; Portunus pelagicus; Early juveniles

⁎ Corresponding author. Tel.: +61 7 47256482; fax: +61 747814585.

E-mail address: [email protected] (N. Romano).

0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.aquaculture.2006.06.019

1. Introduction

Blue swimmer crabs, Portunus pelagicus, alsoknown as sand crabs are native throughout the Indo-West Pacific region (Xiao and Kumar, 2004). Theirharvests support important commercial fisheries in theregion and their popularity is growing fast with

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http://dx.doi.org/10.1016/j.aquaculture.2006.06.019

152 N. Romano, C. Zeng / Aquaculture 260 (2006) 151–162

estimated landings increasing from 947,000 tonnes in1990 to 1,500,000 tonnes in 1998 (Otto et al., 2001).Apart from the traditional sale as hard-shell crabs therehas been a substantial increase in their utilisation forpastuerised canned crabmeat and for the production ofsoft-shell crabs. For example, exportation of pasteurisedblue swimmer crab meat to the United States, Japan andSingapore reportedly generates a multi-million dollarrevenue for Indonesia annually (Muna, 2005; Setyadiand Susanto, 2005). To meet the increasing marketdemands for soft-shell crabs, P. pelagicus crabs, havebeen individually held in compartments within arecirculating system to produce soft-shell crabs inAustralia. The efficient and expedient harvesting ofnewly molted soft-shell crabs is made possible by aperiodic robotic monitoring system (O'Neill, 2003). Inaddition, lined ponds have also been proposed for thefree-range production of soft-shell P. pelagicus whererecently molted crabs are removed via periodicobservation (Walker, 2006). Currently, blue swimmercrabs are largely sourced from fisheries which isunreliable and seasonal (Otto et al., 2001). Thereforethe aquaculture interest of this species is growing due totheir relative ease of hatchery production (Walker, 2006)and fast growth rates (Josileen and Menon, 2005).

As the aquaculture potential for this speciesincreases, an understanding of the basic cultureconditions is necessary to optimise production. Salinityis one of the most important abiotic factors inaquaculture and while many crustacean species exhibitsome degree of euryhalinity (Pequeux, 1995), optimalsalinity levels for growth, survival and productionefficiency are often species-specific (Rouse and Karta-mulia, 1992; Bray et al., 1994; Kumlu and Jones, 1995;Kumlu et al., 2001; Soyel and Kumlu, 2003; Ruscoe etal., 2004). Furthermore, osmotic stress has beenreported to elicit physiological responses such asincreased dissolved oxygen consumption (Chen andChia, 1996; Spanopoulos-Hernández et al., 2005) andammonia excretion (Chen and Lin, 1994b; Lemos et al.,2001; Silvia et al., 2004) that may substantially alter theculture environment in a closed culture system.

Presently, there is no published information on thesalinity tolerance and optimal salinity levels for earlyP. pelagicus crabs (Lestang et al., 2003a), althoughtheir natural distribution appears to be dependent onsalinities (Kangas, 2000; Lestang et al., 2003b). It hasalso been reported that blue swimmer crab juvenilesemigrate in mass numbers preceding seasonal salinityreductions in Western Australian estuaries (Potter et al.,1983). These reports suggest that early juvenile blueswimmer crabs are sensitive to either salinity fluctua-

tions or low salinity levels. Experiments were designedand conducted to obtain information on the effects ofsalinity on the survival and growth of the early P.pelagicus juveniles and to investigate the underlyingphysiological mechanisms. Such knowledge has sig-nificant implications in aquaculture as it can be usedfor farm site selection as well as the manipulation ofsalinity in a recirculating system to maximise produc-tivity. The information will also enhance our under-standing on juvenile habitat preferences and migrationof P. pelagicus juveniles.

2. Materials and methods

2.1. Source of experimental crabs

Blue swimmer crab broodstock were caught fromestuaries in Townsville, north Queensland, Australia,using baited traps. All broodstock were labeled andmaintained in outdoor 1000-l oval recirculating tanks(temperature maintained at 28±2 °C and salinity at 32±2 ppt) at the Marine and Aquaculture Research FacilityUnit (MARFU), James Cook University. Each day, thebroodstock were fed to satiation on an alternation ofprawns, mussels and squid. When a berried female wasfound it was disinfected in a static formalin bath(concentration 50 μl l−1) for 6 h.

After disinfection, the berried female was transferredindoors and individually held in a 300-l tank forhatching at a temperature of 26±1 °C and salinity of34±1.5 ppt. The tank water was aerated and continuallycirculated through three cartridge filters (10, 5 and1 μm) and a UV steriliser. The berried female was notfed and faeces and discarded eggs were siphoned dailyfrom the tank, accompanied with about a 10% waterexchange.

The juvenile crabs used in the first and secondsalinity experiments came from fifth and sixthconsecutive spawns of the same female. Uponhatching, the aeration was turned off and the activelarvae were siphoned from the hatching tank. Larvaewere stocked at approximately 500 larvae l−1 andcultured at a salinity of 25 ppt at 29±1 °C. Newlyhatched larvae were initially fed rotifers (Branchionussp.) at 20–40 individuals ml−1 and the rotifer densitywas maintained by daily additions of microalgaeNannochloropsis sp. From the Zoea II stage onwards,Artemia (INVE, AAA) nauplii were hatched and addeddaily to the larval tanks, without enrichment, and theArtemia density was successively increased from aninitial 1–2 individuals ml−1 to 3–5 individuals ml−1

by the time the megalopa stage was reached. Daily

153N. Romano, C. Zeng / Aquaculture 260 (2006) 151–162

water exchanges were initially between 10% and 15%at the Zoea 1 and Zoea II stages and increased to 20–50% from Zoea III onwards.

Upon larval metamorphosis to the first crab stage(C1) they were transferred to two recirculating 300-lcircular holding tanks at 27±2 °C and 32±2 ppt. Allcrabs were fed a combination of frozen Artemia naupliiand ongrown adults and gradually weaned onto aformulated crumble feed produced by Ridley® for thetiger shrimp Penaeus monodon (43% protein, 6% fat,3% fibre). They were then exclusively fed on the shrimpfeeds until the experiment commenced.

2.2. Experimental design and set-up

Two salinity experiments with similar design and set-ups were conducted in succession. The salinity treat-ments in the first experiment were 10, 15, 25 and 40 pptand for the second experiment, 5, 20, 30, 35 and 45 ppt.Each treatment was triplicated and each replicateconsisted of ten crabs individually cultured in roundblack plastic containers (diameter 16 cm×height19 cm). The black containers had numerous 3.75 mmholes and were bathed in a 300-l tank (diameter1.1 m×height 0.4 m) filled with 200-l of pre-adjustedseawater.

A total of 12 tanks (4 salinity treatments ×3replicates) were used in the first experiment and 15tanks (5 salinity treatments×3 replicates) were used forthe second experiment. All tanks were arranged in arandom block in parallel rows under a shed area. A clearplastic lid was placed on top of each tank to reduceevaporation and heat loss and the water was vigorouslyaerated to maintain adequate dissolved oxygen (DO)levels and water mixing. A thermostat controlledimmersion heater maintained water temperaturesabove 26 °C in each tank throughout the experimentsand a natural photoperiod was maintained.

The salinity experiments commenced when themajority of crabs produced in the hatchery molted tothe crab 4 stage (C4), approximately 2 weeks aftersettlement. Healthy and intact C4 juveniles wererandomly chosen from the holding tanks and weighed(mean weight: experiment 1=0.027±0.001 g andexperiment 2=0.028±0.001 g) prior to being indivi-dually placed in a black container. All containerswere labeled to enable tracking of consecutive moltsfrom each crab. Salinity acclimation involved a step-wise increase/decrease at a rate of 3 ppt h−1 from theoriginal salinity (32 ppt) through the addition ofeither condensed seawater (brine) or de-chlorinatedfreshwater.

At the early stage of the experiments, a 10% dailywater exchange was performed for each 300-l tank.However with the growth of the crabs the waterexchange was successively increased to approximately80% daily by the end of the experiment. Culture waterused throughout the experiments were obtained fromfiltered natural source seawater (32±2 ppt) and adjustedwith either de-chlorinated freshwater or brine producedthrough evaporation. Every 2 weeks a 100% waterexchange was performed and all containers and tankswere sterilised with chlorine solution.

Throughout the experiment crabs were fed theRidley® shrimp feeds daily to satiation. Uneaten food,faeces and debris were siphoned out daily in themorning and checks were made for any mortalities,including incidences of “molt death syndrome” (MDS).MDS was defined as a mortality resulting from aninability of a crab to completely shed the oldexoskeleton during molting and diagnosed as anincomplete disengagement of the old shell from a deadcrab. Successful molts were recorded for the intermoltperiod as well as the carapace length and width of themolt using a Mitutoyo® digital caliper (0.01 mm). Thecarapace length was defined as the distance from thelongest rostral spike to the abdomen and the carapacewidth was defined as the distance between the tips of thetwo lateral spines. To allow adequate water uptake andrecalcification (and therefore stabilisation of weight)(Cadman and Weinstein, 1988) 4-day post molt crabswere measured for wet weight using an AdventurerPro® digital scale (0.001 g). Prior to weighing, each crabwas blotted dry with a tissue and placed on the zeroedscale in a small container filled with the culture water.

At the end of the experiments the final sizes of allsurviving crabs were measured. Unfed intermolt crabsin each salinity treatment were measured for osmolalityin accordance with Lignot et al. (2000). To obtainhaemolymph osmolality measurements, a syringe wasused to draw approximately 10 μl of haemolymph at28 °C through the proximal arthrodial membrane at thebase of the right second walking leg of the crabs. Eachaliquot of haemolymph was immediately analysed on acryoscopic osmometer (Osmomat 030; Gonotec) forosmolality. For dry weight measurements all crabs of atleast 4-day post molt were placed in an oven at 60 °Cfor 48 h and dry weight measurements were madeusing the Adventurer Pro® digital scale.

Every morning the salinity was checked with a handrefractometer (Iwaki, Japan) and adjusted if necessary.Every second day the pH (WP-80; TPS) and dissolvedoxygen levels (WP-82Y; TPS) were digitally measured.Every week the ammonia levels were measured with a


test kit (Aquarium Pharmaceuticals®). Throughout bothexperiments the ammonia ranged from 0.001 to 0.1 mgl−1, pH between 7.7 and 8.3 and the DO remained above6 mg l−1. Temperatures were measured with min./max.thermometers. The mean temperature during the firstexperiment was 30±4 °C and during the secondexperiment was 31±4 °C as the former was conductedduring cooler months.

2.3. Data analysis

To assess differences in growth rates over time thecarapace size or wet weight increase at each crab stagewas obtained by subtracting the new carapace size/wetweight by the old carapace size/wet weight at each molt.The specific growth rate (SGR) of all surviving crabswas calculated using the following formula:

SGR ¼ ðlnWf � lnW0Þ=t � 100

where SGR is specific growth rate; Wf is the final size/weight; t is days of culture; W0 is the initial size orweight.

Final survival, SGR, mean intermolt period, carapacesize, wet and dry weights of the juvenile crabs at eachsalinity treatment were analysed using one-wayANOVA after confirmation of normality and homo-geneity of variance. Log or arcsine transformation ofdata was performed before further analysis whenevervariances were not homogeneous. If any significantdifferences were detected (p<0.05), differences amongtreatments were identified using Tukey's HSD test (Zar,1999). Direct comparisons of survival and growth databetween the first and second experiment were notperformed as the experiments were run at different timesand data could be compounded by batch variations. Alinear regression was used to determine the relationshipbetween haemolymph and medium osmolality (mOsm/kg). The isosmotic line was plotted as points where thehaemolymph osmolality equaled the medium osmolal-ity. All statistical analysis were performed using SPSSstatistical package version 11.0.

3. Results

3.1. Survival

The data from the two experiments showed thatsalinity significantly affected the survival of early P.pelagicus juveniles (Fig. 1). Among the four treatmentsin experiment 1, the survival rate at 25 ppt was thehighest (93.3±6.7%) followed by 40 ppt (86.6±8.8%),

15 ppt (50.0±5.8%) and 10 ppt (10.0±5.8%) (Fig. 1A).Statistical analysis showed that the mortality rate wassignificantly higher (p<0.01) at 15 ppt than those of 25and 40 ppt. No significant differences (p>0.05) weredetected between 25 and 40 ppt (Table 1).

Among the five salinity treatments in experiment 2,the survival rate at 20 ppt was the highest (93.3±6.7%),followed by 30 ppt (63.3±16.7%), 35 ppt (63.3±6.7%),45 ppt (43.3±3.3%) and 5 ppt (0%) (Fig. 1B). A totalmortality occurred at 5 ppt on day 20, which wassignificantly higher (p<0.01) than any other treatment.The mortality rate was significantly higher (p<0.01) at45 ppt compared to 20 ppt, while the 30 ppt and 35 ppttreatments were not significantly different (p>0.05)from either the 20 ppt or the 45 ppt treatment (Table 1).

The majority of mortalities in all salinity conditionswere due to “molt death syndrome” (Table 1), with thehighest incidence of MDS occurring at 15 ppt.

3.2. Growth

In experiment 1, the mean intermolt period of the C5crabs cultured at 10 ppt was significantly longer(p<0.01) compared to that of the 15, 25 and 40 ppttreatments. However, from the C6 stage onwards nofurther statistical tests on growth or development wereperformed for the 10 ppt salinity treatment due to areduced number of surviving crabs. No significantdifference in the mean intermolt period was detected(p>0.05) among the 15, 25 and 40 ppt treatments (Table2). In experiment 2, the mean intermolt period at 45 pptwas significantly longer (p<0.01) at each crab stagecompared to that of 30 ppt. It was also significantlylonger (p<0.01) than those at 20 and 35 ppt at C5 andC6 stage. No significant difference was detected(p>0.05) among the 20, 30 and 35 ppt treatments(Table 2). A complete mortality occurred at 5 ppt andthere were no successful molts.

The mean carapace length and width increase at thefirst molt (molted to C5) in experiment 1 wassignificantly less (p<0.01) at 10 ppt compared to allother salinity treatments (Fig. 2A and B). However, nosignificant difference (p>0.05) in the mean wetweight increase was detected (Fig. 2C). After the C5to C6 stage the mean carapace size and wet weightincrease was often significantly less (p<0.01) at40 ppt than at 25 ppt (Fig. 2A–C). The meancarapace length and width increase at the C7 to C8stage was significantly less (p<0.01) at 15 ppt than at25 ppt (Fig. 2A and C). Among the salinity treatmentsin experiment 2, the mean carapace length and wetweight increase were lowest at 45 ppt for all crab

Fig. 1. Survival of early juvenile P. pelagicus under various salinity conditions in experiment 1 (A) and experiment 2 (B).


stages and the differences from the other treatmentswere often significant (Fig. 3A and C). With theexception of the molt to the C4 to C5 and C5 to C6

Table 1Incidence of “molt death syndrome” and total number of mortalities ofearly P. pelagicus juveniles in each salinity treatment

Experiment1

Incidenceof MDS

Totalmortality

Experiment2

Incidenceof MDS

TotalMortality

Salinity Salinity10 ppt 13 27c⁎ 5 ppt 8 30c

15 ppt 15 15b 20 ppt 2 2a

25 ppt 2 2a 30 ppt 9 10ab

40 ppt 4 4a 35 ppt 11 12ab

45 ppt 7 16b

⁎ The different letters indicate significant differences (p<0.05).

stages, the carapace width increase at 45 ppt wassignificantly less at each molt (p<0.01) compared tothose at 20, 30 and 35 ppt treatments. No significantdifferences in carapace length, carapace width or wetweight increases were detected (p>0.05) among the20, 30 and 35 ppt treatments at any crab stage (Fig.3A and C).

Among the four salinity treatments in experiment 1,no significant difference (p>0.05) in the final dryweight was found (Fig. 4A), although the percentage dryweight was significantly higher (p<0.05) at 40 ppt thanat 15 ppt (Table 3).

Among the four salinities tested in experiment 2, thefinal dry weight was significantly lower (p<0.05) at45ppt than at 20, 30 and 35ppt. The final dry weight was

Table 2The mean intermolt period (days) (±S.E.) of different crab stages of P. pelagicus at different salinities in experiments 1 and 2

Crabstage

Experiment 1 Experiment 2

10 ppt 15 ppt 25 ppt 40 ppt 20 ppt 30 ppt 35 ppt 45 ppt

C5 10.6±0.7b⁎ 8.5±0.2a 7.8±0.2a 8.4±0.2a 6.1±0.1A 5.4±0.7A 5.5±0.1A 7.4±0.2B

C6 7.5±0.2⁎⁎ 7.8±0.2a 7.7±0.2a 8.1±0.2a 6.6±0.2A 6.2±0.2A 6.9±0.2A 8.6±0.5B

C7 11.0±0.5⁎⁎ 8.8±1.1a 8.5±1.1a 8.7±0.9a 10.1±0.4AB 8.6±0.3A 9.6±0.5AB 10.8±0.3B

C8 n/a 10.5±0.3a 10.0±0.3a 10.0±0.3a 9.7±0.3AB 9.2±0.3A 9.5±0.3AB 10.7±0.2B

⁎ The different letters indicate significant differences (p<0.05).⁎⁎ Statistics not performed due to a reduced sample of surviving crabs.


significantly lower (p<0.05) at 20ppt than at 35ppt,while no significant differences were detected (p<0.05)between 30 and 35ppt. The percentage dry weight wasnot significantly different (p>0.05) between all treat-ments of the experiment (Table 3).

The specific growth rates (SGR) of carapace length,carapace width and wet weight in experiment 1 were thehighest at 25 ppt and lowest at 10 ppt, although the 10 ppttreatment was not included in statistical analysis due tothe low number of surviving crabs at the end of theexperiment. The SGR for carapace length or wet weightwas significantly lower (p<0.05) at 40 ppt than both the15 and 25 ppt treatments, while the SGR for carapacewidth was significantly less (p<0.05) at 15 and 40 pptthan at 25 ppt (Table 4). In experiment 2, the SGR forcarapace length, carapace width and wet weight wassignificantly less (p<0.01) at 45 ppt than those of the 20,30 and 35 ppt treatments, while no significant differencewas detected among the 20, 30 and 35 ppt treatments(Table 4).

3.3. Haemolymph osmolality

Fig. 4 shows the mean haemolymph osmolality ofcrabs reared under various salinities at the end of theexperiments. The haemolymph osmolality of thejuvenile crabs showed a clear and significant (p<0.05;r2 =0.950) positive linear relationship (Y=0.752(X)+278) to the osmolality of the culture medium. Theisosmotic point equaled 1106 mOsm/kg, equivalent to asalinity of approximately 38 ppt.

4. Discussion

Results of the current experiments indicate that asalinity range outside 20–35 ppt can significantly reducesurvival, growth and development of early juvenile blueswimmer crabs. Adopting an acclimation rate of 3 ppth−1, initial high mortalities occurred at 5 ppt and 45 ppt.This is in contrast to early juvenile mud crabs (Scyllaserrata) which reportedly experienced no initial mor-

talities at the same salinities when an acceleratedacclimation rate of 5 ppt h−1 was applied (Ruscoe etal., 2004). The high initial mortalities in this study are inagreement with previous studies on other crustaceanswhich suggest early blue swimmer juveniles are highlysensitive to rapid changes to extreme salinities (Kumluand Jones, 1995; McGraw and Scarpa, 2004).

After the initial acclimation period, significantsustained mortalities continued to occur at 5, 10 and15 ppt indicating that early P. pelagicus juveniles havedifficulty in adapting to prolonged exposures to lowsalinity conditions. At a salinity of 5 ppt a completemortality occurred on day 20. Interestingly the majorityof sustained deaths in all salinity treatments were due to“molt death syndrome” (MDS), which has beenpreviously linked with high temperatures, geneticfactors (Rouse and Kartamulia, 1992) and inadequatenutrition (Bowser and Rosemark, 1981; Gong et al.,2004).

The SGR of early P. pelagicus juveniles cultured atthe higher salinities (40–45 ppt) were significantly lessin both experiments. However, the salinity effects of45 ppt on growth and development were moreimmediate as the mean intermolt period was signifi-cantly longer, and the carapace size and wet weightincreases were significantly lower at each molt com-pared to those obtained at 20, 30 and 35 ppt treatments.Similarly, immediate effects on growth and develop-ment were observed at 10 ppt with a significantlyprolonged intermolt period and reduced carapace sizeincrease at the first molt. Salinity experiments on earlyjuveniles of several other portunid crabs, includingCallinectes sapidus, C. similis (Guerin and Stickle,1997) and S. serrata (Ruscoe et al., 2004), have shownno significant growth differences when cultured atsalinities of 10–40 ppt. This suggests that early P.pelagicus juveniles have a substantially lower toleranceto extreme salinities than other portunid crabs, and thisfinding is supported by the fact that early P. pelagicusjuveniles exhibit weak osmoregulatory abilities from theosmolality results obtained in this study.

Fig. 2. Mean increase in carapace length (mm) (A), carapace width (mm) (B), and the wet weight (g) (C) in early juvenile P. pelagicus cultured underdifferent salinities in experiment 1. Statistical tests were not performed beyond the C6 stage for the 10 ppt due to a reduced sample size (n=3).Different letters indicate a significant difference (p<0.05).


Fig. 3. Mean increase in carapace length (mm) (A), carapace width (mm) (B), and the wet weight (g) (C) in early juvenile P. pelagicus cultured underdifferent salinities in experiment 2. Different letters indicate a significant difference (p<0.05).


Fig. 4. Relationship between haemolymph osmolality and medium osmolality (±S.E.) of early juvenile P. pelagicus (mean weight (g)=1.225±0.056)cultured in different salinities.


The osmoregulatory ability of aquatic animals can bedetermined by measuring the haemolymph osmolality atvarious salinity conditions (Lignot et al., 2000) andcomparing with the osmolality of the medium. Thecurrent study showed that the blue swimmer crabhaemolymph exhibited a positive linear relationshipwith the medium osmolality. Similar results have beenreported for penaeid prawns (Chen and Lin, 1994a,b;Sang and Fotedar, 2004; Setiarto et al., 2004) and otherportunid crabs (Chen and Chia, 1997; Guerin andStickle, 1997). While the juvenile crabs exhibited hypo-osmoregulation above the isosmotic line (i.e. >38 ppt)and hyper-osmoregulation below the isosmotic line (i.e.<38 ppt), studies have shown that S. serrata (Chen andChia, 1997), C. sapidus and C. similis (Guerin andStickle, 1997; Li et al., 2006) juveniles are compara-tively stronger hyper-osmoregulators than P. pelagicusjuveniles. This is indicated by the smaller deviation

Table 3The final mean dry weight (g) and percentage dry weights of early juvenile

Experiment 1

Salinity 10ppt 15pptFinal dry weight 0.080±0.002⁎⁎ 0.354±0.% Dry weight 26.1±2.1⁎⁎ 27.9±0.6

Experiment 2

Salinity 20ppt 30pptFinal dry weight 0.295±0.022B 0.361±0.% Dry weight 29.5±0.8a 28.9±0.8

⁎ Different letters indicate significant differences.⁎⁎ Statistics not performed due to a reduced sample of surviving crabs.

from the isosmotic line and steep slope (Chen and Lin,1994a,b; Lignot et al., 2000) between salinities of 10and 15 ppt for P. pelagicus.

It is well known that hyper-osmoregulation incrustaceans requires energy in the form of protein(Rosas et al., 1999; Setiarto et al., 2004; Silvia et al.,2004) or lipids (Lemos et al., 2001; Luvizotto-Santos etal., 2003; Palacios et al., 2004; Sang and Fotedar, 2004).The percentage dry weights may reflect this as it canindicate increased water content and fewer energyreserves (Sang and Fotedar, 2004) that are channeledfor growth. Reduced energy reserves likely occurredwith the juveniles cultured at low salinities (i.e. 10 and15 ppt) as they had lower percentage dry weights andsignificantly reduced survival and growth. Furthermore,it is worth noting that after approximately 2 weeks oflow salinity exposure, the juveniles were observed tohave focal carapace discolorations of various sizes. This

P. pelagicus cultured at different salinities

25ppt 40ppt044A⁎ 0.379±0.029A 0.309±0.029Ab 30.1±0.7ab 31.1±0.7a

35ppt 45ppt028AB 0.411±0.056A 0.130±0.050Ca 31.2±1.0a 31.2±0.8a

Table 4The mean Specific Growth Rates (SGR±S.E.) of the carapace length (mm), carapace width (mm) and wet weight (g) of early juvenile P. pelagicuscultured at different salinities

Experiment 1

Salinity 10 ppt 15 ppt 25 ppt 40 pptSGR carapace length 1.62±0.12⁎⁎ 2.60±0.05ab⁎ 2.75±0.06 a 2.45±0.05b

SGR carapace width 1.94±0.18⁎⁎ 2.88±0.05b 3.14±0.06a 2.82±0.05b

SGR wet weight 5.081±0.381⁎⁎ 8.162±0.110ab 8.614±0.182a 7.692±0.134b

Experiment 2

Salinity 20 ppt 30 ppt 35 ppt 45 pptSGR carapace length 2.67±0.07a 2.81±0.06a 2.77±0.08a 2.04±0.07b

SGR carapace width 3.08±0.08a 3.18±0.07a 3.14±0.07a 2.29±0.06b

SGR wet weight 8.693±0.220a 9.041±0.172a 8.952±0.239a 6.551±0.864b

⁎ The different letters indicate significant differences (p<0.05).⁎⁎ Statistics not performed due to a reduced sample of surviving crabs.


may have been caused by reduced phenoloxidaseactivity at low salinities (Lamela et al., 2005) that isresponsible for melanin synthesis (Adachi et al., 2005).

It may be argued that salinities closer to the isosmoticpoint result in decreased metabolic demands andtherefore increased growth, as seen with penaeidshrimps (Chen et al., 1995). However, portunid crabsincluding C. sapidus, C. similis (Guerin and Stickle,1997), S. serrata (Chen and Chia, 1997; Ruscoe et al.,2004) reportedly have optimal salinity levels below theirisosmotic points. Pequeux (1995) has suggested that athigh salinities the haemolymph osmolality of “weakosmoregulators” may exhibit a close parallel associationwith the isosmotic line due to either a disruption inosmoregulation or a strategy to reduce the osmoticgradient between the haemolymph and environment.The latter appears to be the case in this study. The earlyP. pelagicus juveniles cultured from salinities 35 ppt andabove exhibited a reduced deviation from the isosmoticline. Furthermore, these juveniles had decreased per-centage water contents which have been similarlyreported for Penaeus chinensis (Chen and Lin, 1994b;Chen et al., 1995), P. setiferus (Rosas et al., 1999) andP. latisulcatus (Sang and Fotedar, 2004). Decreasedpercentage water may be the result of increased freeamino acids to counteract high salt loads (Luvizotto-Santos et al., 2003; Silvia et al., 2004; Wang et al., 2004)that are likely to be essential for growth and molting.However, other reports have suggested that reducedgrowth at high salinity conditions may also be attributedto reduced feed assimilation or consumption (Kumluand Jones, 1995; Lemos et al., 2001; Sang and Fotedar,2004). While feeding rates were not quantified in thisstudy, it was noted that the crabs cultured at highsalinities, particularly at 45 ppt, were less active uponthe introduction of food. These two factors may have

synergistically contributed to the significantly reducedgrowth of the early blue swimmer crab at salinitiesabove the isosmotic point.

One of the interesting findings in this experimentwas the high incidence of MDS in all salinitytreatments. It has been demonstrated by Gong et al.(2004) that incorporating dietary trace metals andlipids significantly reduced the incidence of MDS inthe shrimp Litopenaeus vannamei by increasing theirosmoregulatory capacity. Therefore, since early juve-nile blue swimmer crabs exhibit weak osmoregulatoryabilities and their current production largely relies oncommercial shrimp feeds, it appears that designing aspecies-specific diet may be necessary to reduce MDSand improve growth over a broad range of culturesalinities.

The results of the current experiments demonstratethat salinity can have an immediate and significanteffect on survival and growth and a salinity rangebetween 20 and 35 ppt is recommended for the cultureof early juvenile blue swimmer crabs. The naturaldistribution range of P. pelagicus in Western Australianestuaries appears to reflect this salinity range (Lestang etal., 2003b). This suitable salinity range of early P.pelagicus, as revealed by the present study, hassignificant implications for aquaculture, as it can beutilised for various purposes including farm siteselection and maintaining salinity levels within arecirculating system, to maximise productivity.

Acknowledgements

We would like to thank Brendan Fry for hisassistance with final sampling and a special thanks toAdriana Toro-Suarez for her assistance with theosmolality tests and final size/dry weight measurements.


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