The effects of holding space on growth and survival of individually rearedthree-spot crab (Portunus sanguinolentus)

Stephen Nicholson a,b, David Mann b, Ravi Fotedar a, Brian Paterson b,*a Muresk Institute, Curtin University of Technology, Perth, Western Australia 6102, Australiab Bribie Island Aquaculture Research Centre, Department of Primary Industries and Fisheries, PO Box 2066, Bribie Island, Queensland 4507, Australia

Aquacultural Engineering 39 (2008) 30–36

A R T I C L E I N F O

Article history:

Received 8 April 2008

Accepted 11 May 2008

Keywords:

Crab

Intensive culture

Growth

Ecdysis

Survival

A B S T R A C T

The problem of cannibalism in communally reared crabs can be eliminated by separating the growing

crabs into holding compartments. There is currently no information on optimal compartment size for

growing crabs individually. 136 second instar crablets (Portunus sanguinolentus) (C2 ca. 7–10 mm

carapace width (CW)) were grown for 90 days in 10 different-sized opaque and transparent walled acrylic

compartments. The base area for each compartment ranged from small (32 mm � 32 mm) to large

(176 mm � 176 mm). Effects of holding space and wall transparency on survival, CW, moult increment,

intermoult period and average weekly gain (AWG) were examined. Most crabs reached instars C9–C10

(50–70 mm CW) by the end of experiment.

The final survival rate in the smallest compartment was 25% mainly due to moult-related mortality

predominantly occurring at the C9 instar. However, crabs in these smaller compartments had earlier

produced significantly larger moult increments from instar to instar than those in the larger

compartments (P < 0.05). Crabs in the smaller compartments (<65 mm � 65 mm) also showed

significantly longer moult periods (P < 0.05). The net result was that AWG in CW was 5.22 mm week�1

1 for the largest compartment and 5.15 mm week�1 in smallest and did not differ significantly between

compartment size groups (P = 0.916). Wall transparency had no impact on survival (P = 0.530) but a slight

impact on AWG (P = 0.014).

Survival rate was the best indicator of minimum acceptable compartment size (�43 mm � 43 mm) for

C10 crablets because below this size death occurred before growth rate was significantly affected. For

further growth, it would be necessary to transfer the crablets to larger compartments.

Crown Copyright � 2008 Published by Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Aquacultural Engineering

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1. Introduction

Farming of portunid swimming crabs, particularly mud crabs(Scylla spp.), is common to sub-tropical and tropical coastal regionsof Asia and until successful hatchery techniques were developed,has relied upon collection of wild juveniles (Williams andPrimavera, 2001). Focus on hatchery production of Scylla spp.began in the early 1990s due to concerns for depleting stocks ofwild juveniles and led to improvements in hatchery production ofpost-larvae of several portunid species (Mann et al., 1999; Ruscoeet al., 2004). Currently hatchery reared stocks are being tested forsuitability to intensive and semi-intensive farming conditions(Marshall et al., 2005). Two species of the genus Portunus, the blueswimmer crab Portunus pelagicus and three-spot crab Portunus

sanguinolentus, are considered as good candidates for aquaculture

* Corresponding author. Tel.: +61 7 3400 2003; fax: +61 7 3408 3535.

E-mail address: Brian.Paterson@dpi.qld.gov.au (B. Paterson).

0144-8609/$ – see front matter . Crown Copyright � 2008 Published by Elsevier B.V. A

doi:10.1016/j.aquaeng.2008.05.002

(Williams and Primavera, 2001), specifically for production of thehigh value ‘soft-shelled’ crab and both can be reared in hatcheries.

P. sanguinolentus is a tropical/subtropical swimming crabgenerally found in clean sandy, oceanic habitats (Sumpton et al.,1989; Stephenson and Campbell, 1959). No aquaculture researchhas been conducted on this species, let alone its performance underintensive rearing conditions. Cannibalism is a major problemamongst many communally reared clawed lobsters and crabs withthe rate of cannibalism generally being directly proportional to thestocking density (Aiken and Waddy, 1988). Losses as high as 60%have been found when rearing juvenile Scylla serrata and P.

pelagicus communally (Mann et al., 2007; Marshall et al., 2005).Habitat structures and substrates may improve productivity butthey do not stop all cannibalism (Aiken and Waddy, 1988; Tidwellet al., 1998). To counteract these cannibalistic tendenciesindividuals can be separated into compartments or cells. Thehigher costs of individual versus communal rearing of crustaceansrequire that the minimum practical area required for growth ofindividuals is known.

ll rights reserved.

Table 1k-Values (k = SA/CW2) for each compartment size treatment calculated for crab

carapace width at the beginning of the experiment (CW = 10 mm) and at 50 mm CW

# k at CW = 10 W (mm) SA (mm2) # Reps Crabs/m2 k at CW = 50

1 10 32 1,000 16 766.4 0.4

2 13 35 1,250 16 653.3 0.5

3 19 43 1,875 16 450.3 0.8

4 28 53 2,813 16 306.5 1.1

5 42 65 4,219 16 209.3 1.7

6 63 80 6,328 16 141.3 2.5

7 95 97 9,492 16 97.8 3.8

8 142 119 14,238 8 64.5 5.7

9 214 146 21,357 8 44.1 8.5

10 320 179 32,036 8 29.7 12.8

Also tabled are treatment number (#), individual compartment width (W),

compartment base surface area (SA), number of replicates for each block (# reps)

and stocking density equivalent (crabs/m2)

S. Nicholson et al. / Aquacultural Engineering 39 (2008) 30–36 31

Lobsters and crayfish need a comparatively large compartmentfor unrestricted growth (Aiken and Waddy, 1978; Goyert andAvault, 1978; Manor et al., 2002; Van Olst, 1979) but little is knownabout the space requirements of crabs. Aiken and Waddy (1978)theorized a specific density factor or ‘‘k-value’’ indicating the sizean animal could reach (using carapace length as the key dimensionin the case of lobsters and crayfish) in a certain sized compartmentbefore growth was inhibited. Swimming crabs have a radicallydifferent body form to the lobster and crayfish decapods studiedpreviously and therefore may interact with their immediatesurroundings in a different manner.

Growth in aquaculture can be examined in different ways.Crustaceans undergo periodic moulting to discard the old shell andrapidly expand to their new size while somatic growth continuesmore gradually within the new shell. Because different containersizes may become growth limiting at different times it is notappropriate in this study to rely upon a final weighing (for examplecalculating average weekly gain (AWG)). While harvest weight isultimately the only measure of size that matters to a farmer thereare sound reasons for going to the extra effort to monitor carapacesize of each instar during growth experiments as well as thefrequency of moulting. The weight of instar could also be measuredbut it continues to change even after the new carapace hashardened (Aiken and Waddy, 1987). Carapace width (CW) isconveniently measured by timely recovery of the shed exuviae(before it is consumed) and importantly does not involve handlingof the animals.

In addition to compartment size, there may also be biologicalfactors involved in holding crustaceans in adjacent compartments.Juarez et al. (1987) found that intraspecific interaction wasimportant for individually reared freshwater prawn Macrobra-

chium rosenbergii as evidenced by chemically mediated growthinhibition, and enhanced growth when visual communication waspossible. However, studies with American lobsters Homarus

americanus indicate that physical contact between lobsters wasnecessary to bring about ‘social’ growth inhibition (Cobb et al.,1982; Van Olst, 1979). In order to concentrate upon compartmentsize, chemical communication was minimized in this study bydesigning the compartments so that crabs did not share water.

In this study the influence of compartment size on individualgrowth of hatchery-reared P. sanguinolentus was assessed bymonitoring moult CW increment, intermoult period and survivalrate. Additionally, the potential effect of visual interaction amongneighbouring crabs on growth was also investigated.

2. Materials and methods

The experiment was conducted at the Queensland Departmentof Primary Industries and Fisheries (QDPIF) Bribie Island Aqua-culture Research Centre (BIARC) (27850S, 1538110W). The area has asub-tropical climate and P. sanguinolentus occurs naturally in localwaters.

2.1. Experimental animals

Broodstock three-spot crabs were sourced from a trawler inHervey Bay QLD. Larvae and juveniles were raised followinghatchery and nursery methodologies established for mud crabsand blue swimmer crabs by the Bribie Island Aquaculture ResearchCentre (Mann et al., 1999). 136 intact instar 2 (C2) stage crabletswere transferred to the experimental system. Additional siblingcrablets were stocked into a tank with the same water qualityconditions as the experiment to replace mortalities. Replacementwas done to maintain stocking densities and biomass and to keepthe visual communication treatment consistent. Data from

replacement crabs were not used in growth analyses butsubsequent survival of replacement crabs beyond the instar atwhich the original crab died was included in the survival analysis.

The experiment was carried out for 90 days from stocking.Crabs were fed ad libitum approximately 10% body weight per dayover three feeds at 0800, 1200 and 1700 h. Crabs were fed akuruma prawn diet (Ebistar and Vital, Higashimaru, Japan) withparticle size increasing with crab size—C2–C4 were fed 0.36–0.55 mm crumble, C5 and C6 were fed 1.4–1.7 mm crumble, andC7–C9 were fed 2.7–3 mm diameter � 3–8 mm length pellets. Feedsizes were overlapped between the instar stages.

2.2. Experimental system

Trial compartments were partially submersed in a fibreglasswater bath 2.5 m � 1.0 m � 0.03 m high. Seawater sourced from100 m offshore was filtered to 20 mm and heated in a header tankto 27 � 0.9 8C prior to use. Each of 136 trial compartments receivedits own water supply in a single pass flow through at approximately40 mL min�1 on constant flow. The water supply was distributedthrough HDPE pipe and 4 mm silicon tube with 4 mm valvesregulating flow rate. The chosen flow rate equated to two completeexchanges each hour in the largest (176 mm width) and 30 exchangeseach hour for smallest (32 mm width) compartments. On day 38 flowrate to each compartment was increased to 60 mL min�1.

The system was housed in a closed room lit by two fluorescenttubes on a 14:10 h on:off cycle.

2.3. Compartment construction

Ten different-sized compartments were used, increasing in ageometric progression from a starting k-value of 10 (k = SA/CW2)for the initial crab size (Table 1). This gave the starting surface areaof 1000 mm2 which is approximated by a 32 mm � 32 mmcompartment base.

Two sets of compartment blocks were constructed in anidentical configuration; one set with opaque walls and the otherwith transparent internal walls. Both sets of compartments hadopaque black bases and outside walls. The blocks were constructedsuch that in the ‘visual communication’ treatment compartmentswith transparent internal walls, each crab could see three others(Fig. 1). There were eight replicates in a 4 � 2 configuration of theseven smaller sized blocks and four replicates of the three largestsizes in a 2 � 2 configuration, making 136 individual compart-ments in total. Each block was 120 mm high with a 10 mm outflowgap covered by 4 mm gauge oyster mesh on one outer bottom edgeof each compartment. The depth of the water within thecompartments was 80 mm allowing 40 mm freeboard so crabs

Fig. 1. Approximate scaled diagram showing the 10 different-sized compartment

blocks with either eight (blocks 1–14) or four replicates (blocks 15–20). Each size

has two treatments. Opaque walls (indicated by solid lines) and clear internal walls

(indicated by dashed line) are shown.

Fig. 2. Adjusted mean (�S.E.) following the analysis of deviance for proportion of

crablets that survived to moult at each instar stage and compartment size combination.

S. Nicholson et al. / Aquacultural Engineering 39 (2008) 30–3632

could not escape. The compartment blocks were laid out randomlyin the water bath.

2.4. Maintenance and cleaning

Once per day the compartment blocks were lifted to flush wastefeed and faeces through the outflow gap into the water bath. Carewas taken to avoid exposing the crab to air. Waste on the bottom ofthe water bath was then removed by vacuuming via a siphon.

Temperature and salinity were kept within the natural summerlimits for the local wild population. Dissolved oxygen, pH and totalammonia nitrogen remained within levels appropriate for theclosely related species P. pelagicus which has been extensivelycultured at the BIARC laboratory (unpublished data). Temperaturesranged from 25 to 28 8C, salinity 33.3–34.8 ppt, DO% 70.0–82.4, pH7.6–8.0, TAN 0.01–0.12 ppm.

Following eight unexplained mortalities in 4 days, theexperimental system was cleaned thoroughly on day 38. At thistime algal biofilm was found to cause blockage in some of the4 mm tubes. This was corrected by dimming the amount of lightfrom the fluorescent tubes further and increasing the flow rate to60 mL min�1 per compartment.

2.5. Data collection and statistics

Data from the first moult post-stocking (C2–C3; �8–11 mm)were not used in any analyses due to a possible residual influencefrom the previous nursery environment.

Moulting predominantly occurred several hours after darkness(lights off at 1800 h) and shed exuvia were collected and statisticsrecorded each night at about 0200 h. Crabs were not weighedduring the experiment to reduce effects of handling stress. Usingelectronic calipers, measurements of CW from spine to spine (lastanterolateral spines) were measured to 0.1 mm.

Only moults to C4 and onwards were analyzed. Moult data wasused to calculate percentage moult increment (initial CW/(finalCW) � 100) and intermoult period of each instar. AWG wascalculated using the formula: (CW8 � CW4)/weeks where CW4 andCW8 are the CW at fourth and eighth instars, respectively andweeks is the number of weeks between moult to C4 and moult toC8. Of the common measures of growth available, AWG (a lineargrowth measure) produced the best fit with the CW data. Bothdaily growth coefficient (DGC) and specific growth rate (SGR)assumed that the slope or rate of growth accelerated with time,predicting that intermediate instars are much smaller than they

actually were, particularly SGR. In reality, the CW growth ratedecreased slightly with time, so that even linear interpolationsusing the AWG slightly underestimated the size of crabs in instars5–7.

For survival analysis, a binomial generalized linear model(McCullagh and Nelder, 1989) using a logit link function wasemployed to analyze the proportional survival of crabs at eachinstar stage, in each compartment size and wall transparency. Toavoid duplication, survival data for replacement crabs was onlyincluded in the analysis for instar stages after the stage at whichthe crab was replaced. Data are plotted as adjusted mean � S.E.Interaction tables were used to further explore significant interac-tions. To examine the role of k-value in survival, the binomial analysiswas repeated using categories of k-values (1 = 0.25–0.34, 2 = 0.35–0.44, etc.).

Growth measurements, CW, moult increment and intermoultperiod, were analyzed by two way repeated measures ANOVAfollowed by protected least significant difference (LSD) testing.Pre-moult CW was used as a covariate in the analysis of factorseffecting CW. AWG from the fourth to eighth instars was analyzedby two-way ANOVA. All analyses were conducted using Genstat1

8.1th ed. and statistical significance level was set at P < 0.05 withthe exception of the ANOVAs of growth (CW, moult increment andAWG) which used P < 0.017 using Bonferroni’s adjustment to the P

level.

3. Results

3.1. Survival and growth

The accumulated analysis of deviance showed that bothcompartment size (deviance ratio = 3.06, d.f. = 9,675, P = 0.001)and instar stage (deviance ratio = 12.01, d.f. = 5,675, P < 0.001)significantly affected whether crabs of a particular stage survivedto moult in the compartments but transparency of the wall had nosignificant effect on survival (deviance ratio = 0.39, d.f. = 1,675,P = 0,530). Analysis of deviance showed that crab survival was bestexplained by a highly significant interaction (deviance ratio = 1.76,d.f. = 5,675, P = 0.001) between compartment size and the instarstage of crablets occupying the compartment. In other words, atthe end of the trial, the largest instar crablets died in the smallestcompartments (particularly those <43 mm wide) when attempt-ing to moult (Fig. 2). The analysis identified a difficult to interpretinteraction between compartment size and wall transparency(deviance ratio = 2.74, d.f. = 9,675, P = 0.003) however, there wasno simple pattern apparent in the plotted averages (Fig. 3). In somecombinations transparent walled compartments gave a slightlyhigher survival, in other combinations a slightly lower one. Overall,

Fig. 3. Adjusted mean (�S.E.) following the analysis of deviance for proportion of

crablets that survived to moult for transparent and opaque wall treatments at each

compartment size.

S. Nicholson et al. / Aquacultural Engineering 39 (2008) 30–36 33

survival in containers with clear walls (92.5 � 0.93%) was similar tocontainers with opaque walls (91.9 � 1.08%).

3.1.1. Carapace width

Aside from six damaged exuviae, virtually all carapaces (>99%)were recovered intact. The two-way repeated measures ANOVAshowed that compartment size significantly influenced the CW ofcrabs in the experiment (F9,112 = 3.94, P < 0.001), but walltransparency did not (F1,112 = 0.08, P = 0.773). There was nointeraction between compartment size and wall transparency(F9,112 = 0.67, P = 0.736), but there was a significant interactionbetween instar and compartment size (F45,480 = 5.29, P < 0.001).LSD testing showed that there were no differences in mean CW ofcrabs among all compartment sizes for C4 and C5 instars (Table 2)however, significant differences occurred for subsequent instars(P < 0.05). The value of n for each instar in Table 2 reflects thestarting sample number, crab mortality and occasional missingdata. For instars C6–C9, the small compartments produced

Table 2Mean CW for each instar at each compartment width (mm)

Width C4 n C5 n C6 n

32 15.3a 14 21.3a 15 29.4a 15

35 14.9a 16 20.6a 16 28.5ab 16

43 15.0a 15 20.4a 15 28.0abc 16

53 14.4a 15 20.2a 16 27.5abc 16

65 14.9a 16 20.0a 16 27.6abc 15

80 14.4a 14 19.3a 15 25.9c 15

97 14.0a 13 19.5a 15 26.6bc 15

119 14.0a 8 19.2a 8 26.1bc 8

146 15.1a 7 20.2a 7 27.9abc 7

179 14.1a 7 19.7a 8 26.6bc 8

Within columns, like superscript letters show no significance. n = number of crabs. Po

179 = �1.09.

Table 3Percent mean moult increment (%) for CW between successive instars at each compar

Width C3–C4 n C4–C5 n C5–C6 n

32 139.2bc 7 138.3a 13 138.0a 1

35 142.9a 13 138.1ab 16 138.5a 1

43 138.0c 9 136.8abc 15 137.5ab 1

53 139.7abc 11 139.4a 15 136.2ab 1

65 141.5ab 13 134.8bc 16 138.1a 1

80 133.8d 11 134.7c 14 134.2b 1

97 137.4c 11 134.2c 13 136.1ab 1

119 137.3cd 5 137.1abc 8 135.7ab

146 135.9cd 5 134.0bc 7 137.8ab

179 134.6d 7 136.6abc 7 135.4ab

Within columns, like superscript letters show no significance. n = number of crabs. Poo

significantly larger individuals (P < 0.05) (Table 2) (Using dataderived from the experimental crabs and from additional siblingcrabs held in nearby tanks, these CWs for P. sanguinolentus canbe converted to weight using the following formula:weight = 0.0001 � 2.905 length, r2 = 0.98). CW of each instar wasstrongly correlated to CW of the next instar, with a correlationcoefficient (r) for the moult C3–C4 of 0.82, and for subsequentmoults up to C9, r was 0.94–0.96.

3.1.2. Moult increment

The repeated measures ANOVA showed that compartment sizesignificantly influenced the percent moult increment of crabs inthe experiment (F9,112 = 3.31, P = 0.001), but wall transparency didnot (F1,112 = 1.04, P = 0.311). There was no significant interactionbetween compartment size and wall transparency (F9,112 = 0.93,P = 0.504) but there was a weak interaction between instar andcompartment size (F45,437 = 1.58, P = 0.039) which must bediscounted using Bonferroni’s correction to P. The percent moultincrement of CW decreased as the crablets progressed from instarto instar. Increments cannot be calculated for all moults to C4because of missing C3 CW data. Moult increment increased withdecreasing compartment size for the C3–C4 moult (Table 3).Compartment size had only a small quantitative effect on moultincrement at subsequent moults, though significant differencesoccurred (P < 0.05). There is evidence of smaller increments insmaller compartments at the C8–C9 moult (Table 3).

3.1.3. Intermoult period

The repeated measures ANOVA showed that compartment sizesignificantly influenced the intermoult period of crabs in theexperiment (F1,112 = 6.28, P < 0.001), but wall transparency did not(F1,112 = 0.78, P = 0.379). There was no significant interactionbetween compartment size and wall transparency (F9,112 = 0.34,P = 0.961) but there were significant interactions between instarand compartment size (F44,403 = 6.27, P = 0.002) and between

C7 n C8 n C9 n

39.7a 15 51.8a 13 60.9bc 3

39.0ab 16 50.4ab 15 63.4a 7

38.2abc 16 50.4ab 14 64.1a 7

37.4bcd 16 48.9bc 16 62.3ab 11

37.2bcd 16 48.7bc 16 63.1a 12

35.8de 15 46.9cd 12 57.1d 8

34.3e 15 45.3d 14 60.4c 11

37.6abcd 8 47.8bcd 8 56.7d 7

35.7cde 7 45.3d 4 59.4cd 3

34.5e 8 44.8d 6 57.3d 5

oled estimated S.E. (E.S.E.) for widths 32–97 = �0.77; pooled E.S.E. for widths 119–

tment width (mm)

C6–C7 n C7–C8 n C8–C9 n

5 135.1abc 15 130.7a 13 120.2c 3

6 137.1a 16 129.4ab 15 125.1b 7

5 136.3a 16 131.7a 14 127.1b 7

6 135.3abc 16 131.1a 16 128.7a 11

5 135.7ab 15 130.7a 16 129.6a 12

5 132.2c 15 129.7ab 12 130.6a 8

5 134.6abc 15 130.8a 14 129.6a 11

8 132.1bc 8 129.7ab 8 129.3a 7

7 134.7abc 7 128.4ab 4 129.8a 3

8 133.7abc 8 126.1b 6 126.1b 4

led E.S.E. for widths 32–97 = �1.19; pooled E.S.E. for widths 119–179 = �1.68.

Table 4Intermoult period in days between successive instars at each compartment width (mm)

Width C4 n C5 n C6 n C7 n C8 n C9 n

32 5.9a 7 7.1 13 9.1a 15 14.4a 14 20.6a 9 0

35 5.1a 13 6.6 16 9.1a 16 13.8ab 15 20.3a 14 0

43 5.6a 9 6.9 15 9.5a 15 13.1ab 16 20.5a 14 30.2a 4

53 5.6a 11 6.8 15 8.9a 16 12.0bc 16 19.0ab 16 26.6b 7

65 5.2a 13 6.5 16 8.5a 15 11.8bc 15 18.1b 16 25.3bc 8

80 6.2a 11 6.7 14 8.2a 15 10.8c 13 16.4bc 12 23.7cd 8

97 4.8a 11 6.3 13 8.1a 15 10.8c 14 17.9b 13 22.2de 11

119 5.4a 5 6.9 8 8.1a 8 10.8c 8 15.5c 8 21.2e 7

146 5.5a 5 6.0 7 7.5a 7 11.0c 4 15.8c 4 20.7ef 3

179 5.7a 6 6.5 7 8.3a 8 10.7c 6 14.5c 5 18.8f 5

Within columns, like superscript letters show no significance. n = number of crabs. Pooled E.S.E. for widths 32–97 = �.060; pooled E.S.E. for widths 119–179 = �0.84.

S. Nicholson et al. / Aquacultural Engineering 39 (2008) 30–3634

instar, compartment size and wall transparency (F43,403 = 2.12,P = 0.008). Examining the data revealed no fundamental explana-tion for the latter and certainly no role for transparency in growth.There was a very strong positive relationship between intermoultperiod and crab instar. Actual duration of C4 is not known as thiswas the first instar stocked into the experimental system. Mostcrabs moulted to C10 within the 90 days except crabs in the twosmallest compartments (Table 4). Significant differences inintermoult periods did not occur until instar C7 (P < 0.05). Atthat instar, the three smallest compartment sizes had significantlylonger intermoult periods than sizes larger than 80 mm (P < 0.05).Compartment sizes 53 and 65 mm are only significantly differentfrom width 32 mm (P < 0.05). This pattern is repeated at laterinstars and by C9 the effect became more pronounced.

3.1.4. Growth rate

Growth rate was measured up to the moult to C8 because not allcrabs moulted to C9 or beyond. Two way ANOVA showed nosignificant effect of compartment size on AWG (F9,88 = 0.43,P = 0.916) however, wall transparency did make it under theBonferroni corrected P value (F1,88 = 6.23, P = 0.014). Crabs in clearwalled containers apparently had a slightly lower AWG of5.09 � 0.11 mm week�1 than those in opaque containers 5.43� 0.09 mm week�1. There was no significant interaction betweencompartment size and wall transparency (F9,88 = 0.91, P = 0.525).

3.1.5. k-Values

In terms of crabs surviving in the compartments, the significantinteraction noted above between instar class and compartmentsize can also be examined in terms of k-value. Here individualswere assigned to categories where calculated k-value was around0.3, 0.4, and 0.5 and higher values were pooled into one furtherclass. k-Values below 0.25 were not observed. Analysis of devianceshowed that k-value class significantly influenced survival(deviance ratio = 13.54, d.f. = 8.732, P < 0.001) with adjustedmeans given in Table 5. The effect of wall transparency on survivalwas not significant (deviance ratio = 0.61, d.f. = 1.732, P = 0.434)and there was no significant interaction between k-value class andtransparency (deviance ratio = 1.78, d.f. = 8.732, P = 0.075). All

Table 5Effect of k-value on adjusted mean proportional survival of crabs in the experiment

k-Value class Number of

observations

Proportion surviving

adjusted mean

0.25–0.34 16 0.00 � 0.002a

0.35–0.44 14 0.86 � 0.094b

0.45–0.54 17 0.88 � 0.078b

�0.55 704 0.96 � 0.008b

Means with the same postscript are not significantly different

crabs died when k-value was less than 0.35. The minimum k-valueobserved amongst crabs that survived to moult was 0.36.Sequential x2 testing showed that crab survival for the lowestk-value class was significantly different (P < 0.001) from that of thethree other classes (which did not differ significantly from eachother, P > 0.05).

4. Discussion

This research on three-spot crabs P. sanguinolentus indicatesthat survival rather than growth rate should be used to decide theimpact of holding space on animal performance which isconsistent with earlier findings by Van Olst (1979) on Americanlobsters (H. americanus). Mortalities occurred here with a k-valueof <0.35. Lack of space to carry out necessary body and limbmovement during ecdysis was an apparent cause of death in thesmaller compartment sizes. However, some crabs died when theywere unable to remove claws and walking limbs from the exuvium.In some cases the stuck limbs were autotomised.

Growth inhibition did not occur at any compartment size in theexperiment despite the fact that in smaller compartments thecrabs were unable to sit flat on the floor of the compartment andremained at an angle supported by the walls. Growth inhibitionfor other decapods, including H. americanus, M. rosenbergii hasbeen reported at k-values >20 (Aiken and Waddy, 1978; Goyertand Avault, 1978; Jussila, 1997; Manor et al., 2002; Van Olst,1979). In comparison P. sanguinolentus can apparently growsuccessfully in a very confined space with k-values of around1.0–2.0.

The difference in body shape of crabs compared with prawnsand lobsters will affect interspecific comparisons of k-values. Thelow values achieved here reflect the dorsoventrally flattened shapeof crabs (using CW rather than length) and their ability to orientalong the axis of the compartment. The decapods that showpronounced compartment-size effects in the literature all have theelongated reptantian/natantian shape and possess much longerantennae than brachyurans. Previous authors have attributedsevere mortalities before any physical restriction on moulting tocontact ‘‘stress’’ involving the antennae and the compartmentwalls (Aiken and Waddy, 1978; Goyert and Avault, 1978; Manoret al., 2002; Van Olst, 1979).

Because growth in crustaceans involves large intermittentincreases in volume it is worth examining the outcome of eachmoult. While AWG was not significantly affected by compartmentsize, the smaller compartments actually produced crabs of largerCW from instar 6–9 (Table 2). Size at instar, moult period andincrement shows that this result is not contradictory within ourown experiment. AWG is not affected because the longer moultperiod is compensated for by a larger moult increment. Previouscompartment studies with decapods have not collected these types

Fig. 4. The relationship between duration of the preceding intermoult (days) and

moult increment (mm) for three-spot crabs raised from instar 4–9 in small (43–

97 mm walls, open symbols) and large compartments (119–179 mm walls, closed

symbols). Data for compartments less than 43 mm wall size not included because of

mortalities at instar 9.

S. Nicholson et al. / Aquacultural Engineering 39 (2008) 30–36 35

of data (Aiken and Waddy, 1978; Goyert and Avault, 1978; Manoret al., 2002; Van Olst, 1979).

Moult increment in crustaceans tends to increase by a relativelyfixed percentage of the pre-moult size (Botsford, 1985), sopercentage moult increments are fairly uniform across crabs indifferent compartment sizes. However, the LSD comparisons ofpercentage moult increments showed that they were higher in thesmaller compartment sizes for the moult to C4 (Table 3). This earlyeffect on moult increment had a long-term impact on relative sizesbecause pre-moult size strongly influences the size achieved aftersuccessive moults (Cadman and Weinstein, 1988).

The increase in CW increment when early instars were housedin very small compartments may be due to reduced lightpenetration to the compartment floor, providing a shaded zonethat reduced stress. Alternatively the crabs in small compartmentshave less floor area to search for and gather food and may haveutilized the food ration provided more efficiently.

Intermoult period in crustaceans becomes longer with succes-sive instars and must build up sufficient physiological reserves forcrabs to undergo ecdysis (Aiken and Waddy, 1987). In thisexperiment, from instar C7 onwards, crabs in smaller compart-ments showed significantly longer intermoult periods (Table 4)indicating that in the later instars the crabs in smaller compart-ments took longer to gain the reserves. Observations at the end ofthe study revealed that large crabs in small compartments weremore likely to accidentally push food through the mesh and out ofthe compartment. Also crabs oriented to the walls (k-values ofaround 1.5 or less) may have more difficulty reaching and graspingfeed on the floor. Klein Breteler (1975) observed that growth wasdependant on food uptake for individually reared shore crabsCarcinus maenas so that increased food availability reduced theintermoult period. However, even instar C9 crabs in a compara-tively generous compartment size of 119 mm (k = 4.3), showed anintermoult period significantly longer than found at a width of179 mm (Table 4). This indicates that other factors besides feedintake may be causing variation in moult period. Designingcompartments that prevent food loss by crabs held at extremelylow k-values will be an important practical step in clarifyingaspects of this issue.

Normally in a growth trial, changes in moult period orincrement lead to real changes in growth rate. Reduction in thequantity and quality of food is widely recognised to lengthen theintermoult period in crustaceans and perhaps reduce or notchange the increment (Hartnoll, 1982). In this experimentgrowth rate did not change because the longer moult periodalso saw a compensatory relatively large moult increment. Moultperiod is well known to increase with successive instars, that is,with size in crustaceans (Hartnoll, 1982). Plotting the moultincrement for these crabs against the intermoult period leadingto the moult shows that all the crabs conform to a similar slope,linking preparation time for the moult (the period) and the sizeoutcome (the increment) (Fig. 4). While bigger crabs areexpected to moult less often, one explanation could be thatthere is a mechanism allowing crabs to moult less often underunfavourable circumstances without actually compromisingtheir growth rate.

Moult period is not always the main driver of growth incrustaceans and there must be some biological purpose behind thereported variation. For example, growth can be inhibited bydifferences in moult increment without a change in period in M.

rosenbergii and H. americanus (Cheng and Chang, 1993; D’Abramoet al., 2000; Juarez et al., 1987). Differences in growth can also arisefrom changes in intermoult period rather than moult increment asreported for H. americanus, Cancer magister and Mithrax spinosissi-

mus (Cheng and Chang, 1993; Kondzela and Shirley, 1993; Wilber

and Wilber, 1989). However, both growth parameters can alsochange simultaneously as in Callinectes sapidus and C. maenas

(Cadman and Weinstein, 1988; Mohamedeen and Hartnoll, 1989)as also seen in this experiment.

Aside from compartment size, other factors such as visual,chemical or tactile factors may influence growth in a high-densitygrowout system. No dramatic effect of visual communication ongrowth or survival was demonstrated in this study, similar to thefinding of Van Olst (1979) for H. americanus. That visualinteraction could influence growth rate is perhaps understand-able, but the slight difference in AWG is at odds with thecomprehensive moult by moult repeated measures analysis andshould be treated with some caution. In a further study ofAmerican lobsters, Cobb et al. (1982) found that visual andchemical stimuli were not enough to form a dominant/sub-ordinate relationship: and actual contact between animals wasnecessary to affect growth. In contrast, Juarez et al. (1987) showedsignificant differences in growth where visual communicationwas possible in the freshwater prawn M. rosenbergii. Further studywill be required to establish how serious interactions betweenneighbouring crabs can become, particularly if perforated wallsenable physical and chemical contact to occur.

In early instar three-spot crabs small compartments (<80 mmwidth) conferred a growth advantage and this instar sizeimprovement followed through to later instars. In mid-sizedjuveniles, instar C5–C9, the crabs can be grown successfully incompartments to the point where the individuals cannot sit flat onthe floor (k < 1), making very high production densities possible inintensive compartmentalised systems. However, within the sizeranges examined in this experiment the crabs should be moved tolarger compartments (>43 mm width) before C9 to allow furtherunobstructed moulting. The larger juveniles up to C9 took longer tomoult but their growth was not affected. It is unknown whateffects individual rearing may have on later stage crabs past C9particularly as the crabs attain maturity at C11–C12. Transferringcrabs to larger compartments may be beneficial in other ways thanjust ensuring survival. Cheng and Chang (1993) found that movingAmerican lobsters from small to larger compartments promotedmoulting, so similar experiments with this and other species ofcrab are warranted.

S. Nicholson et al. / Aquacultural Engineering 39 (2008) 30–3636

Acknowledgements

The authors would like to thank Beverley Kelly and TomAsakawa at the Bribie Island Aquaculture Research Centre forhatching and rearing of the crab larvae and Robert Dean forassistance with design and construction of the experimental set-up. Biometrician David Mayer (DPI&F) provided advice andassistance with statistical analysis. The three-spot crab rearingwas carried out as part of the Soft-shell Crab Innovation Venture, ajoint Queensland/Industry partnership.

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