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Aquaculture 238 (2004) 239–247

The combined effects of temperature and salinity on

growth and survival of juvenile mud crabs

(Scylla serrata Forskal)

Ian M. Ruscoe*, Colin C. Shelley, Graham R. Williams

Darwin Aquaculture Centre, Fisheries Group, Department of Business, Industry and Resource Development,

GPO Box 3000, Darwin, NT 0801, Australia

Received 12 December 2003; received in revised form 29 March 2004; accepted 17 May 2004

Abstract

An experiment was conducted to determine the effects of temperature and salinity on the growth

and survival of the mud crab Scylla serrata, and the combined optima for these parameters during the

nursery phase. Instar 2 crablets, mean weight 18.43F 0.42 mg were individually reared at

combinations of temperature (20, 25, 30, and 35 jC) and salinity (0x, 5x, 10x, 20x, 30x, and

40x) for 18 days. Crablets were fed an artificial penaeid diet to satiety each day and at harvest were

weighed, and the carapace width was measured. All crablets held at 0x, regardless of temperature,

were dead the morning after stocking and were therefore excluded from any data analysis. Survival

was significantly different among treatments due to temperature but not due to salinities between 5xand 40x. The mean survival for the various temperatures were 36.0%, 98.0%, 96.0%, and 94.0% for

temperatures of 20, 25, 30, and 35 jC, respectively. Growth, as measured by individual wet weight at

harvest, carapace width, instar 3 intermoult duration, and weight-specific growth rate (WSGR

percentage per day), was influenced more by temperature than by salinity, although both had

significant effects on all parameters examined. Analyses indicate that maximum WSGR of around

16% day� 1 was achieved at 30 jC and 10–20x salinity. Based on the findings of this experiment,

S. serrata juveniles from the Northern Territory of Australia should be reared at approximately 30 jCwith salinities in the range of 10–25x in order to achieve maximal production.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Mud crab; Scylla serrata; Temperature; Salinity; Growth; Survival; Crablet

0044-8486/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.aquaculture.2004.05.030

* Corresponding author. Tel.: +61-8-8924-4265; fax: +61-8-8924-4277.

E-mail address: ian.ruscoe@nt.gov.au (I.M. Ruscoe).

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247240

1. Introduction

Mud crabs of the genus Scylla have recently been the focus of international

collaborative research, and progress has been made in hatchery technology in order

to supply seed stock (Ruscoe et al., 2004). In the Northern Territory of Australia, these

relatively large crabs are usually found in intertidal and subtidal zones of estuaries and

in mangrove systems (O’Grady et al., 2003). This shallow water tropical environment,

characterised by a 5-month warm, wet season and a 7-month cooler, dry season,

exposes the crabs to wide annual, and often daily, fluctuations in temperature and

salinity.

The apparent wide temperature and salinity tolerances of S. serrata, as evidenced

by its natural range and preferred environment, makes this species an attractive can-

didate for aquaculture. However, it is necessary to characterise the effects of tem-

perature and salinity on crab performance to optimise industry development and farm

practise.

The biological effects of variations in temperature and salinity are considered to be

among the most important factors influencing marine organisms (Ponce-Palafox et al.,

1997). Furthermore, temperature is considered to be the most important modifier of energy

flow and hence growth, while salinity imposes the greatest additional load on metabolic

requirements of aquatic animals (Brett, 1979).

There are numerous studies which focus on the effect of a single environmental variable

on growth and survival of crustaceans (Hill, 1980, S. serrata—temperature; Vijayan and

Diwan, 1995, Penaeus indicus—temperature, salinity, pH; Hai et al., 1998, S. serrata—

salinity; Coman et al., 2002, Penaeus japonicus—temperature; Hamasaki, 2003, S.

serrata—temperature), but relatively few which study the interrelationship of two or

more factors (Hill, 1974, S. serrata—temperature and salinity; Chen and Chia, 1996, S.

serrata—temperature and salinity; Ponce-Palafox et al., 1997, Litopenaeus vannamei—

temperature and salinity), and additionally most studies have used larval animals. The

optimal salinity for survival of S. serrata zoea larvae was reported to be 32x, and

additionally, this variable did not affect the zoeal stage duration (Baylon et al., 2001). It

has also more recently been found that the best temperature for survival of the same

species larvae was 29 jC, and that there was a significant shortening in the duration of

larval rearing with increasing temperature between 23 and 32 jC (Hamasaki, 2003). It is,

however, the immature and stable juvenile stages of development, those not subject to

differentiation, that are considered the best candidates in which to study the governing

mechanisms of growth (Brett, 1979).

The interaction of temperature and salinity on the growth and survival of juve-

nile stone crabs Menippe mercinaria and Menippe adina has been assessed (Brown

et al., 1991). These authors found that temperature (5–40 jC), but not salinity

(10–40x), significantly affects moult frequency, and that both parameters affect

survival.

The aim of this study was to investigate the effect of temperature and salinity on growth

and survival of S. serrata juveniles, and to establish any interactive effects of the same.

The information generated will be a useful guide to industry development by elucidating

optimal water requirements for nursery rearing.

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247 241

2. Materials and methods

2.1. Experimental animals

The instar 2 crablets were produced from a larval-rearing experiment carried out at the

Darwin Aquaculture Centre (DAC). Megalopa were settled in a 3.8-m-diameter flat-

bottomed tank and were fed on-grown Artemia and an artificial crumble diet (500–750 Amsieved, Vital 12 P. japonicus diet, Higashimaru, Kagoshima, Japan) until the second crab

instar. The crablets were harvested by draining onto a submerged screen. Only fully intact

and active crablets were used in the experiment. Fifty-six individuals, which were not then

used in the experiment, were weighed on the day of stocking to obtain a mean stocking

weight. The crablets were gently dried using blotting paper and were individually weighed

on a Denver AA-250 electronic balance (accuracy to 0.0001 g). All subsequent weighing

occurred in the same way.

2.2. Facilities and protocol

This experiment was carried out in 20 100-l plastic tanks inside an environmentally

controlled room at the DAC. These tanks were arranged in 2 parallel rows of 10.

The 100-l tanks acted as water baths, which were maintained at four experimental

temperatures. These were 20, 25, 30, and 35 jC. The room was cooled so that the water in

the tanks was maintained at 20 jC, unless otherwise heated. All temperatures above 20 jCwere maintained with a 300-W thermostat-controlled immersion heater, and gentle

aeration ensured mixing of the water bath. There were five 100-l tanks maintained at

each temperature in a randomised block design. Within each 100-l tank were spaces for 12

partially submerged 600-ml plastic containers (105 mm diameter� 105 mm deep), which

were used to hold a single crab each. Crabs were stocked at 30 jC and were increased or

decreased at a rate of not more than 3 jC h� 1.

Six salinities were tested. These were 0x, 5x, 10x, 20x, 30x, and 40x. In each

100-l water bath, there were two 600-ml containers at each salinity. Each container held a

single individual and had a lid to prevent evaporation and subsequent changes in

temperature and salinity. In total, 240 individual crabs were used in this experiment.

The various salinity treatments were randomised within each 100-l water bath. Salinity

was either increased or decreased at a rate of not more than 5x h� 1, concurrently with the

temperature adjustment.

Water used to create the various salinities was made from filtered seawater and a

commercial scientific grade marine salt ‘‘Coralife’’ (Energy Savers Unlimited, Carson,

CA, USA). The filtered seawater for all treatments was made up to slightly higher than 40

ppt by the addition of the Coralife salt and was then adjusted with carbon-filtered

freshwater to the various salinities required, except for the 0-ppt treatment, where only

carbon-filtered freshwater was used. This was done so that each salinity received some

commercial salt mixture, some filtered seawater, and some freshwater.

Three times a week, each crab was removed from its container and was placed

immediately into a prepared (temperature- and salinity-adjusted) batch of new water. The

rearing container was cleaned in freshwater, refilled with the appropriate temperature- and

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247242

salinity-adjusted water, and the crab was then restocked. In this way, a 100% water

exchange was facilitated.

Each day at 1000 h, the crablets were fed 1400–1700 Am sieved Vital 12 artificial diet

(as above) to excess. The following morning, the containers were vacuumed to remove

uneaten food and faeces, and the occurrence of moulting and mortality recorded. Crabs

were grown for 18 days. At harvest, crabs from all tanks were individually weighed (0.001

g), and carapace width measurements were taken (0.1 mm) using vernier calipers.

2.3. Water quality

Temperature and pH of randomly selected containers in each water bath was recorded at

0800 daily with a WP-90 digital water quality meter (TPS, Brisbane, Australia). Salinity of

randomly selected containers within each water bath was measured with a refractometer.

Oxygen was measured with an OxyGuard H01G-Handy Gamma, digital meter (OxyGuard

International, Birkerød, Denmark). Ammonia was measured every 3 days in several

containers that were observed to have the greatest feed consumption using the salicylate

method (Bower and Holm-Hansen, 1980) incorporating a colorimeter.

2.4. Data analysis

Residuals of all data sets were examined to determine a requirement for data

transformation. Block replicates were used to calculate survival, hence these data were

analysed using a two-way ANOVA with a main effects design for the factors temperature

(20, 25, 30, and 35 jC) and salinity (5x, 10x, 20x, 30x, and 40x). Therefore,

comparison of means was not possible for these data set. The proportional survival data

were transformed (arcsine-square root) prior to analysis, although percentage data are

shown in the table for ease of interpretation.

Growth data, recorded as mean weight and carapace width at harvest, as well as weight-

specific growth rate (WSGR, percentage per day; Hopkins, 1992; Evans and Jussila,

1997), was analysed using ANOVAwith a split-plot design where the main plot-factor was

temperature (20, 25, 30, and 35 jC), and the subplot factor was salinity (5x, 10x, 20x,

30x, and 40x). The instar 3 intermoult duration was also recorded and analysed in the

same manner as the growth data, although the data from the 20 jC treatments were not

included in the analysis as so few of the animals in this treatment moulted to instar 4

during the experiment. The third instar was chosen because the majority of crabs reached

the fourth instar. Significant differences between treatments were separated by Tukey’s test

(Sokol and Rohlf, 1981).

3. Results

The mean weight (F S.E.) of the instar 2 crablets at stocking was 18.43F 0.42 mg.

Mean temperature (jC; F S.E.) of the treatments at 0800 h was 20.0F 0.1, 24.9F 0.1,

29.8F 0.1, and 34.8F 0.1. No salinity change from the set values was measured

with the refractometer within the experimental units throughout the experiment.

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247 243

Ammonia–nitrogen ranged between < 0.1 and 2.0 mg l� 1. Oxygen remained above

4.0 mg l� 1, and pH ranged between 7.78 and 8.16.

Table 1 shows the summarised data for the various treatments in this experiment. All

crablets held at 0x, regardless of temperature, were dead the morning after stocking.

These animals were excluded from any data analysis.

For the remaining data, there was a highly significant difference in survival among

treatments due to temperature (P < 0.0001), but not due to salinity in the range 5–40x(P= 0.1137). The mean survival for the various temperatures were 36.0%, 98.0%, 96.0%,

and 94.0% for 20, 25, 30, and 35 jC, respectively.There were highly significant differences in the mean weight at harvest attributable to

both temperature (P < 0.0001) and salinity (P= 0.0027), but with no interaction effect

(P= 0.38). The crabs held at 30 jC in salinities ranging from 5x to 20xwere the heaviest at

harvest. Similarly, highly significant differences in carapace width at harvest were

attributable to both temperature (P < 0.0001) and salinity (P= 0.027) but with no interaction

Table 1

Means of crablet survival (%), harvest weight (g), carapace width (mm), instar 3 intermoult duration (days), and

weight-specific growth rate (%/day; F S.E.) for various combinations of temperature and salinity used in this

experiment

Temperature

(jC)Salinity

(x)

Survival

(%)

Harvest

weight

(g)

Harvest

carapace

width (mm)

Instar 3

intermoult

duration

WSGR

(%/day)

(N) (days)

20 0 0

5 30 0.042F 0.024 c 6.9F 3.9 f 1 13 4.37F 1.01 ef

10 40 0.045F 0.023 c 7.0F 3.5 f 0 4.99F 0.26 ef

20 40 0.080F 0.040 c 8.1F 4.1 ef 2 7.50F 1.5 7.75F 1.28 def

30 40 0.042F 0.020 c 6.9F 3.4 f 0 4.49F 0.32 f

40 30 0.052F 0.030 c 7.1F 4.1 f 0 5.76F 0.38 ef

25 0 0

5 100 0.142F 0.045 bc 9.9F 3.1 def 10 7.5F 0.31 cd 10.89F 0.72 bcd

10 100 0.156F 0.049 b 10.2F 3.2 cdef 10 7.2F 0.29 bcd 11.48F 0.66 bcd

20 100 0.200F 0.063 b 10.9F 3.4 bcde 10 7.7F 0.45 cd 12.68F 0.77 abc

30 90 0.135F 0.045 bc 9.8F 3.3 def 9 7.6F 0.41 cd 10.56F 0.77 cd

40 100 0.120F 0.038 bc 9.5F 3.0 def 10 8.2F 0.63 d 10.35F 0.23 cde

30 0 0

5 80 0.291F 0.103 a 13.4F 4.7 ab 10 4.3F 0.21 a 15.24F 0.36 ab

10 100 0.337F 0.105 a 13.7F 4.3 a 10 4.3F 0.21 a 16.06F 0.34 a

20 100 0.333F 0.105 a 13.7F 4.3 a 10 4.9F 0.23 ab 15.98F 0.36 a

30 100 0.291F 0.919 a 12.9F 4.1 abc 10 5.5F 0.22 abc 15.10F 0.54 ab

40 100 0.215F 0.068 ab 11.7F 3.5 abcd 10 6.9F 0.28 bcd 13.16F 0.87 abc

35 0 0

5 80 0.195F 0.068 b 11.5F 4.1 abcd 10 5.3F 0.45 abc 12.84F 0.41 abc

10 100 0.197F 0.062 b 11.6F 3.7 abcd 10 4.8F 0.42 ab 12.90F 0.59 abc

20 90 0.197F 0.066 b 11.5F 3.8 abcd 10 6.1F 0.28 abcd 12.59F 0.60 abc

30 100 0.172F 0.054 b 10.9F 3.5 bcde 10 5.3F 0.40 abc 11.93F 0.79 bcd

40 100 0.135F 0.042 bc 10.0F 3.1 def 9 6.6F 0.44 abcd 10.40F 0.93 cde

Means followed by the same letters are not significantly different from one another ( Pz 0.05).

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247244

effect (P= 0.86). A response surface plot showing harvest carapace width of mud crab

juveniles reared at combinations of temperature and salinity is presented in Fig. 1.

There were also significant differences among treatments for the instar 3 intermoult

duration attributable to both temperature (P < 0.0001) and salinity (P= 0.0075) but with

no interaction effect (P= 0.06). Generally, crabs held at 30 and 35 jC had a shorter

intermoult duration than at 25 jC.The WSGR of crablets in this experiment was also significantly affected by temperature

(P= 0.0002) and salinity (P= 0.0366) but with no interaction effect (P= 0.84). Maximum

specific growth rate of around 16% day� 1 was achieved at 30 jC and 10–20x salinity. A

response surface plot showing WSGR is presented in Fig. 2.

4. Discussion

Several treatment combinations yielded 100% survival over the experimental period

with apparently good growth rates of up to 16% day� 1. This would indicate that the

experimental facilities and procedures represented appropriate experimental culture con-

Fig. 1. Response surface plot showing carapace width of mud crab juveniles reared at combinations of

temperature and salinity for 18 days. Isobars show carapace width in millimeters.

Fig. 2. Response surface plot showing weight-specific growth rate (WSGR) of mud crab juveniles reared at

combinations of temperature and salinity for 18 days. Isobars show WSGR (percentage per day).

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247 245

ditions for this species. The results have shown that juvenile mud crabs (S. serrata) can

tolerate a wide range of temperature and salinity, although variations from the optimal of

both of these factors will have substantial effects on growth, survival, and production.

Temperature was by far the most influential factor affecting survival and growth in this

experiment (for salinities between 5x and 40x). For every indicator of growth assessed

in this experiment, the temperature effect accounted for greater variance in growth than the

salinity effect. Similar to the results of this experiment, Cadman and Weinstein (1988)

found that both temperature and salinity affected growth of the blue crab Callinectes

sapidus, although temperature had a greater effect.

Crabs held at 20 jC in this experiment were observed to have a reduced activity and

food consumption, although these variables were not quantified. Conversely, crabs held at

35 jC were hyperactive especially at the introduction of food, and their consumption

levels, along with the crabs held at 30 jC, were observed to be higher than at 20 and 25

jC. These observations are consistent with those of Ponce-Palafox et al. (1997) for L.

vannamei, and Niu et al. (2003) for Macrobrachium rosenbergii. Caution should be

exercised however in making use of this temperature optima information, as Wyban et al.

(1995) suggested that the temperature optima for L. vannamei is size-specific and

I.M. Ruscoe et al. / Aquaculture 238 (2004) 239–247246

decreases as the prawns grow. In addition, for some species, there may be genotypes

within a population with temperature optima significantly different from the population

mean (Coman et al., 2002). This study utilised siblings from a single mating. Given the

large geographic range of S. serrata, the potential for site specificity in regard to

temperature optima needs to be considered. Larvae of South African S. serrata suffered

high mortality at temperatures above 25 jC (Hill, 1974), whereas Japanese S. serrata

larvae had the best survival at 29 jC (Hamasaki, 2003).

Hai et al. (1998) found that a reduction in salinity affected moult frequency of S.

serrata juveniles, whereby crabs held between 18x and 30x moulted more frequently

than crabs held at 6x and 12x, while all crabs held at 0 ppt died. This is similar to the

results of this experiment. A difference between the two studies however is that our

animals were reduced to zero salinity within 6 h, whereas Hai et al. (1998) reduced salinity

at a rate of 2x day� 1. During the 15 days of acclimation, the crablets in that experiment

actually reached instar 3, whereas in this study, mortality occurred overnight, suggesting

that the rapid dilution undertaken in this experiment was too stressful for the animals. A

slower dilution should be considered for future experiments, especially if salinities below

5x are to be tested. Interestingly, salinity in the range 5–40x had no impact on survival,

hence reducing salinity to as low as 5x in 6 h can occur without mortality, especially at

optimum temperatures.

Haemolymph osmolality of subadult S. serrata (198.58F 35.98 g) has been shown to

reach constant levels after 5 days when crabs are transferred to salinities ranging from 14xto 44x (Chen and Chia, 1997). In that study, osmoregulation was largely accounted for by

ion regulation in salinities below 33.4x (968.6 mOsm kg� 1). Above this salinity, crabs

were osmoconformers. In another experiment, adult S. serratawere held at salinities ranging

from 1x to 42x and did not exhibit any ability to discriminate between various salinities

(Davenport and Wong, 1987). Further experimentation could assess the growth capacity of

this species at salinities below 5x, although this experiment has established that the optimal

salinity for growth of juveniles is around 10–20x. Chen and Chia (1996) found that oxygen

consumption and nitrogen excretion of juvenile S. serrata (0.49F 0.11 g), indicators of

metabolic effort, were both lower at 25x compared to 15x, 20x, and 30x.

Based on the findings of this experiment, S. serrata juveniles from theNorthern Territory of

Australia should be reared at approximately 30 jC and at salinities in the range of 15–25x in

order to achieve optimal growth and survival in nursery culture prior to pond stocking.

Acknowledgements

This work was funded by the Fisheries Research and Development (FRDC) under grant

number 2000/210. The authors would like to thank Keith Newman for valuable technical

assistance throughout this project.

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