Influence of salinity and cadmium on the survival and ...

This article was downloaded by: [CSIR Information Services] On: 24 September 2013, At: 21:05 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

African Journal of Aquatic Science Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/taas20

Influence of salinity and cadmium on the survival and osmoregulation of Callianassa kraussi and Chiromantes eulimene (Crustacea: Decapoda) M Thwala

a b

, BK Newman

a c

& DP Cyrus

a

a

Department of Zoology and Coastal Research Unit of Zululand, University of Zululand, Private Bag 1001, KwaDlangezwa, 3886, South Africa b

CSIR, Natural Resources and the Environment, PO Box 395, Pretoria, 0001, South Africa

c

CSIR, Natural Resources and the Environment, PO Box 17001, Congella, 4013, South Africa Published online: 20 Jul 2011.

To cite this article: M Thwala , BK Newman & DP Cyrus (2011) Influence of salinity and cadmium on the survival and osmoregulation of Callianassa kraussi and Chiromantes eulimene (Crustacea: Decapoda), African Journal of Aquatic Science, 36:2, 181-189, DOI: 10.2989/16085914.2011.589115 To link to this article: http://dx.doi.org/10.2989/16085914.2011.589115

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

African Journal of Aquatic Science 2011, 36(2): 181–189 Printed in South Africa — All rights reserved

Copyright © NISC (Pty) Ltd

AFRICAN JOURNAL OF AQUATIC SCIENCE ISSN 1608–5914 EISSN 1727–9364 doi: 10.2989/16085914.2011.589115

Influence of salinity and cadmium on the survival and osmoregulation of Callianassa kraussi and Chiromantes eulimene (Crustacea: Decapoda) M Thwala1,2, BK Newman1,3* and DP Cyrus1 Department of Zoology and Coastal Research Unit of Zululand, University of Zululand, Private Bag 1001, KwaDlangezwa 3886, South Africa 2 Current address: CSIR, Natural Resources and the Environment, PO Box 395, Pretoria 0001, South Africa 3 Current address: CSIR, Natural Resources and the Environment, PO Box 17001, Congella 4013, South Africa * Corresponding author, e-mail: [email protected]

Downloaded by [CSIR Information Services] at 21:05 24 September 2013

1

Received 20 October 2009, accepted 18 January 2011 This study investigated the influence of salinity and cadmium on the survival and osmoregulatory capability of two decapod crustaceans, Callianassa kraussi and Chiromantes eulimene. Callianassa kraussi was able to survive in salinities of 5–55 over 96 h, whilst C. eulimene survived in 0–55 over the same time period. The 96-hour cadmium LC50 for both species decreased progressively at salinities above and below their respective isosmotic conditions, with the decrease being slightly more pronounced below compared to above isosmotic salinity. A hypo-iso-osmoregulatory strategy was followed by C. kraussi as it hyper-osmoregulated at salinities between 5 and 25 and osmoconformed at salinities greater than 25. Chiromantes eulimene followed a hyper-hypo-osmoregulatory strategy; it hyper-regulated in salinities from 0 up to isosmotic conditions at about 28 (c. 780 mOsm kg–1), followed by hypo-regulation up to 55. The effect of cadmium exposure on the osmoregulatory capacity of C. kraussi was more pronounced at hyper-regulating salinities (5–25) whilst on C. eulimene the influence was more pronounced at salinities above the isosmotic point (28). The influence of salinity and cadmium on both survival and osmoregulation of the two crustaceans are discussed by outlining the chemical and physiological mechanisms involved. Keywords: hyper-osmoregulation, hypo-iso-osmoregulation, LC50, osmolality, osmotic capacity, toxicity

Introduction The toxicity of cadmium and its influence on the physiology of crustaceans has received considerable attention. Apart from the fact that in soluble form, cadmium is toxic to some aquatic biota at relatively low concentrations (Taylor 1984, Wong and Rainbow 1986, Zyada and Abdel-Baky 2000, Singh et al. 2006), the toxicity of cadmium is often strongly inversely related to salinity. Furthermore, it appears that all crustaceans accumulate all cadmium taken up from solution without significant excretion, storing accumulated cadmium in the body in detoxified form (bound to metallothionein; Rainbow and White 1989, Rainbow 1998, Nuñez-Noguiera and Rainbow 2005). Not surprisingly, considerable attention has been directed at understanding the mechanisms for increased cadmium toxicity at low salinity, since this has potential implications for crustaceans that inhabit transitional waters (see numerous references cited in this study). Osmoregulation is one of the most important regulatory functions required by many aquatic organisms. In estuaries, efficient osmoregulation permits organisms to withstand often wide variations in salinity, and is a crucial determinant of survival in such environments. Monitoring the physiological wellbeing of crustaceans through osmoregulation has potential use as a biomarker (Lignot et al. 1997, 1998, 2000), and thus as an early warning sign of further ecological impairment. Lignot et al. (2000) summarised the effects

of different stressors on osmoregulation in crustaceans, and showed that the stress applied by exposing animals to chemical contaminants often reduces osmoregulatory capacity, thereby affecting the animal’s health and its success in an ecosystem. This study had two main objectives. The first was to determine the interaction between salinity and cadmium on the survival and osmoregulatory ability of the decapod crustaceans Callianassa kraussi (sandprawn) and Chiromantes eulimene (crab), with the aim of determining whether their physiological response (specifically osmoregulation) provides a potential (sublethal) biomarker for monitoring the pollution status of estuaries. There is currently a lack of health effects monitoring for coastal waters in South Africa and such a toxicity investigation was undertaken with such an underlying factor in mind. Apart from the fact that they both inhabit estuaries, C. kraussi and C. eulimene were identified as test organisms due to the fact that, while they show comparable tolerances to salinity, the osmoregulatory strategies underlying this tolerance are different. This provided an opportunity to examine the influence of osmoregulatory strategy on tolerance to cadmium. Tolerance to cadmium at salinities above that of seawater (about 35) was also examined, for the reason that most previous studies have focused on tolerances at

African Journal of Aquatic Science is co-published by NISC (Pty) Ltd and Taylor & Francis


182

or below the salinity of seawater. Apart from providing a point of interest, C. eulimene may be exposed to salinities in excess of seawater. This crab does not construct burrows (BKN and S Khanyile. University of Zululand, Zoology Department, pers. obs.), and during low tide must therefore rely on water remaining in surface depressions after rain or tidal inundation, on water in other crabs burrows (which they routinely enter), and possibly on sediment porewater (fine tufts of setae present at the base of pereiopods may be used to take up porewater). The salinity of these potential water sources can vary widely depending on prevailing meteorological conditions, ranging from near freshwater to hypersaline and extreme salinity up to 65 was used to define the upper osmoregulatory physiology at such salinity. Hypersaline conditions (>35) are not altogether impossible in marine-dominated estuaries during extreme droughts. For example, Gillikin (2000) recorded sediment porewater salinities of 1–90 in a mangrove ecosystem in Kenya. The second main objective was to provide an increased understanding of the salinity tolerance and osmoregulatory strategies of decapod crustaceans in South African estuarine waters. Materials and methods Organism collection and laboratory maintenance Adult C. kraussi were collected from the intertidal zone of the Richards Bay harbour and C. eulimene from Phragmites reed stands in the Mhlathuze River estuary on the subtropical north-east coast of South Africa (28°48′ S, 32°02′ E), using a hand-held pneumatic pump and small hand-held nets, respectively. The collection site for C. kraussi was presumed to be minimally influenced by contaminants introduced to the harbour by port activities, since it supports an abundant population of this prawn and is situated >2 km from areas where port activities are concentrated. Prawns and crabs were rinsed with site water in the field to remove adhering sand and mud, and then transported to laboratories at the University of Zululand in 40-litre tanks filled with seawater (salinity 35) collected from the Richards Bay harbour. In the laboratory, the prawns and crabs were held in tanks (about 40 individuals per tank) containing approximately 30 litres of filtered (0.45 μm) aerated seawater (salinity 35) under a 12 h light:12 h dark photoperiod at 22 °C for 24 h before experiments started. The prawns and crabs were not fed during this holding period. Experimental procedure Prawns and crabs were individually exposed to experimental media in 400 ml high-density polyethylene containers washed with 10% HNO3 and filled with 300 ml of exposure medium. A cadmium stock solution (2 000 mg l –1) was prepared by dissolving 3.26 g cadmium chloride (as CdCl2⋅5H2O) in one litre of deionised water, and stored in a glass Schott® bottle. Test concentrations were prepared by pipetting appropriate volumes of stock solution to appropriate volumes of exposure water to achieve nominal cadmium concentrations of 0 (control), 2.5, 4, 6.3, 10, 16, 25, 40 and 63 mg l–1. Chiromantes eulimene was exposed to all of these concentrations, whereas C. kraussi was

Thwala, Newman and DP Cyrus

exposed to concentrations of up to 16 mg l–1. The ranges of concentrations used in experiments were identified from the findings of pilot experiments, and include one cadmium concentration that resulted in 100% mortality of prawns and crabs after 96 h in the pilot experiments. Experimental salinities were 5, 15, 25, 35, 45 and 55 for C. kraussi, and 0, 4, 9, 18, 26, 35, 45 and 55 for C. eulimene. Callianassa kraussi was not exposed to a salinity of 0 since the prawns did not survive during acclimation to this salinity. Salinities below 35 were prepared by diluting seawater (salinity 35) with filtered (0.45 μm) dechlorinated tap water, while higher salinities were obtained by adding Instant Ocean Synthetic Sea Salts (Aquarium Systems Inc.) to seawater. Salinity was measured with a temperature-compensated Atago refractometer. Experiments were performed in an environmentally controlled chamber at 22 °C and a 12 h light:12 h dark photoperiod. Prawns and crabs were acclimated to experimental salinities by transferring them between containers filled with water of the appropriate salinity at a rate of 4–10 every 24 h. Once acclimated, 10 prawns or crabs were exposed individually to experimental media. Experimental containers were loosely covered to minimise evaporative losses. Experiments lasted 96 h, with exposure media renewed after 48 h and survival monitored every 24 h. Experimental individuals were not fed during the exposure period. Determination of haemolymph osmolality Haemolymph osmolality was determined for all C. kraussi and C. eulimene alive after 96 h, the experimental period. Haemolymph was withdrawn from the pericardium in C. kraussi and from a ventral haemocoel in C. eulimene by inserting a sterile 19-gauge hypodermic needle through the arthrodial membrane immediately behind the carapace or between the basis and coxa of the fifth pereiopod, respectively. The haemolymph was immediately expelled onto a sheet of Parafilm®, a 20 μl aliquot drawn into an applicator, and osmolality measured with a freezing point depression micro-osmometer (Advanced Instruments 3300, IEPSA MED/BK, South Africa). The time that elapsed between expelling the haemolymph onto the Parafilm® and its analysis in the osmometer ranged between 8–10 s. The osmolality of experimental media was measured following a similar procedure. The osmometer was calibrated prior to each set of experimental measurements, using commercially available standards. The osmolality of distilled water ranged between 2 and 10 mOsm kg–1 (mean = 7.75 mOsm kg–1). Data are expressed either as haemolymph osmolality (in mOsm kg–1), or as osmoregulatory capacity (OC), the latter being the difference between haemolymph osmolality and the osmolality of the exposure medium. Data analysis Survival data are summarised as the 96 h Cd LC50 (with 95% confidence limits), calculated using United States Environmental Protection Agency Probit Analysis software. The influence of exposure to cadmium on osmoregulation in C. kraussi and C. eulimene was evaluated by comparing the osmoregulatory capacity between treatments at each salinity. Osmoregulatory capacity is the difference

African Journal of Aquatic Science 2011, 36(2): 181–189

Survival Callianassa kraussi In the controls, C. kraussi tolerated the full range of exposure salinities (5–55), with survival ≥80% after 96 h, except at 15 where survival was at 70% after 96 h. Exposure to cadmium reduced survival at every salinity level, with the adverse effect becoming increasingly pronounced with increasing cadmium concentration. At any salinity no prawns survived 96 h of exposure to a cadmium concentration of 25 mg l–1. The highest tolerance to cadmium was evident at a salinity of 35. The LC50 for other salinities decreased either side of 35, but more sharply below this salinity compared to above it (Figure 1). Chiromantes eulimene In the controls C. eulimene tolerated salinities of from 0 to 55, all crabs surviving for 96 h, except at 0 and 9, where survival was 70% and 90%, respectively. Exposure to cadmium reduced survival at every salinity, with the adverse effect becoming increasingly pronounced with increasing cadmium concentration. The highest tolerance to cadmium was evident at salinities of 26 and 35. The LC50 decreased below and above salinity 26, but more sharply below compared to above this salinity (Figure 1). At salinity 0, all crabs in all cadmium concentrations died within 48 h of exposure, except at 6.3 mg Cd l–1, where a single crab died at between 48 and 72 h of exposure. Osmoregulation Callianassa kraussi In the controls C. kraussi showed a hyper-iso-osmoregulatory strategy (Figure 2). The haemolymph was maintained hyper-osmotic to the external medium at salinities between 5 and 25. Although C. kraussi essentially osmoconforms at salinities ≥25, the haemolymph was maintained slightly hyper-osmotic to the external medium at a salinity of 25 and 35 and slightly hypo-osmotic at salinity 45 and 55 (Figure 2). The influence of exposure to cadmium on osmoregulatory capacity was clearly evident only at salinities of 5 and 15 (Figure 3). At a salinity of 5, the single prawn that survived exposure to cadmium at 2.5 mg Cd l–1 for 96 h had an osmoregulatory capacity of about 47% of the average for prawns in the control treatment (Figures 2 and 3). Thus, in the single instance when exposure to cadmium at a salinity of 5 did not lead to mortality, a severe disruption of the

60 50

45.05

42.12

40 29.76

30 20 LC50 (mg Cd l−1)


Results

71.46

70

22.85 16.00

11.95

10 4

9

18

26

35

45

55

9.15

9

7.73

8 7 6

5.01

5

4.27

4

3.49

3 2 1 15

25

35 SALINITY

45

55

Figure 1: LC50 values for Chiromantes eulimene and Callianassa kraussi after 96 h exposure to cadmium at various salinities

HAEMOLYMPH OSMOLALITY (mOsm kg−1)

between the osmotic pressure of the haemolymph and the external medium at a given salinity (Charmantier et al. 1989). In situations where data were normally distributed and variances were homogeneous, data were compared using one-way ANOVA, followed where appropriate by a Tukey test to identify treatments that were statistically significantly different. In situations where these assumptions of parametric tests were not met, the data were compared using Kruskal-Wallis ANOVA followed by a Dunn’s multiple comparison test where appropriate. Statistical inference was at the α = 0.05 level of significance. Data were only statistically compared if there were three or more measurements per treatment.

183

mg Cd l−1 0 2.5 4 6 10 16

1 500 1 300 1100 900 700 500 5

15

25

35

45

55

300 500 700 900 1100 1 300 1 500 MEDIUM OSMOLALITY (mOsm kg−1) Figure 2: Haemolymph osmolality (mean ± SD) of Callianassa kraussi after 96 h exposure to combinations of salinity and cadmium. The diagonal line represents the isosmotic line (internal osmolality = external osmolality). Numbers above the x-axis indicate the salinity of the exposure medium

ability to regulate haemolymph osmolality was evident. A similar effect on osmoregulatory capacity was evident on exposure to cadmium concentrations of 2.5, 4 and 6.3 mg l–1


5


1 600

15

25 35 SALINITY

45

HAEMOLYMPH OSMOLALITY (mOsm kg−1)

mg Cd l−1 0 2.5 4 6.3 10 16

400 350 300 250 200 150 100 50 0 −50

Figure 3: Osmoregulatory capacity of Callianassa kraussi after 96 h exposure to combinations of salinity and cadmium. Different superscript letters indicate that osmoregulatory capacity differed significantly between cadmium treatments at a particular salinity (one-way or Kruskal-Wallis ANOVA, p < 0.05)

at salinity 15 (Figures 2 and 3). Here, the osmoregulatory capacity of prawns exposed to 2.5 and 4 mg Cd l–1 was statistically significantly lower than that for the controls (Figure 3). Only one individual exposed to 6.3 mg Cd l–1 at salinity 15 was alive after 96 h, so its osmoregulatory capacity could not be statistically compared to other treatments. Nevertheless, this prawn had an osmoregulatory capacity comparable to the average for prawns exposed to 2.5 and 4 mg Cd l–1 (Figure 3). Although the osmoregulatory capacity of prawns exposed to cadmium at salinities ≥25 differed, the response was not consistent between salinities and cadmium treatments. A statistically significant difference in osmoregulatory capacity was evident only at salinity 55, and then only for one cadmium treatment (Figure 3). Chiromantes eulimene In the controls C. eulimene showed a strong hyper-hypoosmoregulatory strategy (Figure 4). Following an initial increase in haemolymph osmolality between salinities of 0 and 4, haemolymph osmolality was maintained within a narrow range as the salinity of the external medium increased to 26. Haemolymph osmolality then increased with each further increase in the salinity of the external medium. The change from hyper- to hypo-osmoregulation (i.e. the isosmotic point) was reached at an external medium salinity of about 28 (c. 780 mOsm kg–1). Osmoregulatory capacity was greater at salinities below compared to above the isosmotic point. Thus C. eulimene was a more efficient osmoregulator at salinities below than above the isosmotic point (Figure 4). At a salinity of 0 no crabs were able to tolerate exposure to Cd. At salinity 4 the osmoregulatory ability of C. eulimene was not affected by exposure to 6.3 mg Cd l–1 (Figure 5), whilst exposure to higher cadmium concentrations severely impaired their osmoregulatory ability. Thus, the osmoregulatory capacity at 10 and 16 mg Cd l–1 was statistically significantly lower compared to that in the control and 6.3 mg Cd l–1

1 450 1 300 1150 1 000 850 700 550 400

55

mg Cd l−1 0 6.3 10 16 25 40 63

0

4

0

9

18

250

26

35

45

55

500 750 1 000 1 250 1 500 MEDIUM OSMOLALITY (mOsm kg−1)

Figure 4: Haemolymph osmolality (mean ± SD) of Chiromantes eulimene after 96 h exposure to combinations of salinity and cadmium. The diagonal line represents the isosmotic line (internal osmolality = external osmolality). Numbers above x-axis indicate the salinity of the exposure medium

675 600 525 450 375 300 225 150 75 0 −75 −150 −225 −300 −375 −450 −525

mg Cd l−1 0 6.3 10 16 25 40 63

OSMOREGULAORY CAPACITY (mOsm kg−1)

OSMOREGULAORY CAPACITY (mOsm kg−1)

184

0

4

9

18 26 SALINITY

35

45

55

Figure 5: Osmoregulatory capacity of Chiromantes eulimene after 96 h exposure to combinations of salinity and cadmium. Different superscript letters indicate that osmoregulatory capacity differed significantly between cadmium treatments at a particular salinity (one-way or Kruskal-Wallis ANOVA, p < 0.05)

treatments. An increase in the Cd concentration to 25 mg l–1 led to a further substantial decrease in haemolymph osmolality and, hence, to the osmoregulatory capacity of the single crab that survived for 96 h (Figure 5). At salinity 4 no crabs were able to tolerate exposure to cadmium concentrations >25 mg l–1 for 96 h. At salinities of 9–26, exposure to cadmium had far less influence on haemolymph osmolality, and hence on osmoregulatory capacity. Haemolymph osmolality and osmoregulatory capacity generally increased with increasing Cd concentration, although the difference in osmoregulatory capacity between control and cadmium exposed crabs was usually not statistically significant (Figure 5).


At external medium salinities above the isosmotic point the trend for cadmium-exposed crabs to have a higher haemolymph osmolality, but now lower osmoregulatory capacity, compared to crabs in the control continued, but here the differences were often statistically significant (Figure 5). The most pronounced influence of cadmium exposure on osmoregulation was at 63 mg Cd l–1, where the crabs that survived 96 h of exposure had haemolymph osmolalities that were not very different to the osmolality of the external medium (Figures 4 and 5). In other words, these crabs had a trend in haemolymph osmolality that was characteristic of a weak hyper-hypo-osmoregulator.


Discussion The toxicity of cadmium to C. kraussi and C. eulimene was strongly influenced by salinity. For both species the 96 h cadmium LC50 followed a similar trend; it decreased progressively at salinities above and below isosmotic conditions, with the decrease being slightly more pronounced below compared to above isosmotic salinity. Thus, cadmium became increasingly toxic to C. kraussi and C. eulimene as salinity deviated from 35 and 26, respectively. High survival percentage of both C. kraussi and C. eulimene in the controls at salinities of 35 and 26, respectively, indicated that at these salinities mortality on exposure to cadmium was not solely due to salinity-driven haemolymph dilution. Callianassa kraussi was far more sensitive to cadmium than C. eulimene, as evidenced by the lower LC50 values at comparable salinities. Callianassa eulimene showed its highest tolerance to cadmium at salinities spanning about 28, where the haemolymph was isosmotic with the surrounding medium. An isosmotic point could not be defined for C. kraussi but, since the highest tolerance of this prawn to cadmium occurred at a salinity of 35, the rate of cadmium uptake is probably lowest at this salinity. An increase in the toxicity of cadmium at low salinities similar to that observed for C. kraussi and C. eulimene has been reported for numerous crustaceans (e.g. Rainbow et al. 1993, Guerin and Stickle 1995, Hall and Anderson 1995, Wright 1995, Verslycke et al. 2003, Rainbow and Black 2005). Similarly, the lowest toxicity of cadmium and other trace metals at salinities corresponding to or approximating the isosmotic point has been reported for numerous crustaceans (e.g. De Lisle and Roberts 1994, Guerin and Stickle 1995, Hall and Anderson 1995, Wildgust and Jones 1998). Various explanations for the influence of salinity on cadmium toxicity to crustaceans have been provided and probably apply also to C. kraussi and C. eulimene. It is relevant to discuss these explanations relative to the osmoregulation of C. kraussi and C. eulimene observed here. The increased toxicity of cadmium at low salinities is associated with an increased uptake of this metal in many crustaceans (e.g. Campbell 1995, Zander and Rojas 1996). The most widely accepted explanation for this trend (Wright 1995) is that the free metal ion (Cd2+), which is the most bioavailable and hence most toxic form of cadmium (Sunda et al. 1978, Blust et al. 1992, Tessier et al. 1994, Rainbow 1995, 1997), is most abundant at low salinities (e.g. Turner et al. 1981, Bruland 1992, Rainbow et al. 1993, Rainbow 1997). Cadmium readily forms complexes with chloride

185

ions, e.g. as CdCl2 (Sunda et al. 1978, Rainbow et al. 1993, Rainbow 1995, Wright 1995). In seawater, most cadmium will therefore be complexed and, hence, unable to exert toxicity. As salinity decreases so too does the abundance of chloride ions, leading to a (non-linear) increase in the abundance of the Cd2+ ion (Mantoura et al. 1978, Sunda et al. 1978, Sadiq 1992). Verslycke et al. (2003) modelled cadmium speciation at salinities of 5 and 25 and reported that Cd2+ comprised 19.7% and 3.4%, respectively, of total cadmium. These proportions are in agreement with the findings of other workers (Rainbow et al. 1993, De Lisle and Roberts 1994, Hall and Andersen 1995, Roast et al. 2001). Although the Cd2+ concentration was not determined or modelled as part of the present study, it seems reasonable that the abundance of this free ionic form of cadmium was similarly related to salinity and that this, in part, explains the increasing toxicity of cadmium to C. kraussi and C. eulimene with decreasing salinity below 35. At salinities above 35, cadmium becomes increasingly complexed with chloride ions, further reducing the abundance of the Cd2+ ion (Rainbow et al. 1993). This physico-chemical model does not, however, account entirely for the increased toxicity of cadmium at low salinities, since cadmium toxicity is often not linearly related to salinity. Rather, it appears that the toxicity of cadmium to crustaceans is also mediated by organism physiology (Rainbow 1995). At or near the isosmotic point the osmolality of the haemolymph and that of the external medium are closely similar, and therefore minimal osmotic movement of water or solutes into and from the haemolymph takes place, although ionic movements still occur as the medium and the haemolymph are isosmotic and not isionic (Roast et al 2002). In many crustaceans, the lowest toxicity of cadmium and other trace metals has been reported at salinities that correspond to or approximate the isosmotic point (e.g. De Lisle and Roberts 1994, Guerin and Stickle 1995, Hall and Anderson 1995, Wildgust and Jones 1998). This was the situation for C. eulimene in the present study, with the lowest cadmium toxicity measured at salinities straddling the isosmotic point. An isosmotic point could not be determined for C. kraussi, and a similar conclusion for this prawn is not possible. However, considering that the lowest toxicity of cadmium to C. kraussi was recorded at salinity 35, it is reasonable to assume that the uptake of cadmium was lowest at that salinity. For many crustaceans the lowest toxicity of cadmium at salinities that correspond to or approximate the isosmotic point suggests that osmoregulatory ionic exchange influences metal uptake from solution, and hence that metal uptake may be mediated by osmoregulatory mechanisms. The lowered cadmium toxicity towards C. kraussi and C. eulimene around the isosmotic point recorded in this study strongly indicate an osmoregulation-mediated uptake of cadmium. Legras et al. (2000) highlight osmoregulation as an important physiological adaptation controlling dissolved metal uptake in euryhaline organisms. However, osmoregulation is not the only factor driving metal uptake, which may not always be lowered around the isosmotic point, as shown by Roast et al. (2002); this varies between species and health status. The strongest evidence for this comes from the observation in numerous crustaceans (Wright 1977, Bjerregaard and Depledge 1994, Lucu and


186

Obersnel 1996), other invertebrates and fish (Wicklund Glynn and Runn 1988, Bjerregaard and Depledge 1994, Rasmussen and Andersen 2000, Kalay 2006) that cadmium uptake is dependent on the external calcium concentration. This suggests that there is a common mechanism for the uptake of cadmium and calcium (Wright 1977, Rainbow and Black 2005). At salinities below the isosmotic point the difference in osmotic pressure between the internal (haemolymph) and external environment causes crustaceans to gain water by osmosis. In most crustaceans the excess water is excreted, predominantly as urine (Mantel and Farmer 1983, Péqueux 1995). However, major and essential ions (e.g. Na+) are lost in the urine, since the urine of crustaceans is isosmotic with the haemolymph (Roast et al. 2002b). To counteract this loss, ions are actively transported from the surrounding medium into the haemolymph by transport proteins (pumps) situated in the gill epithelium (Mantel and Farmer 1983, Péqueux 1995); the Na+K+ATPase being the main membrane transporter system (Van Kerkhove et al. 2010) whose functioning is widely understood (Rainbow and Black 2005). Ion-transporting proteins in the gill epithelium are not, however, selective for specific ions but rather for ionic radius and charge (Rainbow 1995, 1997). Since the Cd2+ ion has a similar radius (Williams and Frausto da Silva 1996) and the same charge as the Ca2+ ion, it is inevitable that some cadmium ions will incidentally be transported across the gill epithelium by calcium-transport proteins (Rainbow 1995, Simkiss and Taylor 1995, Rainbow 1997). Evidence that cadmium is taken up across the gills via calcium-transport proteins has been provided from experiments in which the calcium concentration in exposure media was altered, and by spiking exposure media with lanthanum, which is an unspecific calcium-channel blocker in crustacea (Pedersen and Bjerregaard 1995, Lucu and Obersnel 1996, Pedersen and Bjerregaard 2000, NOrum et al. 2005). Other studies (Hille 2001, Barbier et al. 2004, Gagnon et al. 2007, Wang et al. 2009) also reported the uptake of cadmium via calcium channels with Cd2+ blocking Ca2+ uptake. Additional evidence of competitive uptake between cadmium and calcium was given by Jackson et al. (2000) and Yu and Wang (2002) who reported decreasing LC50 and cadmium uptake, respectively, with increasing medium calcium concentration. Therefore, at low salinity during increased calcium pump activity, more bioavailable cadmium (Cd2+) ions are taken up via calcium pumps. Many euryhaline crustaceans are able to vary their integument permeability at low salinities (Mantel and Farmer 1983, Rainbow 1997, Rainbow and Black 2002), and this represents a further possible physiological control over metal uptake by crustaceans that is independent of the effects of varying physico-chemical factors (Rainbow and Black 2002, 2005). A reduction in apparent water permeability at low salinities reduces the uptake of both cadmium and calcium. Thus, physico-chemical and physiological factors interact to influence the uptake rates of dissolved cadmium by crustaceans. Which of these mechanisms is more important for inducing cadmium toxicity appears to depend on the ecology and physiology of the species (Rainbow 1995, 1997). Physico-chemical parameters determine


bioavailability and speciation of a metal whilst the physiology of an animal regulates uptake activity and routes of uptake. Wright (1977) demonstrated that cadmium uptake by Carcinus maenas is increased at low salinity and that this increase is sensitive to the external calcium concentration independently of the overall salinity effect. De Lisle and Roberts (1994) showed that the speciation of cadmium is primarily responsible for the effect of salinity on the toxicity of cadmium to Americamysis (≡ Mysidopsis) bahia, but that calcium exerts a sparing effect on cadmium toxicity. Blust et al. (1992) concluded that salinity affected the uptake of cadmium by Artemia franciscana through a combined effect on permeability of the shrimp, as well as on bioavailability of cadmium. By keeping the Cd2+ concentration to which the crab Carcinus maenas was exposed constant at two salinities (32 and 10.5), Burke et al. (2003) were able to attribute an increased accumulation of cadmium at low salinity (10.5) to physiological effects. Although an increase in the Cd2+ ion concentration led to an increase in cadmium accumulation, there was a proportionally larger increase in tissue cadmium levels in crabs in low salinity compared to that in high salinity, even though all crabs were subject to the same 10-fold increase in Cd2+. Burke et al. (2003) suggested this indicates that crabs may have a physiological preference for Cd2+ ions at low salinity, or that other physiological mechanisms are switched on at low salinity to increase the absorption of other cadmium species. The mechanism for cadmium toxicity at salinities above the isosmotic point for crustaceans that hyper-hypoosmoregulate is less certain. At these salinities, the haemolymph has a lower osmotic pressure than the surrounding medium and water is lost through osmosis, primarily through the gills. To counteract this loss, water is taken up, mainly by drinking but possibly also through the gills (Mantel and Farmer 1983). However, drinking leads to an unintended gain of major ions and, in order to regulate haemolymph osmolality, the ions are actively excreted across the gill epithelium. Cadmium uptake at salinities above the isosmotic point thus presumably occurs across the intestinal lining via passive or some form of facilitated diffusion. The absorbed cadmium cannot be excreted across the gill epithelium, leading to its accumulation in tissues (Rainbow and White 1989, Rainbow 1998, Nuñez-Noguiera and Rainbow 2005). Although the toxicity of cadmium to C. eulimene was lower at salinities above compared to those below the isosmotic point, the difference was not particularly pronounced. The effect of cadmium exposure on the osmoregulatory capacity of C. eulimene was, however, far greater at salinities above the isosmotic point, leading to a substantial and often statistically significant increase in haemolymph osmolality, and hence decrease in osmoregulatory capacity, relative to the controls with increasing concentration. This suggests that the active excretion of major ions was increasingly inhibited on exposure to increasing cadmium concentrations. It seems improbable that drinking was inhibited. Cadmium accumulation by Eriocheir sinensis was shown to be increased relative to zinc above the isosmotic point, indicating that physiological intervention during hyporegulation might not necessarily reduce uptake or increase elimination (Roast et al. 2002). In their work



with bivalves Lee et al. (1998) also reported that the rate of cadmium uptake increased with increasing salinities relative to zinc uptake. Although crabs exposed to cadmium at salinities between 15 and 26 also showed an increase in haemolymph osmolality relative to the controls, the difference was small and usually not statistically significant. A similar increase in haemolymph osmolality, on exposure to cadmium at salinities below the isosmotic point, has been reported for other crustaceans (e.g. Bjerregaard and Vislie 1985, Rodriguez et al. 2001), although this response is by no means universal (e.g. Vitale et al. 1999, Silvestre et al. 2005). At 15–26, in addition to a salinitymediated change in the abundance of Cd 2+ ions, the uptake of cadmium by C. eulimene was probably also mediated by osmoregulation, with the cadmium entering via protein pumps. However, the magnitude of difference in haemolymph osmolality across the latter salinities was far less pronounced than the difference in LC50, suggesting that while exposure to cadmium did not lead to a marked difference in osmolality, death was additionally influenced by some other processes. The ability of C. eulimene to control its integument permeability at low salinities, at least to a certain extent, may also contribute to this reduced influence of cadmium exposure on osmoregulatory capacity at salinities of 15–26. The haemolymph osmolality and osmoregulatory capacity of crabs exposed to salinities of 4 and 9 decreased significantly with exposure to increasing cadmium concentrations. This suggests that, in contrast to higher salinities, the ability to excrete excess water gained through osmosis and/or the inability to actively take up major ions from the external medium was compromised, and that the crabs died due to excessive dilution of their haemolymph. High energetic costs of osmoregulation, associated with the active transport of major ions into or out of the gills at extreme salinities might also have contributed to the crabs’ inability to regulate haemolymph osmolality efficiently, ultimately leading to mortality. The probable oxidative damage of the mitochondria, which are the cellular energy centres, would have worsened the loss in active ionic regulation of the gills. Since C. kraussi shows a different osmoregulatory strategy at salinities ≥25 relative to salinities below 25, the same explanation for cadmium toxicity to C. eulimene cannot account for cadmium toxicity to this prawn. In the control treatments at salinities 25 and 35 C. kraussi maintained its haemolymph slightly hyper-osmotic to the surrounding medium. This was presumably to facilitate the uptake of water through osmosis for urine production. However, because the gradient in osmotic pressure between the internal and external environments is small, the rate and volume of water uptake via osmosis is low. By implication, the volume of urine produced, and hence of major ion loss via the urine, is also low. Therefore the rate of major ion uptake to counteract this loss, and hence also of cadmium uptake, is also low. The observed difference in LC50 for prawns at salinity 25 and 35 must, therefore, be predominantly due to the decreased abundance of cadmium at the higher salinity. At lower salinities (i.e. 5 and 15) C. kraussi maintains its haemolymph increasingly hyper-osmotic to the surrounding medium. The significant decrease in osmoregulatory capacity relative to the

187

controls at these salinities suggests, as was the case for C. eulimene, that the ability to excrete excess water gained through osmosis and/or the inability actively to take up major ions from the external medium was compromised, and that the prawns died due to excessive dilution of the haemolymph. In the controls, at salinity 45 and 55, C. kraussi maintained haemolymph osmolality slightly hypo-osmotic to the external medium. However, in other experiments we performed with C. kraussi, the haemolymph was maintained slightly hyper-osmotic to the medium at these salinities and only became slightly hypo-osmotic at a salinity of about 65. Nevertheless, for the prawns examined in this study at salinities 45 and 55, water loss to the surrounding medium would need to be replaced by drinking, and thus would lead to the uptake of cadmium. Here too, however, the rate of uptake would not have been inordinately different to that at 25 and 35. A further factor that might account for the increasing toxicity of cadmium to C. kraussi at salinities above 35, and that may also apply to other salinities and to C. eulimene, is damage to the structure of the gill epithelium that occurs on exposure to cadmium, and indeed to other metals (Péqueux 1995, Rodríguez et al. 2001, Silvestre et al. 2005). The damage may manifest as a thickening of the gill epithelium, vacuolisation, cellular hyperplasia and necrosis, and has consequences for osmotic and ionic regulation, as well as for circulatory and respiratory physiology (Lignot et al. 2000, Rodríguez et al. 2001, Silvestre et al. 2004, 2005). Cadmium-mediated generation of reactive oxygen species is known to affect the mitochondria, thereby potentially affecting any active cellular pathyway (Wang et al. 2009). Failure to mediate or control cadmium uptake and toxicity can also be related to a collapse of active osmotic and ionic regulation. In conclusion, the lack of direct measurements of cadmium uptake, gill epithelium histology, haemolymph ionic composition and energy budgets for C. kraussi and C. eulimene in this study makes it difficult to determine the mechanisms that induced cadmium toxicity. Nevertheless, both the physico-chemical and physiological mechanisms reported in the literature for cadmium toxicity to crustaceans are explanations applicable to both cadmium toxicity and the accompanying salinity influence on C. kraussi and C. eulimene observed here. The lowest experimental concentration of 6.3 mg Cd l–1 used here is well above concentrations that can be expected in estuaries that are ‘significantly’ contaminated. For example, Fatoki and Mathabatha (2001) reported a range of