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Toxicity of cadmium on larvae of Palaemon adspersus Rathke, 1837 from the Black Sea

*Corresponding author: Levent Bat, Fisheries Faculty, Department of Hydrobiology, Sinop University, TR57000 Sinop, Turkey. Tel: +90 3682876254 E-mail: leventbat@gmail.com Foundation Project: Supported by the University of Sinop, Fisheries Faculty, Department of Hydrobiology (Grant No. S.049). The journal implements double-blind peer review practiced by specially invited international editorial board members.


Introduction
Environmental contaminants especially heavy metals have detrimental effects on aquatic organisms [1]. Heavy metals as non-degradable contaminants are considered as a major problem for marine ecosystems that receive them through different anthropogenic and natural sources [2,3]. Non-essential metals such as Cd are very important in marine environments and many studies have focused on different aspects of their toxicity for marine organisms [4][5][6][7][8][9][10][11]. Crustaceans are widespread and important components of marine coastal ecosystems [12] and are thus interesting candidates for toxicological studies [13][14][15][16]. A variety of toxicity test methods have been developed and 4 and 10 days bioassays have a considerable status helping to measure the susceptibility and survival potential of organisms to particularly toxic contaminants such as heavy metals [13].
Baltic prawn, P. adspersus is a common inhabitant of the Black Sea and is known to have omnivorous feeding on small crustaceans, polychaetes, algae and detritus [18,19]. The growth and biology of the coastal population of P. adspersus has been studied in the Black Sea [19], and it has been shown that there is an association between this species and phanerogam beds which are considered as soft bottom habitats [17,20]. It tolerates a wide range of temperatures and salinities and it is distributed in shallow water, often in estuarine conditions [17].
The current experiments were carried out using the individuals of P. adspersus obtained from the Sinop Peninsula

Collection of animals
The crangonid shrimps appear during the winter and spring and the numbers of individuals increase to reach their highest abundance in mid-winter and mid-spring [21]. Specimens of the Baltic prawns were collected from Sinop Peninsula in 2013

Experimental protocol
Four and ten days experiments were designed. CdCl 2 ·2½H 2 O was dissolved in distilled water and a stock solution was made. The larvae of Baltic prawns were exposed to nominal concentrations of 0, 0.02, 0.05, 0.1, 0.5, 1.0 and 5.0 mg/L of Cd for 4 days. The 4-day LC 50 was calculated by the probit analysis [22]. At 10 days bioassays, the concentrations of 0.05, 0.1, 0.5, 1.0, 5.0, 10.0 and 20.0 mg/L were introduced into each of the jars in triplicate treatments and 0 mg/L as control [4,5]. Each tank containing 20 larvae was exposed to test solutions.
The pH of the solutions was daily measured with the pH meter, temperature with mercury-in-glass thermometer, dissolved oxygen with a digital DO 2 . The exposure lasted for 96 h. Animals mortality was monitored and recorded hourly for the first 3 h and for the next 24 h and subsequently every 24 h for the next 4 and 10 days.
If there was any dead individual within first 3 h, the experiment was terminated. The inability of the larvae to respond to external stimuli was used as an index of death. Dead organisms were removed immediately with a scoop net to avoid contamination due to rotting. Water temperature, pH and dissolved oxygen were determined every hour for the first 3 h and for the next 24 h and once every 24 h before the end of the experiment.

Statistical analysis
Statistical analyses were performed using SPSS software (ver. 21). Data were analysed by One-way of analysis of variance (ANOVA). Means were compared by Duncan multiple comparison test (P < 0.05).

Results
Control animal groups did not show significant differences during the present study. Figure 2 shows Zoea -I stage of P.
adspersus, and Figure 3 shows mean size of P. adspersus during the course of the experiment.

Discussion
The primary criterion of a toxicity test is the survival after exposure to water with contaminated Cd concentrations and control [23,24]. Environmental conditions which affect the toxicity of heavy metals include salinity, temperature, and dissolved oxygen. These factors may either influence the physiology of the organism especially larvae or may alter the form of the Cd in seawater. None of the control larvae died, indicating that the holding facilities and handling techniques were acceptable for conducting such bioassays, as required in the standard EPA/ COE protocol where mean survival should be 90% [23]. Mortality of P. adspersus increased in parallel with the increasing Cd concentrations in the seawater.
Results obtained from the 4 and 10 days bioassays obviously demonstrated that Cd is highly toxic to larvae of P. adspersus.
The most important effects of sub-lethal exposure of organisms to the metals include changes in growth, development, morphology and behaviour [4,5]. It was found in the present study that, when the larvae were exposed to concentrations of Cd, they become slightly excited and swam erratically, probably due to stress. However behavioural and swimming patterns in control groups were normal, and there was no mortality during the course of the experiment. These results agree with the toxicity studies conducted on other invertebrates [25][26][27]. Moreover, Bat et al. [17] showed that Cu was 4.25 times more toxic than Pb to P. adspersus. In this study, the toxicity of Cd for larvae of P.
adspersus increased with increasing concentration and exposure time. The 4 day LC 50 values of Cd for aquatic organisms differ from species to species and according to the type of heavy metals [13]. These results also agree with the finding by other authors [4,5,[25][26][27]. Chandurvelan et al. [7] found that acute 96 h lethal concentration of 0.405 mg/L was derived for Paratya curvirostris, placing it among the most tolerant of fresh water shrimp species.
It is known that fish feed on these contaminated benthic invertebrates, thereby accumulating heavy metals. These toxic contaminants are then further accumulated by fish consuming people. Therefore, these toxicants should not be allowed to enter the water bodies of the coastal marine ecosystems.

Conflict of interest statement
We declare that we have no conflict of interest.