Evaluation of Lead and Cadmium pollutions in morphometric characteristics of Hermit crab (Coenobita scaevola)

Introduction:
Marine ecosystems are impacted by a large amount of pollutants from different sources. The Persian Gulf is a semi-enclosed tropical sea with limited water exchange with the Indian Ocean through the Hormuz Strait. In recent decades, the Gulf countries have grown increasingly in terms of industrial and demographic trends, and one of the consequences of this growth has been the significant increase in oil, urban, agricultural and industrial pollution such as heavy metals with both short and long-term impacts.
Heavy metals as major contaminants have been drastically increased in marine ecosystems with anthropogenic activity. This causes increasing extinction rate or complete extinction of some sensitive marine organisms, and biochemical and physiological damage to the organisms, subsequently be transferred to human through the food chain. Lead and cadmium are considered as important toxic pollutants for aquatic animals and the adverse effect of them multiply as they move up the food chain.
Evaluating the flexibility of the morphological properties of species populations inhabiting areas with different habitat characteristics and pollution levels would enhance our knowledge the effects of environmental changes on species characteristics (Kuliev, 1988). Like other organism, crabs adopt a species adjustment mechanism to develop similar morphological characteristics under similar environmental conditions. Crabs’ morphological characteristics are highly sensitive to changes in major environmental factors such as habitat type, environmental pollution, water flow, vegetation, competition, predation, and availability of food resources.
Morphometric studies generally use traditional or geometric approaches (Eigdari and Nasri, 2012). Traditional morphometries apply multivariate statistical methods to examine morphological variations within or between groups (Webster et al., 2010). Traditional morphometric studies usually use multivariate statistical tools to assess a set of distance measures. Hermit crabs (Coenobita scaevola) are more noteworthy in recent years because of the following: they are included in aquatic and shore birds’ food chain, they have an important role in cleaning the environment and also they are one of the most important animal communities in tidal zones.
The purpose of this study is to investigating the effects of heavy metal pollutions on the morphological specifications of hermit crabs in two different pollution contexts by using traditional morphological method.

Material and Method:
In June 2016, sixty hermit crabs were collected from two different stations in south and southeast of Kish Island (30 sample from each station) and then transferred to the lab. At first, the morphological variables of each sample have been measured by the traditional morphological techniques.
Morphological variables
Total Length TL
Abdomen Length AbL
Hard Length HL
Carapace Length CL
Anterior Shield Length ASL
Eyestalk Length El
Antennal Length AL
1st Walking Leg Length 1WLL
2st Walking Leg Length 2WLL
Cheliped Length CHL
Carapace Width CW
Eyestalk Width EW
Cheliped Propodus Width CPW
Total Width TW
Sex Sex
Table 1: Morphological Variables.
c b a

Figure 1. (a) Eyestalk Length (b) Carapace Length (C) Total Length
Due to the fact that in crustaceans, heavy metals accumulate more in hard tissues, the hard tissue was removed by cutter from the soft tissue. Then, after extraction wet digestion technique according to AOAC, 1980, lead and cadmium concentration were measured by atomic absorption spectrometry. The recovery rates ranged from 96% to 101% for both investigated elements.

Finally, in order to compare the lead and cadmium concentrations at each station with global standards, data normality and comparison of lead and cadmium concentrations between two stations, SPSS Software and for the Principle Component Analysis (PCA), the CANOCO software were used.

Results and Discussion:
According to the results, the mean lead concentration in hard tissue in the southeast station is 2.38 ppm and in the south station is 22.49 ppm. As a result, the lead concentration in both stations is more than the world health organization(WHO) standard and in the south station the lead concentration is significantly more than the southeast station.
The mean of cadmium concentration in the south station is 2.38 ppm and it is more than the southeast station which is 1.8 ppm and in both cases it is more than WHO standard (0.2 ppm). On the other hand, despite the higher mean concentration of cadmium in the south station, but based on Mann Whitney U test, this difference, is not significant. In the southeast station the carapace length, the eyes stem length, the antenna length, the upper moving legs and the total weight variables are significantly more than the south station. According to the fact that the length of the moving legs in hermit crabs is very important for escaping from the enemies, finding the perfect pair, finding an appropriate oyster and finding food, the effects of lead concentration on this parameter can decrease the survival rate and making intercross, movement and nutrition disorders in these species in south station.
On the other hand, results show that the south station samples have less flexibility and we predict less abundance for them in the future. Also, according to the PCA results and with regard to the focus of 57% of the important and effective components correlation in first axis, we can link the average low of the morphological variables of the samples in south station is due to high lead and cadmium concentration in hard tissue in this station.
Total Length(TL), Hard Length(HL), Abdomen Length(AL), the Anterior Shield Length(ASL) and the 1st and 2st Walking Leg Length (1,2 WLL), there is a positive correlation with the cadmium concentration in the hard tissue of the hermit crab. Cadmium has a direct impact on the 1st and 2st walking leg in hermit crabs and hermit do activities such as finding food, pairs and oysters (to protect the abdomen) and escape from the enemies with walking leg. So the cadmium has a negative effect on nutrition, growth, survival and reproductive activity.
Thus, due to the presence of hermit crab in the aquatic and shorebirds' food chain as well as its significant role in environmental purification (Seiffabadi et al., 2014), it is also possible to use it as a bio indicator of contaminated water. Also habitats and populations monitoring of this species are proposed on the southern coasts of Iran.

Keywords: Hermit crab, Kish Island, Traditional morphometry, Lead, Cadmium