Introduction

Fluctuating asymmetry is a differential development of a bilateral character between the sides of an organism (Van Vallen, 1962; Leary and Allendroff, 1989). Developmental instability can be reflected by fluctuating asymmetry which is a random deviation from perfect bilateral system. In another words, it is the inability of an organism to compensate for disturbances during development (Zakharov, 1992), and such incapability can be affected by stress related to environmental or genetic conditions. As a result, high fluctuating asymmetry could indicate the worst condition of fish that experience unfavourable environments. Therefore, the bilateral asymmetry measure could show an environmental influence on the fitness of the organism.

Several adult fish species were used to study the relationship between fish condition and fluctuating asymmetry. In such a studies, numbers of measurements have been proposed, including the number of gill arks, pectoral fin rays, fish body proportions, eye spot area, or otolith size and shape (Escós et al., 1995; Somarakis et al., 1997; Jawad, 2001, 2003, 2004; Gonçalves et al., 2002; Jawad et al., 2012).

In having an asymmetrical otolith, fish will face some consequences that might change its habit or behaviour (Gagliano et al., 2007; Gagliano and McCormick, 2004). The survival of young individuals will be threatened as it will be difficult for them to find a proper place for settlement. On the other hand, fluctuating asymmetry in the otolith would be of interest to fisheries managers as it can supply a sort of measurement of past individual and population fitness due to the possibility to study otolith of specimens from old populations. Such information will assist in the collapse of a present time stock and throws light on the future management strategies (Díaz-Gil et al., 2015).

As fluctuating asymmetry studies were never performed on the otolith dimensions of the species in question or on that of the same species from other localities in Egyptian waters the present study represents the first study on fish otolith asymmetry of the Egyptian waters.

The present work studied fluctuating asymmetry in the otolith length, width of the two parrotfish Chlorurus sordidus and Hipposcarus harid collected from Hurghada, Red Sea coast of Egypt. It aims to provide information related to the detection of suitable settlement habitats by the larvae of those two species.

1 Materials and Methods

Description of sampling area

Hurghada lies at the northern part of the Red Sea between latitude 27⁰ 10’ N- 27⁰ 33' N and longitudes 33⁰ 70' E – 33⁰ 85' E (Figure 1). It is located on the western coast of the Red Sea, 500km south-east of Cairo, and stretches for about 36 kilometres along the seashore, and it does not reach far into the surrounding desert. The vicinity of Hurghada was chosen as it represents one of the main fishing grounds for the two species in question and asymmetry study for Chlorurus sordidus and Hipposcarus harid is important to show the effect of this phenomenon on the settlement of the larvae in this important fishing ground.

Sample collection

Specimens of parrotfish were obtained during the fishing season 2012-2013 through monthly sampling from the commercial landings. The fishes were caught using gill nets of 60 to 100 m long with mesh size of 2 – ¼ mm. Standard length (SL) was measured to the nearest 1.0 mm using digital caliper. Both left and right sagittae were removed through a cut in the cranium to expose them and then cleaned and stored dry in glass vials. Sagittae specimens were collected from 30 specimens (150-260 mm SL) of Chlorurus sordidus and 30 specimens (160-260 mm SL) of Hipposcarus harid of different fish length groups. Specimens with obvious evidence of calcite crystallization or other aberrant formations were not considered for this study. Each sagittae were systematically placed with the sulcus acusticus oriented towards the observer and the length was determined using ordinary light microscope. The maximum length (OL) and the maximum height (OW) were measured to an accuracy of 0⋅01 mm, recording the greatest distance from the anterior tip to the posterior edge (OL) and the greatest distance between the otolith dorsal and ventral margins (OW), according to Harvey et al. (2000) and Battaglia et al. (2010).

Statistical analysis

The statistical analysis was based on the squared coefficient of asymmetry variation (CV² _a) for the three otolith dimensions according to Valentine et al. (1973):

CV² _a = (S r-l X 100/X r + l)²

Where S r-l is the standard deviation of signed differences and X r + l is the mean of the character, which is calculated by adding the absolute scores for both sides and dividing by the sample size.

2 Results

The results of asymmetry data analysis of the otolith length, width of C. sordidus and H. harid collected from Hurghada, Red Sea coast of Egypt are shown in Table 1. The results showed that the level of asymmetry of the otolith length was higher than that of the otolith width. For C. sordidus and for both otolith variables, the lowest and highest values are found in the fish total length groups 150-170 mm and 240-260 mm. The asymmetry coefficient has zero value for other length groups. In both species, the results showed that asymmetry was correlated with fish length (Table 2).

The percentage of the individuals showing asymmetry in the otolith length character was the highest among the percentages obtained for the two otolith characters in both species (Table 1).

3 Discussion

Abnormal swimming and behaviour in some fish individuals resulted in interference with the sound localization and lead for these individuals to decrease their ability to integrate with the environment that they living in (Lychakov and Rebane, 2005). Such consequences are results of the bilateral asymmetry in otolith mass.

The variations in the dimensions of the otolith of C. sordidus and H. harid, can affect the capability of the young individuals to locate and settle down in their suitable habitats (Gagliano and McCormick, 2004; Gagliano et al., 2007). Due to the asymmetry observed, settlement of larvae might be affected. Since the species in question are among the commercial species in Egypt, such studies are considered relevant to the ecology of these species in order to assess their stock in the area.

Due to unavailable data on the correlation between different environmental pollutions and the morphology of the two species dealt with in the present study to evaluate asymmetry along a pollution gradient, or from fish taken from impacted vs control (non-impacted) sites, it is not possible at this stage to have precise indication on the significance of this phenomenon. However, based on previous studies in this field, it is possible to suggest a correlation between environmental stress due to pollution and asymmetry in the morphology of this species. Such environmental factors are present in the waters of Hurghada, Red Sea coast of Egypt (Mansour et al., 2000; Mansour et al, 2005; Madkour et al., 2006; Madkour and Dar, 2007; Madkour et al., 2008, Mansour et al., 2011).

The environmental causes might be natural events, and several factors are known to produce nutritional deficiencies such as various pathogens and various population phenomena (Bengtsson and Hindberg, 1985), and it is highly possible that these factors may be in action in Oman Sea as they seem to be common in the aquatic environment.

Several authors have shown a relationship between the coefficient of asymmetry and fish length (Al- Hassan et al., 1990; Al- Hassan and Hassan, 1994; Al- Hassan and Shwafi, 1997; Jawad, 2001, 2013) where there was a trend of increase in the asymmetry value with the increase in fish length. The otolith morphological characters studied were identical and gave zero value for the asymmetry coefficient in several length groups studied. The results also show a trend of increase of otolith length and width asymmetry value with fish length.

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