On the Asymmetry of Some Morphological Characters of the Silver-Cheeked Toadfish Lagocephalus sceleratus (Gmelin, 1789) Collected from the Sea of Oman

Laith Jawad1

 


1 4 Tinturn Place, Flatbush, Manukau, Auckland, New Zealand; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

Asymmetry analysis has been carried out for some bilateral characters of silver-cheeked toadfish Lagocephalus sceleratus (Gmelin, 1789) (Family: Tetraodontidae) collected from the Oman Sea. The pre-orbital length analyses demonstrated the highest incidence of asymmetry among the characters studied. The lowest asymmetry value was recorded for the head length. An increasing trend in the asymmetry value with fish length is also obtained for pre-orbital length, number of pectoral fins. The result of the asymmetry direction showed that all the characters studied are dextral, where the right side showed higher value over the left side, except for pectoral fin ray count where shown to be sinistral, where count on the left side is larger than that of the right side. The possible causes of asymmetry in this species are herein discussed in relation to different pressures.

Keywords: fish bilateral characters, pre-orbital distance, post-orbital distance, orbital diameter, head length, pectoral fin ray count

 

 

Introduction

Fluctuating asymmetry (Van Vallen, 1962; Palmer and Strobeck, 1986; Leary and Allendorf, 1989) are profound bilateral asymmetries in phenotype which is a widespread phenomenon and comprises an emerging but controversial direction in evolutionary studies. It generally originates from developmental instability (Waddington, 1942) that has received significant attention due to negative correlations with fitness, including associations with sexual selection (Møller and Hoglund, 1991; Watson and Thornbill, 1994) and trait function (Mather, 1953; Moodie, 1977; Palmer and Strobeck, 1986; Balmford et al., 1993, Gummer and Brigham, 1995; Clarke, 1998). Asymmetry has shown to have detrimental effects on the biomechanics of the principle function of the organism (Moodie and Reimchen, 1976; Reimchen, 1983, Bergstrom and Reimchen, 2003). Nonetheless, the consequences of asymmetry in such effects varies among habitats (Bergstrom and Reimchen, 2003), suggesting a complex and site-dependent relationship with fitness. Thus, fluctuating asymmetry appears to be a useful biomarker for stress-induced developmental instability.

The inability of individuals to undergo an identical development of characters on both sides of the body will result in fluctuating asymmetry. The sensitivity of development to environmental stress can be indicated by fluctuating asymmetry (Møller and Pomiankowski, 1993).

Silver-cheeked toadfish Lagocephalus sceleratus (Gmelin, 1789) (Tetraodontidae) is found in all tropical to warm temperate seas. It lives in shallow water, but some species of this family occurs at moderate depths (Randall, 1995). Fifteen species of the family Tetraodontidae have been observed in the area and along the coasts of Oman (Randall, 1995) and according to the species richness this family is considered to be one of the principal components of the coastal fish community.

Meristic and metric characters are sensitive to environmental factors and could show a significant instability once they became exposed to environmental stress (Fowler, 1970).

This work studied the bilateral asymmetry phenomenon in selected morphological characters of the tetraodontid species, L. sceleratus collected from the Oman Sea in order to determine: (1) if bilateral asymmetry occurs in the chosen characters; (2) the extent of asymmetries; (3) the direction of the asymmetrical development, as shown by one side tending to have a larger number of elements; (4) the possible usefulness of the information in future taxonomic studies of L. sceleratus; and (5) possibility of use of selected bilateral characters as biomarker (stress induced bilateral instability).

Material and methods

One hundred and thirty three specimens of L. sceleratus were collected from the Oman Sea on March 3rd, 2011. Five bilateral characters were used to compare asymmetry, as follows: (1) Length of the pre-orbital distance (mm), measured from the mouth to the anterior edge of the orbit; (2) length of the post-orbital distance (mm), measured from the posterior edge of the eye to the posterior edge of the operculum; (3) orbital diameter (mm), measured from the anterior to the posterior edges of the eye; (4) head length (mm), measured from the mouth to the edge of the preoperculum; and (5) number of pectoral fin rays, a count of the total number of pectoral fin rays, including the uppermost ray.

Most characters were counted and measured under a binocular dissecting microscope. For specimens too large to fit under a microscope, a magnifying glass was used. The square coefficient of asymmetry variation (CV2a) for meristic and metric characters was calculated according to Valentine et al. (1973) as:

CV2a = (S r+l X 100/X r+l)

where S r+l is the standard deviation of the signed difference, 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.

To eliminate scaling problems associated with growth in morphometric characters (non-discrete, measurable), each measurement was divided by a conventional standardizing measurement (e.g., head length was used in the present study). Every metric record was treated in a similar manner and the squared coefficient of asymmetry was determined.

Results

The results of the asymmetry data analysis of the selected characters of L. sceleratus collected from the Oman Sea are shown in Table 1.

The highest value of asymmetry was recorded for length of the pre-orbital distance and the lowest value for the head length. The contribution of individuals showing asymmetry in length of the pre-orbital distance was the highest among the percentages recorded for asymmetry of the five characters (87.98 % of the total fish studied), while the lowest share was recorded for individuals with asymmetry in the head length (33.33 % of the total fish studied).

The result of the asymmetry direction analyses have shown that all the characters studied are dextral, where the right side showed higher value over the left side, except for pectoral fin ray count where shown to be sinistral, where count on the left side is larger than that of the right side. For pectoral fin ray count, bilateral asymmetry occurred in 87.9% of the total of 133 L. sceleratus examined. Out of specimens showing asymmetry, 58.2% had larger left side counts. As to the pre-orbital, post-orbital lengths, orbital diameter and head length, manifested bilateral asymmetry was 72.7%, 75.8%, 51.5% and 33.3% respectively. Of these asymmetric specimens, 68.5%, 79.5%, 55.7% and 48.9% are right sided respectively.

Individuals of L. sceleratus were grouped into length classes (Table 2). A trend of increase in the asymmetry values was noticed in the pre-orbital length and pectoral fin ray count.

 

Table 1: Squared coefficient asymmetry (CV2a) values and character means (X r+l) of Lagocephalus sceleratus
Tab1

 

Table 2: Squared coefficient of asymmetry and character means (X r+l ) by size class of Lagocephalus sceleratus
Tab2