International Journal of Aquaculture, 2018, Vol.8, No.1, 1-7
2
surprisingly we noticed some alteration in external morphology and level of ‘left-right asymmetry’, where some
morphological characters (metric and meristic) of left and right sides differed asymmetrically in some specimens.
These observations prompted us to measure and evaluate the morphological asymmetry (Fluctuating asymmetry,
FA) in the three species of
Hydrocynus
. The aim of the present study is to determine and evaluate the level of FA
of morphological characters (9 metric and 5 meristic) of the three species of
Hydrocynus
. This study is the first of
its kind in species of the Nile fish. It is an attempt to explain the causes and relationship of this morphological
asymmetry with the environmental stress caused by different environmental conditions in the Nile. The result of
the study is expected to be a base line for further study of fluctuating asymmetry in fish and can be employed to
detect the level of pollutants in the Nile environment, in evaluating the condition of the environment and the
organisms and as a potential indicator for describing developmental instability of the Nile fishes.
2 Materials and Methods
2.1 Fish collection
The study was carried out for thirty specimens of
Hydrocynus
species, were obtained from the White Nile at Jebel
Aula Dam, 45 Km South of Khartoum. Comparative material included 10 from each of
H. vittatus
,
H. brevis
and
H. forskalii
. Each individual specimen was identified according to the original description of Boulenger (1907).
The institutional abbreviations followed Daget and Grosse (1984).
2.2 Morphological characters
Nine paired bilateral morphometric characters (Length of the head (HL), eye diameter (ED), post-orbital length
(POL), length of the snout (SNL), length of pectoral fin (PFL), ventral fin (VFL), pelvic fin (PVFL), pectoral
spine (PSL) and caudal peduncle (CPL); and five meristic (number of pectoral fin rays (PFR), number of scales
along the lateral line (LLS), scale rows above (DLS) and below lateral line (VLS), and number of gill rakers (GR)
were used to estimate fluctuating asymmetry of the three species of
Hydrocynus
, following Holcík et al. (1989).
Metric characters were measured to the nearest 0.01 mm, using a fine dial caliper. Every character was estimated
as mean of three repeated measures of each variable. Meristic characters were chosen due to their functional
importance as sensory (lateral line), structural (scale rows), locomotive (fins) and feeding (gill rakers) traits.
2.3 Fluctuating asymmetry (FA) analysis
Character size was calculated as the average between both sides [(R+L)/2]. Measure of individual fluctuating
asymmetry (FA) was calculated as the signed mean differences between left and right measures and estimated
using the formula of Palmer and Strobeck (1986): (FA = Ri–Li), where (Ri) is right side and (Li) left side of
morphological character. Because metric characters and FA values may be positively influenced by body size
(Pertoldi and Kristensen, 2015), the relationship between each metric character and body size was analyzed by
performing regressions of FA against standard body length. The magnitude of absolute FA was assessed as the
unsigned difference between the measurements of the right and left sides (
∣
R−L
∣
). Spearman rank and Pearson
correlations were performed between absolute values of FA for meristic and morphometric characters,
respectively and character size. An index of asymmetry was calculated as follows (R−L/R) %, according to (Rossi
et al., 2003). For individuals with R>L their right sides dominated over the left, and the index was assigned a
positive value. In contrast, for individuals with R<L the index was assigned a negative index value.
3 Results
The results of asymmetry analysis for the measured morphological characters of the population of
H. vittatus
(1),
H. brevis
(2) and
H. forskalii
(3) showed that most investigated individuals of the three species expressed some
degree of asymmetry as shown in (Figure 1; Figure 2). The magnitude of fluctuating asymmetry was different in
the three species although
H. forskalii
showed more tendency towards the left side. The snout of all specimens of
the three species was longer on the left side compared to right side, indicating fluctuation to the left side “lefty”.
More scales were recorded along the lateral line on the right side of
H. vittatus
and
H. brevis
compared to
H.
forskalii
, while more gill rakers were also recorded on the left side of
H. vittatus
compared to the right one. The
smallest asymmetry index (0.1 to 0.4%) was found for PVFL of the three species and the largest index (4.5%) was
found for POL of
H. brevis
(Table 1). Although the three species had almost same means of characters size, mean
