International Journal of Marine Science, 2017, Vol.7, No.5, 37-50
46
Table 3 The mean ±standard deviation of morphometric distances which gave high loadings on the principal components
Morphometric distances
t-statistic
Degrees
of freedom
Red Sea
Mediterranean Sea
Caudal Peduncle Length (
CPL
)
-18.4121
91.711
1.464 ±0.27
a
2.625 ±0.35
b
Caudal Peduncle Depth (
CPD
)
-16.5746
81.273
0.710 ±0.17
a
1.464 ±0.27
b
Eye Diameter (
ED
)
-10.6802
91.082
0.690 ±0.16
a
1.001 ±0.12
b
Note: The superscripts in the same row indicate that their means are significantly different (Welch’s t-test, p ≤ 0.001) between the
samples collected from Red Sea and Mediterranean Sea
3 Discussion
Our study detected significant shape difference among
U
.
moluccensis
populations of the Mediterranean Sea and
the Red Sea. The detected pattern of phenotypic variation is probable given the geographic separation and the
extent of fish migrations are concerned (Pazhayamadom et al., 2015). A study based on genetic data does not
support the phenotypic differentiation of
U
.
moluccensis
observed in this study (Hassan and Bonhomme, 2005).
However, phenotypic variations are not directly under genetic control but are sensitive to
phenotypic plasticity
(
West-Eberhard, 1989
). When compared to other vertebrates, fishes show greater variation in morphological traits
and are vulnerable to environmentally induced differences through adaptations to feeding environments, prey
types and food availability (Thompson, 1991; Wimberger, 1992). Many studies also show that environmental
factors such as temperature, salinity, diet content or water velocity may determine the phenotypic differentiation in
fish populations (Beacham, 1990; Wimberger, 1992; Pakkasmaa and Piironen, 2000; Mazzarella et al., 2015).
Hence, morphological differences do not prove genetic distinctness between populations, but do suggest that the
fishes in each group may not mix with those in other groups (Hassan and Bonhomme, 2005). Environmental
conditions also cause ontogenetic changes in morphology (Cheverud, 1988) therefore phenotypic distinction could
also indicate a sign of long separation between populations either in time, space or both (Pazhayamadom et al.,
2015). Such findings are important in fisheries management because a lack of detectable genetic heterogeneity
does not mean that self-recruiting fish stocks do not exist (Carvalho and Hauser, 1994). In the following sections,
we discuss the phenotypic discreteness of
U
.
moluccensis
populations in relation to the environmental factors that
persist in the habitats of Red Sea and the Mediterranean Sea.
Results indicated that one reason for morphometric differentiation between samples was the shape difference in
length and depth of caudal peduncle. The shape difference in caudal peduncle is related to their swimming
abilities and hence it is important to review the hydrological conditions where the fish live in. Researchers have
found that the water turbulence in the Syrian coasts of the eastern Mediterranean Sea is considerably higher
(Yolsal et al., 2007; Coppini et al., 2011) than the Egyptian coasts of the Red Sea (El Nemr et al. 2007; Dar and
El-Saharty, 2013). A deeper caudal peduncle has been observed for fishes from turbulent waters (McLaughlin and
Grant, 1994; Walker, 1997) in many studies. Therefore, the morphometric variation observed in the caudal
peduncle could be a consequence of phenotypic plasticity in response to the ocean turbulence prevailing in these
habitats, with a deeper caudal peduncle for specimens collected from the Mediterranean Sea (Table 2 and Table 3).
An elongated and wide caudal peduncle maximizes the thrust and thus enhances the ability of fish to control its
swimming process (McLaughlin and Noakes, 1998; Blake, 2004).
Results from our study also indicate that the eye diameter is greater for specimens collected from the
Mediterranean Sea (Table 2). This is likely due to the effect of light intensity at the respective environments
because fishes may reduce or devote more resources to eye if they grow in environments with low visibility
(Pankhurst, 1987; 1989; Kotrschal et al., 1998; Remington, 2008). Many studies indicate that the turbidity of
Mediterranean Sea is relatively higher when compared to the Red Sea (Lakkis and Novel-Lakkis, 1981; Riegl and
Piller, 1999; Bianchi and Morri, 2000; Mansour et al., 2007). This likely resulted in reduced light intensity and
thus hampering the vision of fish populations. Studies indicate that the allometry of eye diameter (positive or
negative) generally depends on the behaviour and biology of the fish in the new environment (Remington, 2008;
Dugas and Franssen, 2012). For example, the eyes may reduce or degenerate if they are no more useful for
survival of the population (Jeffery, 2005). Similarly, the eye size may increase in a turbid environment if they
