ournal of Animal and Veterinary Advances, 6 (12): 1409-1415 (2007)
Sexual dimorphism in Artemia urmiana Günther, 1899 (Anostraca: Artemiidae) from the Urmia Lake (West Azerbaijan, Iran)
Alireza Asem*1 and Nasrullah Rastegar-Pouyani2
1) Protectors of Urmia Lake National Park Society (NGO), Urmia, Iran
2) Department of Biology, Faculty of Science, Razi University, 67149 Kermanshah, IranAbstract:
Sexual dimorphism is an important subject in biosystematic and evolutionary studies of the animal kingdom. In this survey, four geographical stations have chosen from the Urmia Lake (west Azerbaijan, Iran). Males and females of Artemia urmiana were examined in order to study sexual dimorphism. The t-Test was used to find out significant differences between means. For classified the male and female samples in each station, Principal Components Analysis (PCA) and Discriminant Function Analysis (DA) were used. PCA analysis let us to separate male and female groups in each station. Also by using DA we can find out that %100 of original groups were correctly classified. This research shows that A. urmiana in each four different station is a sexually dimorphism. Also size difference between male and female can be interpreted as mating advantage; according to Artemia breeding mechanism, female carry male during copulation process then large size of female is necessary for this breeding system and can prove mating process.
Key word: Sexual dimorphism, Artemia urmiana, body size, mating advantage, breeding mechanism
Introduction:
The genus Artemia (Crustacea: Anostraca) is complex of bisexual and parthenogenetic species, which has wide distribution in the five continents which include salt lakes, coastal lagoons and solar salt-works (Van Stappen, 2002); and also the Urmia Lake is the second large lake of Iran (by the total average surface of 5000 km2), which situated in West Azerbaijan Province. It is the habitat of the endemic Artemia urmiana Günther 1899. This species was confirmed by Clark & Bowen (1976) when it is reproductive isolation with other bisexual species was confirmed.
Brine shrimp Artemia is an economical taxon which is the main and important usage of it has been shown in aquaculture industry (Bengtson et al., 1991).
Sexual dimorphism (S.D.) between adult male and female sex in the same species is a different pattern. These differences in pattern include: size, color, and the present or absent a part of the body. This character is a phenomenon which occurs among a variety of animal taxa. In the most vertebrates, the male is the largest sex. Dimorphism has been attributed to sexual selection for larger male and the competitive advantages which confers during competition for females (Andersson, 1994). Although in some taxa, reversed size dimorphism has been regarded as a problem (Figuerola, 1999). For example Artemia female is larger than male. But up to now you couldn’t see any research or critique about S.D. in Artemia genus. In this essay, we were discussed sexual dimorphism of A. urmiana from the Urmia Lake.
Materials and Methods:
Field and Morphological study
Four stations were chosen from the middle, northern and southern parts of the Urmia Lake (Fig. 1) and four primary ecological factors for each station were measured (Table 1):
1) Salinity (0.5 m from surface and 0.5 m from depth)
2) PH (0.5 m from surface and 0.5 m from depth)
3) Depth and
4) Transparency
Also thirty male and female specimens of Artemia urmiana were randomly collected from each station. Twelve morphometric characters were measured which include: total length, abdominal length, head width, distance between compound eyes, diameter of left eye, diameter of right eye, abdominal width, distance between 3rd abdominal segment to 8th abdominal segment, length of telson, length of furca, length of right antenna, length of left antenna) were measured, and the number of setae per furca (left and right branch) was counted (Hontoria & Amat, 1992; Pilla & Beardmore, 1994; Triantaphyllidis et al., 1997b; Cohen et al., 1999; Zhou et al., 2003; Camargo et al., 2003; Amat et al., 2005). Morphometric characters have been shown in Table 2.
Statistical analysis
Sexual dimorphism, between male and female individuals of A. urmiana was investigated by t-test (p<0.05) from four different geographical locations in the Urmia Lake, also all of these analyzed via Principal Components Analysis (PCA) and Discriminant Function Analysis (DA).Testing has been done SPSS computer program.
Results
The values for the morphometric and meristic characters as well as the statistical comparison of the results are summarized in Table 3.
Station N2:
As we can see in Table 3, in the first station, N2, the means of HW, AW and number of setae on left branch of the furca do not have significant differences (P<0.05). Based on the PCA, male and female groups were separated in the factor1, but these two groups don’t show any separation in the factor2 (Fig. 3-a). In companent1, ed_le, ed_ri, la_ri, la_le, ed, al and tl are the most important characters causes to exist the sexual dimorphism in station N2 (Table. 4). The first two components in orderly 61.32% and 21.10% show of the total variation respectively (Table. 4). Also, according to in the DA, 100% of the main groups of male and female were correctly classified (Table. 5).
Station M1-2:
According to Table 3 the length of LTE and number of setae on right branch of the furca do not show significant differences between both sexes (P<0.05) in station M1-2. In the PCA, male and female groups with regard to factor 1 were relatively divided, but these two groups are not separated in factor 2 (Fig. 3-b). In the first component, la_le, ed_le, la_ri, ed_ri and LF orderly have the most important role in sexual dimorphism in station M1-2 (Table. 4). Percentages of first and second components are 56.49% and 28.48% respectively, and two components show 84.97% of variation in total the (Table. 4). 100% of the original male and female groups were correctly recognized in the DA (Table. 5).
Station M3-2:
With regard to Table 3, the length of HW, AW and the number of setae on right and left branch of the furca don’t show any significant differences (P<0.05) in station M3-2. male and female groups were divided relatively with regard to the first factor in PCA, but in the second factor they were not separated (Fig. 3-c). In the component1, la_le, ed_le, la_ri, ed_ri, al and tl are important in sexual dimorphism in station M3-2 (Table. 4). The first and second components, show 61.71% and 17.64% of the total variation orderly, 79.36% of variation is presented in the first two components in the whole (Table. 4). 100% of the original male and female groups were correctly classified in the DA (Table. 5).
Station S2:
As shown in Table 3, all morphometric and meristic characters show significant differences (P<0.05) in station S2. In the PCA, male and female groups were divided with regard to factor 1, but factor 2 did not show considerable separation (Fig. 3-d). In the first component, al_le, ed_le, ed_ri, la_ri, al, de and tl are most important in sexual dimorphism (Table. 4). The first and second components show 67.59% and 14.76% of the total variation respectively; in total the two components show 82.35% of variation (Table 4). As in the other stations, 100% of the original male and female groups were correctly classified in the DA (Table 5).
Discussion
Sexual dimorphism is the phenotypic difference between the two sexes of a species (Mayr & Ashlock, 1991; Anderson, 1994). This concept is an important subject in biosystematic and evolutionary studies. According to biosystematic and taxonomic concepts, if species show sexual dimorphism, the separate morphological analyses for male and female data must be performed; otherwise, male and female samples must be combined (Manley, 1994; Fowler et al., 1998).
Pilla & Beardmore (1994), Triantaphyllidis et al. (1997b), Zhou et al. (2003), Camargo et al. (2003) and Amat et al. (2005) separately analyzed male and female samples of different Artemia populations, but they do this before they survey the of sexual dimorphism between male and female samples.
Hontoria & Amat (1992) and Cohen et al. (1999) studied morphometric characteristics of different bisexual populations of A. franciscana and A. persimilis. In these surveys, only female samples were used by Discriminant Function Analysis and male samples were not analyzed. But Gajardo et al. (1998) employed a different method for the study of morphological differentiation of Artemia populations. They analyzed the data combining both males and females for the individuals grouped by each population to which they belonged.
Zhou et al. (2003) have shown that the overall percentage of correctly classified cases of females was a little higher than that of males in A. sinica and A. tibitiana from China. The result of Camargo et al. (2003) show that the classification based on male characters provides better group membership than females of A. franciscana populations from the Colombian Caribbean. The Recent studies on A. urmiana shows that morphological differentiation between A. urmiana male samples is higher than female samples (Asem, 2005).
Now according to the upon topics account, two important questions poses:
1. If males show more morphological differentiation than females in some populations of Artemia, Why do male samples remove from the analysis? However using of male samples can show clear biosystematical results.
2. If male and female samples have morphological differentiation; why do the data of males and females combine with together? This method presents ambiguous results. For example, A. urmiana males have high morphological differentiation than females (Asem, 2005), therefore if these two groups are mixed with together then the variation will decrease because female’s low variation lessening total morphological differentiation.
The results of this study have shown that A. urmiana is a sexually dimorphic taxon in all the four different stations from the Urmia Lake and for each morphological study, male and female samples must analysis separately. This method should be considered for other bisexual population and species of Artemia.
Sexual dimorphism (S.D.) is a pervasive phenomenon. Researchers have considered patterns of sexual dimorphism and have provided theories which explaining its existence (Lappin & Swinney, 1999). And also S.D. is related to mating systems in many vertebrate taxa (Lovich & Gibbons, 1992; Promislow, 1992; Stamps, 1993).
Several theories can be applied to define the origin and maintenance of sexual dimorphism in animal. Sexual selection can be explained as directional selection of certain characteristic, large body (size) associated with an increase in reproductive fitness. It can occur by two different mechanisms: intersexual selection and intrasexual selection (Darwin, 1874). Intersexual selection occurs via female choice of mates with particular characteristics. Intrasexual selection occurs when a morphological charecter increases an individual's ability to get and keep mates or territories. For example, male-male combat for territories during the breeding season, it can be selected for enlarged or otherwise modified characteristics in males. Males try to hold the largest territories possible which females are present in this place typically. Protecting for a large territory which has overlaps with several female home-ranges increases access to breeding privileges and the probability of successful copulations. For example, in mammals S.D. has an important function in the mating system; so the males which are larger than females try to find many mates in this system (Heske & Ostfeld, 1990). Similarly, the birds with the lek mating systems rather than birds without lek system often show more sexually size dimorphic (Oakes, 1992). In snakes (Shine, 1978) and amphibians (Shine, 1979), S.D. is a function of male-male competition. Snelson (1972) proved that in North American cyprinids (minnows), males are usually larger than females in species that males keep territories. If natural selection for female be less than sexual selection, males should be larger than females in mating systems with male-male contest and female choice for larger male size. This assumes that any effects of increased size on male fitness are greater than comparable effects on female fitness.
In lizards, having a larger body and head size has been shown that it can give the advantageous during male-male combat for keeping breeding territories (Carothers, 1984; Cooper & Vitt, 1989). This is most likely the mechanism responsible for the cranial sexual dimorphism observed in Gambelia sila, (Tollestrup, 1982 & 1983). A lack of selection for raised male size could be related to the loss of territoriality (Lappin & Swinney, 1999). Loss of territoriality may define why males are not larger than females, but it does not make clear why females are larger than males (Tinkle et al., 1970). Selection for intensified fecundity, therefore, may select for large female body size (Tollestrup, 1983).
Body size comparison of male and female belong to each bisexual Artemia species generally show a size sexually dimorphic which female individuals have larger body than males (Triantaphyllidis, 1997a). Brine shrimp, Artemia doesn't show a complex sexual behavior and elaborate mating system, then Size difference between sexes can be interpreted as mating advantage because with regards to Artemia breeding system, female carry male during copulation process therefore large body of female is necessary for this breeding mechanism and can prove mating process. This phenomenon portrays an evolutionary relation among mating system of Artemia and its size sexually dimorphic.
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