INTRODUCTION

Comparison of body structure characteristics and morphometry in living organisms is used as one of key factors of biological studies (^{Reyment, 2010}). Morphological studies of sturgeon fishes started from early eighteen century using taxonomical studies (^{Ruban et al., 2008}) and morphological variation of different populations (^{Walsh et al., 2001}). However, there is scarce literature on the use of comparative morphological studies of otolith for discrimination of most sturgeon species, especially Caspian sturgeon fish, despite the fact that otholoiths are useful tools in identifying teleost fishes (^{Torres et al., 2000}; ^{Chalupnicki & Dittman, 2016}). The otoliths are versatile structures that have applications in ecology, fish biology, and fisheries science (^{Zischke et al., 2016}). Differences in otolith morphology among species can occur for genetic differences, variability in habitat and behavior, changes in environmental conditions and differences in food habits (Cardinale et al., 2004). Otolithis including three types entitled lappillus, asteriscus and sagitta. Sagittal otoliths is the largest otoliths in most species (Torres et al.). Sagittal otoliths have been used to investigate age and growth, movement and habitat, population structure, and trophic ecology (^{Rooker et al., 2008}). Sagittal otoliths correlated with genetics and environmental factors, and have special shape, so its characteristics used for identyfing of species and populations (^{Campana, 2005}) and evolutional relationships (^{Lombarte et al., 2003}). The aim of this work was to provide measurements of otoliths of Great sturgeon (Huso huso), Persian sturgeon (Acipenser persicus) and starry sturgeon (Acipenser stellatus) species by means of morphological characteristics of sagittal otoliths and also study of relationships between this calcified structure and their ecological characteristics.

MATERIAL AND METHOD

A total of 30 great sturgeon, 30 Persian sturgeon and 30 starry sturgeon were sampled from the south Caspian Sea using commercial beach seines. Total length and total weight recorded for all fish samples were measured to the nearest 1 cm and 0.01 g. Right sagittal otoliths removed and washed by distilled water and then dried in a dark room (^{Reñones et al., 2007}). Sagittal otoliths weighted to the nearest 0.0001 g and then photographed using stereo microscope (20-30) equipped with a digital camera. The shape indices were calculated according to ^{Tuset et al. (2008}) to describe shape of otoliths.

Relationships between sagittal otolith weight and body weight, and also between sagittal otolith length and body length estimated using linear regression. Sagittal otolith weight/body weight and sagittal otolith length/total length ratios and also condition factor among studied species compared using one-way analysis of variance. Means were compared using the Tukey test when there were significant differences. Prior to the statistical analysis, all data were examined for normality and homogeneity of variance and nonparametric analysis was used, and classical data transformations was used (^{Zar, 1996}).

RESULTS

Total length for Great sturgeon, Persian sturgeon and Starry sturgeon species ranged 50-101, 55-134, and 28-123 cm, respectively. Three different shapes of sagittal otolith were found in the studied species (Fig. 1). Sagittal otolith was trian- gular with irregular edges for Great sturgeon, an elongated triangle with smooth surface for Persian sturgeon, and relatively circular with smooth surfaces for Starry sturgeon (Fig. 1). In all species, when the body weight increased, then sagittal otolith weight increased. Regression slope in Persian sturgeon and Great sturgeon were significanly higher (P <0.05) than Starry sturgeon (Table I).

Fig. 1 Sagittal otoliths (lateral view) of (a) A. persicus (total length, LT = 52.5 cm) (b) H. huso (total length, LT = 85 cm) and (c) A. stellatus (total length, LT =101 cm). Scale bars = 1mm 

Table I Regression equations between body weight-otolith weight and total length-otolith length. BW, body weight (g); WO, otolith weight (g); OL, otolith length (mm); TL, total length (cm); r2, coefficient of determination. 

The sagittal WO/BW ratio in Starry sturgeon was significantly higher ( P <0.05, Table II) than Persian sturgeon and Great sturgeon. Meanwhile, the condition factor varied significantly among the three studied species (P <0.05) and in great sturgeon was significantly higher (P <0.05) than Persian sturgeon and Starry sturgeon.

It was found that when body length increased, sagittal otolith length increased in all three studied species. Regression slope in Persian sturgeon and Great sturgeon were higher than Starry sturgeon (Table I). As result, 80.0 %, 90 % and 80 % of total variation of sagittal otolith length in Persian sturgeon, Starry sturgeon and Great sturgeon, respectively are related to variation of body length.

Table II Comparison of sagittal otolith weight/body weight ratio (WO/BW), sagittal otolith length/total length ratio (OL/TL) and condition factor (CF) in Great sturgeon, Persian sturgeon and Starry sturgeon*. 

^{*Means within each row with no common superscript differ significantly at P <0.05.Values in rows with different superscripts are significantly different}.

DISCUSSION

The sagittal otoliths are widely used in comparative studies because their form, consistency, chemical composition, weight and growth have a degree of interspecific variation and difference in the shape of otoliths between closely related species of the same habitat has been widely found (^{Zorica et al., 2010}). Differences were found in the geometric forms of otoliths in all three species of sturgeon. The differences in the patterns of otolith growth among the species may also be associated with differences in their body growth and condition factor (^{Bani et al., 2013}), so greater weight of Starry sturgeon sagittal otolith is related to low growth rate. The three species have different habitats (^{Bakhshalizadeh et al., 2013}a, 2013b). Lombarte et al. demonstrated that the great otolith mass is related to shallow and warm waters. Persian sturgeon is a typically benthic inhabitant of coastal waters in seas and it remain close to the bottom (^{Vlasenko et al., 1989}), while the Starry sturgeon utilizes the middle and upper water layers (^{Shubina et al., 1989}), and Great sturgeon inhabits the pelagic zone of marine waters in comparison to other sturgeon species (^{Pirogovskií et al., 1989}).

According to Campana, total variation due to length growth in Persian sturgeon and Starry sturgeon is based on their phylogenetical relationships compared with Great sturgeon. Sagittal OL/LT ratio have same trend in all three studied sturgeons (Table I). Since sagittal otolith growth is related to body growth (^{Vallisneri et al., 2008}), relative variations in length and weight of otoliths among species, can therefore be affected by their growth rate (Bani et al.) which is associated with environmerntal factors such as temperature and depth of their habitats. Different habitats and feedig patterns affect fish biological and ecological behaviours (Lombarte et al.).

It was found that the otolith mass variations are related to body weight. This shows that the otolith mass is a more sensitive measure of the variations in the sturgeon growth rate, as has been found in other species (^{Fletcher, 1991}). Residues of variation are correlated with habitats, hearing system function (Lombarte et al.) and cranial shape (Bani et al.) in each species.

In conclusion, it was found that based on our findings, description of sagittal otolith morphological characteristics can be used for identification and discrimination of different sturgeon species and understanding of their phylogenetical and ecological relationships.