Influence of Hydraulic pressure on Morphometric Variability of Riffle Beetle Elmis maugetii Latreille, 1802 (Coleoptera: Elmidae)

The paper presents the results of morphometric study of the riffle beetle Elmis maugetii Latreille, 1802 in response to hydraulic pressure. The following species morphometric characters were analyzed: head length, prothoracic width and total body length with respect to measured hydrological parameters (water level, sampling depth and current discharge). A total of 155 E. maugetii adult specimens from six typical riffle beetle streams in Serbia were examined during investigated period (2014-2015). Among measured hydrological parameters, it was found that only current discharge affects specimen size-a larger specimens were found in streams characterized by higher flow velocity. The main goal of the study is to assess the influence of current discharge on species morphometric variability.


INTRODUCTION
Riffle beetles in the family Elmidae are frequent members of the invertebrate community of running water. Most elmid species occur in well-aerated streams and rivers, but some also occur on wave-washed lake shores. Adults of a few species are terrestrial, but most are aquatic with plastron respiration [1].
Hydraulic conditions can play a key role in the distribution of organisms in streams and rivers, through direct effects, i.e. flooding and drag forces [2,3], or indirect effects, such as those of stream discharge on the substrate particles [4] and food availability [5,6]. Changes in hydraulic conditions, particularly current discharge, could also disturb the fauna by remving or killing organisms [7] and affecting reproduction rates, thus creating new opportunities for individuals to settle [8,9]. The vulnerability of orgnisms to intense current discharge can be reduced by morphological adaptations of the body or behavioural mechanisms such as the use of refugia [9,10,11,12]. Reduced body size is favoured in adverse hydraulic conditions because smaller larvae can penetrate sub strates of low porosity, in small interstitial spaces that serve as refugia against current displacement [13]. In adverse conditions, organisms in these shelters are more likely to survive and later recolonise the affected areas. The active search by macroinvertebrates for shelter during high-flow events was confirmed in observations made during simulated floods in tanks [14].
Phylogenetic relationships might also influence the body shape of beetles. However, all other ecological and behavioral factors have relationship with phylogeny so it is difficult to take them into account separately. Moreover, contribution of diet, flying ability and other behavioral factors should be considered to explain the underlying evolutionary processes of this shape variation [15]. On the other hand, some studies explain geographical variation considering shape differences [16].
Elmis maugetii was selected as a model for this study due to it is the most common riffle beetle species in the investigated area. The main goal of the study is to assess impact of hydrological regime parameters to morphometric characters of this riffle beetle species.

MATERIAL AND METHODS
Aquatic beetle samples were taken using a hand net (25x25 cm, 500 μm mesh size) or specimens were manually collected during the Annual Water Quality Monitoring Programme conducted by the Serbian Environmental Protection Agency (SEPA) in 2014 and TEHNIKA -KVALITET IMS, STANDARDIZACIJA I METROLOGIJA 20 (2020) 1 2015. The multi-habitat sampling procedure [17] and the AQEM protocol [18] were applied. The samples were preserved using 70% ethanol and further procssed in the laboratory. Species determination was carried out by using the stereomicroscopes Leica MS 5 and Carl Zeiss StereoDiscovery V8 with AxioCam ICc5 and riffle beetle taxonomic key [19]. Ecoregion delineation was given according to [20].
General data on sampling sites are provided in Table 1.

Table 1. General data on sampling sites
A total of 155 adult specimens were examined in this study. Additionally, some of physico-chemical parameters relevant to the riffle beetle community occurrence [water temperature ( o C) and oxygen saturation (%)] are given ( Table 2). The following linear morphometric characters were selected (dorsally): head length, prothoracic width and total body length ( Figure 1).
The following hydrological regime parameters were measured in situ: water level, sampling depth and current discharge during Annual Hydrological Programme conducted by the Hydrometeorological Service of Serbia (HMSS). A water depth is measured on sampling date for each sampling sites. A discharge was measured using different types of rotating-element current-meters depending on flow gradient. cimen: selected morphometric characters In order to estimate correlation between investigated morphometric characters and measured hydrological parameters, a linear regression analysis with 95% confidence interval for regression slope was used [21].

RESULTS AND DISCUSSION
Morphometric characters of adult E. maugetii specimens with respect to hydrological regime parameters (water level, sampling depth and current discharge) and their significance in terms of species morphometric variability were analyzed.
A one-way ANOVA showed significant differences (p<0.05; Table 3) in specimen body length between different populations. Among hydrological parameters it was obtained that only current discharge is correlated with each riffle beetle morphometric character (P<0.05) as shown in Table 4. The water level as well as the sampling depth are found to be insignificant with respect to analyzed morphometric characters (Table 5 and Table 6). The results of this study are in accordance with the [22]. In this study streams in southern France were covered and larger body sizes were found predominantly under extreme hydraulic stress. They argued that morphological and behavioural characteristics such as the ability to cling to the substrate may be enhanced, according to the size of the individual thus, overcoming the disadvantage due to larger size [22].
On the other hand, the opposite results to [22] as well as to this study were provided by [9]. They investigated patterns of body size of Phanocerus clavicornis Sharp, 1882 (Elmidae: Larainae) larvae along a gradient of change of stream discharge in the Atlantic Rainforest. The effects of such disturbance can vary in relation to the body size of each organism: smaller individuals are more able to resist being washed away and/or survive under conditions associated with increased discharge. Their results suggest that in third-order streams may be a balance between a better use of refugia by smaller larvae and greater ability to settle on the substrate by larger larvae.
Besides they assumed a direct relationship between the velocity of stream discharge and seasonality, the discharge being slower during the dry season than the rainy. This study stated the hypothesis that P. clavicornis larvae have different body sizes in response to various current discharge in streams of low order in the Atlantic Rainforest. Specifically, in the period of faster discharge-the rainy season-the larvae of P. clavicornis would have a smaller body than in the period of slower discharge -the dry season. Similar results were also reported in a previous study in a river in Australia, where, after a major flood, the invertebrates were smaller [23].
Avariety of biological processes produce differences in shape between individuals or their parts, such as disease or injury, ontogenetic development, adaptation to local geographic factors, or longterm evolutionary diversification. Differences in shape may signal different functional roles played by the same parts, different responses to the same selective pressures (or differences in the selective pressures themselves), as well as differences in processes of growth and morphogenesis. Shape analysis is one approach to understanding those diverse causes of variation and morphological transformation [24]. Size analysis plays an important role in many kinds of biological studies. Avariety of biological processes produce differences in size between individuals, amount of food resource, physico-chemical factors, adaptation to local geographic factors. But also, ecological factors is very important. The assumption of the present study is also that differences in ontogenic development of riffle beetle larva are varied with respect to flow disturbance.

CONCLUSIONS
Hydraulic disturbance in water is one of a key factor for aquatic invertebrate community composition and structure. Extreme weather conditions, such as heavy rainfalls and torrents influence on aquatic taxa richness and their spatial patterns of habitat distribution.
The major morphological adaptation of riffle beetle species to rapid discharge are long legs with prominent tarsal claws allowing them to hardly cling the substrate. Furthermore riffle beetle body shape is not so hydrodynamic.
Besides, extreme weather conditions can also change the riffle beetle species microhabitat preference; riffle beetles retreat in well-aerated stretches of watercourses (provided by the oxygen saturation measured) with optimal microhabitat conditions. Hydromorphological changes influence on microhabitat structure, predominantly by devastation of perennial microhabitats, relevant as refugia for some riffle beetle species, particularly those limited to inhabit upper stretches of watercourse with specific physico-chemical and hydrological conditions (trout zone).
In streams, the irregular surface of inorganic substrates [14], leaves and wood debris [25] often provide refuges from the current discharge and enable survival during a disturbance [26]. In adverse conditions, refuges can protect organisms that will later reoccupy the areas affected [27]. Research on the influence of current discharge and the search for refugia and how they affect the functional attributes of invertebrates has recently become more prominent [11,28,29,30,31]. These studies suggest that the use of refugia plays a key role in determining the species composition and functional characteristics of benthic communities. The event of high discharge in streams is known as the main disturbance affecting the structure of macroinvertebrate communities [32].
This study provides hydraulic stress, predominantly discharge intensity influences on morphometric parameters of the E. maugetii. Future research should provide more precise data on impact of parameters investigated as well as other hydrological parameters to aquatic beetle size and shape including both larval and imago specimens.

ACKNOWLEDGEMENT
The author is grateful to colleagues from the Serbian Environmental Protection Agency (SEPA) and the Hydrometeorological Service of Serbia (HMSS).