Endemic spring snails Terrestribythinella ( Mollusca ) as unusual material for larval case of Crunoecia irrorata ( Trichoptera : Lepidostomatidae ) in Transcarpathian Ukraine

An unusual larval case of Crunoecia irrorata (Curtis, 1834) (Trichoptera, Lepidostomatidae) was collected in a small stream in Ukrainian Transcarpathia. This larval case consisted mainly of living snails of extremely rare endemic gastropod mollusk of the genus Terrestribythinella Sitnikova, Starobogatov & Anistratenko, 1992 (Gastropoda: Terrestribythinellidae). The larval case, distinguishing characters of C. irrorata larva and its habitat are illustrated. Ecological relations between shell-utilizing caddisfly larvae and mollusks used as material for larval cases are discussed.


Introduction
The spring-dwelling caddisfly, Crunoecia irrorata (Curtis, 1834) (Trichoptera, Lepidostomatidae), is widely distributed throughout entire Europe, though in Scandinavian countries it occurs in the southern regions mainly (e.g.Bergan 2015).Species inhabit mainly small streams and springs in forest zone, where the larvae graze among the humid moss pillows, in the interlayer of water among the fallen leaves or wet accumulations of wood detritus.
Larvae of the family Lepidostomatidae pick out a certain kind of material available in the biotope to build the larval cases.Crunoecia irrorata larvae usually use the plant material (pieces of leaves, sticks) for case building, and less often other types of material (sand grains) used for posterior part of case (e.g.Lepneva 1966).Sometimes the materials of different types are mixed.
In the present paper the larva of C. irrorata from the Ukrainian Transcarpathia (Zakarpattia oblast, Ukraine) is described and illustrated.The record of C. irrorata in the region is quite common, but the larval case of the specimen found appeared to include 22 living snails of extremely rare endemic gastropod mollusk of the genus Terrestribythinella Sitnikova, Starobogatov & Anistratenko, 1992 (Gastropoda: Terrestribythinellidae).This paper contributes some details in the case-building strategy practiced by this caddisfly species.Here we also consider this uncommon case of specific selection of rare snails for the case construction in terms of general ecological relations between shell-utilizing caddisfly larvae and mollusks used as material for larval cases.

Results and Discussion
The recorded larval case of Crunoeсia irrorata is about 13.5 long and 6.0 mm wide, slightly flattened dorsoventrally (Fig. 2, A-C).All distinguishing larval characters of the species are clearly visible on the head and thorax, including the characteristic prosternal horn (Fig. 2, D, E).Anterior half of the case is built of two pieces of beech leaves, while the remaining part is constructed of 22 alive specimens of endemic gastropod mollusk genus Terrestribythinella; any other building materials in the case are absent (Fig. 2, A-C).The larva was found in a small stream among the wet leaves.Since the territory of Ukrainian Transcarpathia within the Rakhiv district is exclusively rich with waterbodies of different types, we provide here the images of microhabitat showing the environmental conditions where the discussed larva was found (Fig. 3).
Snails used by Crunoeсia irrorata larva for case building belong to the endemic genus Terrestribythinella Sitnikova, Starobogatov & Anistratenko, 1992.These small gastropods are rare and restricted in distribution to Ukrainian Carpathians (Sitnikova et al. 1992;Anistratenko & Stadnichenko 1995).Mollusks inhabit the litter of deciduous forests, they hold close to waterbodies: different small streams and reocrens; they stay at the boundary of terrestrial and aquatic environments (Fig. 2, C).Syntopic occurrence of Terrestribythinella with terrestrial gastropods on the wet litter leaves or stones reflect their amphibiotic ecological preferences (Anistratenko 1995;Anistratenko et al. 2017).The ecological characteristics of Crunoeсia irrorata larvae match these of Terrestribythinella snails since they occur together.Species diversity of macroinvertebrates in the springs and small streams of Ukrainian Transcarpathia is not high and their inhabitants often produce the populations of a very low density.The monospecific composition of shells in the larval case suggests that no other alive mollusks or their shells suitable in "caliber" could be found by the caddisfly larva in the habitat.Shells of gastropod and bivalve mollusks, similar to Terrestribythinella in size and bigger, are used by some caddisfly species for building the larval cases (e.g.Limnephilus flavicornis (Fabricius, 1787)).Small-sized mollusks in fifteen genera of Gastropoda (11) and Bivalvia (4) were recorded in the cases of caddisfly larvae that inhabit small Ukrainian rivers in the middle part of Dnieper River basin, Cherkassy Region (Anistratenko 1987).
Unexpectedly, the fact of being incorporated into caddisfly larval cases does not have a visible negative effect on the basic vital functions of mollusks.Approximately 40% of sphaeriid and 20% of valvatid mollusks were found to be alive while being incorporated in the cases from the Dnieper basin.Some individuals of Valvata cristata O.F. Müller, 1774 even produced the egg capsules (spawn) and attached them to surface of the same larval case (Anistratenko 1987).As mentioned above, all the snails of Terrestribythinella herein discussed were alive too (Fig. 2, A-C).Moreover, sediment-feeding Terrestribythinella being inbuilt into the larval case probably could grow and develop afterwards.These slow-moving snails obviously gain certain benefits in terms of spreading, at least within the limits of a waterbody.This is especially important for mollusks of the genus Terrestribythinella considering their limited ability to spread.
The passive dispersal (zoochory) of freshwater mollusks by more mobile organisms (usually by fishes, birds, ect.) is well known (e.g.Zhadin 1952;Boeters 1982;Davis & Gilner 1982;Coughlan et al. 2017).However, such a vector for dispersal seems as not commonly used by the snails of Terrestribythinella due to their hidden mode of life.This might be suggested because all known populations of Terrestribythinella are limited to a few locations in the Zakarpattia and Ivano-Frankivsk regions (Sitnikova et al. 1992;Anistratenko 1995;Anistratenko & Stadnichenko 1995;Anistratenko et al. 2017).Nevertheless, the including of Terrestribythinella viable snails into the caddisfly larval cases provide them a chance to settle other parts of the same spring/small stream or even move to other close waterbodies.This may further result in a formation of a new population(-s).
It is also interesting that the rare snails with a low population density were the main material for the C. irrorata larval case.Similarly, in the Kaliningrad Region (Western Russia) the shells of locally rare amnicolid gastropod Marstoniopsis scholtzi (A. Schmidt, 1856) were reported in the cases of caddisfly larvae (Manakov 2017).This may suggest that the rarity of some mollusk species in the natural communities is likely a result of subjective human treatment and actually the caddisfly larvae are much more successful in searching for tiny snails, that the malacologists.
Depending on abundance of different natural resources caddisfly larvae can either select a preferable kind of material for their cases or just use a material available in the biotope.According to our data, caddisfly larvae have no selectivity to mollusks of certain species (genera) used for cases building.We assume that only the size of mollusk shells and their abundance (i.e.accessibility) in the habitats of caddisfly larvae are of decisive importance for these insects (Anistratenko 1987).
Small-sized mollusks in their turn can gain the benefits from being included in the larval cases of caddisflies; the caddisflies can provide them the settling along the waterbody, changing the stations, reaching for the area of waterbody richer in oxygen, etc.It is important that such benefits can be gained by only smallsized mollusks, able to live only by filtration (bivalves of the family Sphaeriidae) or sedimentary nutrition, e.g.gastropods of the families Valvatidae, Bythinellidae, Terrestribythinellidae, etc.Thus, the relations between some caddisflies from families Lepidostomatidae, Limnephilidae, etc. and small-sized water and/or amphibiotic snails, when the latter are being included in the larval cases of the former, might be considered in some cases as commensalism.
Conservation status of C. irrorata in Ukraine is not yet assessed.All efforts on investigation of caddisflies in Ukraine (see Szczęsny & Godunko 2008) are insufficient for assessment a conservation status of C. irrorata within the country.Meanwhile, C. irrorata is classified as a rare species at the European scale.It is red-listed in Sweden as vulnerable (VU) (Gardenfors 2005), it is classified as endangered (EN) in Finland (Rassi et al. 2001).Also it is the only caddisfly species legally protected in Poland (Rychła et al. 2015).Being considered as a species requiring conservation efforts in the adjacent country (Poland), the C. irrorata might also require protection both in Ukrainian Transcarpathia and Carpathians