Contribution to the knowledge on supralittoral macroinvertebrates of the northwestern Black Sea

A checklist of the northwestern Black Sea supralittoral fauna is presented. It includes 18 species: Ophelia bicornis, Namanereis pontica, Cryptorchestia cf. garbinii, Deshayesorchestia deshayesii, Orchestia montagui, Orchestia gammarellus, Orchestia montagui, Armadilloniscus ellipticus, Halophiloscia cf. couchii, Ligia italica, Tylos ponticus, Chthamalus stellatus, Microeuraphia depressa, Thalassomyia frauenfeldi, Donacilla cornea, Myosotella myosotis, Truncatella subcylindrica, and Melarhaphe neritoides. This paper provides distribution maps for this species in the northwestern Black Sea, based on field studies. The changes that occurred in species composition are discussed. The regional IUCN categories are proposed.


Introduction
The Black Sea differs from most seas or oceans in its nontidal conditions. Its littoral zone (known as the "pseudolittoral" or "midlittoral") is reduced (Arnoldi, 1948) and formed not by regular tides, but by wave oscillations (Mokievsky, 1949). As a result, the ranges of specialized littoral and supralittoral species are limited by narrow belts of the corresponding biotopes, often no more than tens of centimeters wide.
This makes it vulnerable, especially given the maximum littoral concentration of many human activities: industry, agriculture, hydropower engineering; municipal, fishery and shipping infrastructure, recreational activities, and coast protection works. Such spatial coincidence with strong human impact causes different types of ecological and economic conflicts (Zaitsev 2006).
At the same time, the supralittoral fauna is not covered by official monitoring in any of the Black Sea countries and is usually also relatively little studied in scientific works. Only a few studies in the Black Sea (Mokievsky, 1949;Caspers 1951;Surugiu, Giurgiu, 2006) covered supralittoral species of different taxonomic groups in the complex.
The northwestern Black Sea (from the Cape Kaliakra to the Cape Tarkhankut) has additional specificity, because, firstly, it is desalinated by the largest European rivers (especially, the Danube and the Dnieper) and, secondly, it includes large shallow semi-closed bays, which are absent in other areas of the sea .
In this paper, we summarize data on the current distribution of marine supralittoral species in the northwestern Black Sea and its changes. Both many taxa of terrestrial origin (beetles, flies, spiders, etc.) and sea crabs distributed in the sublittoral, which spend part of the day on land, were excluded from this analysis. Large Enchytraeidae also are excluded. Recent studies show high cryptic diversity in this group, but materials from the Black Sea basin were not included (Erséus, et al, 2019). The revision of the Black Sea species will also fundamentally change its taxonomy and species composition.

Material and Methods
The paper uses both results of our field research in the region between the Danube Delta and the Cape Tarkhankut (2005-2020) and literature sources (retrospective data and data on the region south on the Danube to the Cape Kaliakra).
It was used both qualitative (manual sampling) and random stratified quantitative (with 10 cm 2 frame) approach. The specimens were stored in 96% ethanol solution in the Institute of Marine Biology of the National Academy of Science of Ukraine (IMB NASU) collection.
Additionally, several museum collections were studied; among them, usable materials were found at the National Museum of Natural History at the National Academy of Sciences of Ukraine (NMNH).
The studied localities are listed in Table 1.   The main geographic names used in this work are pointed in fig. 1. We do not use the EUNIS names of habitats, because a significant part of the specific habitats in this region remains uncodified (prepared by us as a separate publication). Therefore, we use informal names of habitats, the main of which are shown in fig. 2.
The IUCN categories of conservation status were assessed for the studied species on a regional level for the region of our field study: Least Concern (LC), Near Threatened (NT), Vulnerable (VU), Endangered (EN), Critically Endangered (CR). This regional assessment of conservation status was carried out in a two-step process. The global IUCN approaches were used in the first step for the regional populations and, in the second step, this initial assessment was supported by specific elements of regional level assessments (Gärdenfors et al, 2001). In these cases, it includes analyses of the «rescue effect» (the process by which immigrating propagules result in lower extinction risk for the target population) (Gärdenfors et al, 2001). It was taken into account for species with pelagic larvae that could be carried by currents from other regions of the Black Sea as a downgrading of the conservation category.

Results
Seventeen species were analyzed in our study. For each, the numbers of the stations where it was found during our research are given, as well as literature data on other locations. The regional conservation status covers the coast between the Danube Delta and the Cape Tarkhankut.

Discussion
Regarding habitat distribution, most species are highly specialized and colonize mobile substrates (Fig. 6, A-F) (burrowing species), exposed rocks, or shelters (Fig. 6, G-L) (lower surfaces of stones, algal deposits, or Zostera mats). In separate locations with mosaic habitats, species populate a wider range of habitats, for example, D. deshayesii and T. ponticus not only burrow in the sand but can also be present under shelters.
The species that burrows in the sand or lives under shelters were colonized the most of estuaries in the studied region. This fact was almost not observed before our studies (Zakutsky, 1967).
The exception is C. cf. garbinii, which alternatively, associated with fresh and brackish waters, set apart from the typically marine, supralittoral Talitridae such as Orchestia spp. (Rewicz et al, 2020). In the first half of the XX century, it was not recorded from open coasts of the northwestern Black Sea, where Orchestia spp. were reported, but now is widespread not only in the estuaries but also in the marine supralittoral zone. For the two districts within the study region we can compare distributional data with retrospective information: Karkinit Bay (Arnoldi, 1948;Mokievsky, 1949) and Odessa Bay (Zagorovsky, Rubinstein, 1916;Pauli, 1954;Kaminskaya et al, 1977). The first is relatively kept in the natural conditions, except for sandy stretches, which are places of concentrations of recreation activities. As a result, in our studies, we do not see significant changes in species composition and distribution along the Tarchankut Penninsula. Only on several small stretches of public beaches, the supralittoral ecosystem was destroyed. The only species that has sharply declined in this district, is D. cornea.
Alternatively, the ecosystem of Odessa Bay is completely transformed. Natural rocky coast in 1950-1960s was turned to the mosaic of hard-substrate infrastructure and artificial sandy beaches. As a result, most of the locations are barren habitat without shelters for invertebrates, where among supralittoral species only Th. frauenfeldi are abundant. Interesting, that this species has appeared in Odessa Bay after the construction of numerous seawalls, inhabiting the smooth surfaces, which became the optimal biotope for this species. Probably, at the same time, the turnover of Talitridae species (key components of supralittoral ecosystems) took place. O. gammarellus, which, in the Odessa Bay, occupied the habitat of algal deposits (Zagorovsky, Rubinstein, 1916), disappeared from this area, and the more eurytopic C. cf. garbinii began to be recorded in its vicinity (Kaminskaya et al, 1977).
Also, several other species were extirpated: isopods, T. ponticus, and L. italica, which have been present there in the XIXearly XX centuries (Pauli, 1954) and gastropod M. myosotis, which was sampled in Odessa Bay last time in 1986 (collections of NMNH).
In general, the assessment results show a sharp difference in conservation status between species confined to different types of coasts and habitats.
Species that are buried in the sand (O. bicornis, D. cornea, T. ponticus, D. deshayesii) belong to the "problem" categories CR, EN, and VU. This is since the expansion of recreational activity on the sandy beaches leads to the trampling of the supralittoral species. As a result, they tend to completely disappear on city beaches and resort areas. Alternatively, species confined to rocky shores or living under shelters are predominantly in low risk NT and LC.
However, for such species in this region, there is a specific threat that is not related to human impact. Strong desalination in the area makes it possible for severe icing and freezing of rocky shores. This is critical for species inhabiting exposed rocks. This is directly proved for the disappearance of Ch. stellatus in Romania (Surugiu, Giurgiu 2006) and it is quite possible regarding the disappearance of some species in the Odessa Bay.
The regional IUCN status of CR A1abc; B1ac(i, ii, iii, iv)+2ab(i, ii, iii, iv) was proposed for D. cornea in the Romanian Black Sea (Micu and Micu, 2006), which is consistent with our data and the status of CR can be attributed to the entire Black Sea.
Apparently, for the species of O. bicornis, M. neritoides, Th. frauenfeldi, M. depressa, Ch. stellatus, C. cf. garbinii, D. deshayesii, O. gammarellus, and Orchestia spp. our estimates will coincide with the general situation for the entire northwestern Black Sea. For other species, more research is needed.