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Timestamp: 2019-04-26 08:08:09+00:00

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The bathypelagic larvae of the polychaete, Vigtorniella zaikai, were found in the southern part of the Black Sea for the first time. The findings were made in the depths from 85 - 110 to 150 - 180 m at 14 stations. Maximum density (119 ind m-3) of the larvae was registered in suboxic zone (<10mM dissolved oxygen content, DOC), immediately above the hydrogen sulphide seawater layer. Vertical distribution of pelagic larvae of other polychaetes associated with V. zaikai was also studied. The deepest seawater layer adjoining redox zone was inhabited by V. zaikai, Protodrilus sp. and the larvae of Phyllodocidae gen. sp. These taxa compose the bathypelagic polychaetes fauna.
Victoriella; therefore, Kisseleva  had to re­name her finding as Vigtorniella.
the seawater above the continental slope of Turkey has not been investigated until the present study.
Sea in the south-western (Table 1) and in south­eastern (Table 2). The pelagic larvae of V. zaikai were found on 14 stations (Fig. 1). Samples of meroplankton were collected with a Nansen net with 135 j mesh size.
Results. Larvae of V. zaikai were found at 11 of 13 stations made during the expedition on board of the R/V Knorr (Table 1). At stations 5 and 8 located in shallow seawater areas (the depths of 112 and 38 m, respectively) the larvae were absent in the samples collected from the 30 -0 and 100 - 0 m depths. At station 11, where the depth was 2168 m, the total numbers measured in the sampled seawater layer (0 -130 m) was the highest - 250 ind. (Table 1).
Meroplankton was studied using samples collected from 13 stations. For the majority of the stations the pelagic larvae of the bivalve mollusc, Mytilus galloprovincialis, were the most abundant component of meroplankton. Second in the 58 prevalence were polychaete larvae which divided into two communities. The first community was represented by the neritic warm-water larvae of Harmothoe aff. reticulata, Pholoe synophthalmica, Phyllodoce maculata, Microspio meznikowianus, and the second by the bathypelagic cold-water larvae of Vigtorniella zaikai, Protodrilus sp. and larvae of the family Phyllodocidae not identified to species. At stations 6 and 8, planulas of Hydroidea were enormously abundant. The rest of the meroplankton -Polititapes aures (Bivalvia), Morhensternia (=Rissoa) parva (Gastropoda), Upogebia pusilla (Decapoda) and Balanus improvisus (Ciripedia) -were only rarely found.
120 - 145 Phyllodocidae g. sp.
85 - 110 Protodrilus sp. Harmothoe sp.
At stations 7 and 11, the share that V. zaikai contributed to the total abundance of larvae was highest (Table 1).
L30T28, were located in the close proximity to the southern coast of the Black Sea (fig.).
As regards vertical distribution, V. zaikai occupied the depth with the upper boundary going at 85-110 m depth at the deep-water station M30V45; at two other, relatively shallow-water, stations it descended deeper (Table 2). The lower depth of the occurrence had not been determined because the samples of zooplankton were collected only down to hydrogen sulphide zone. It should be noted that the lower depth at which these polychaetes occurred depended upon the station; the difference was especially pronounced between the depth of 140 m at the open-sea station M30V45 and the depth of 180 m at two stations in the coastal zone of sea water. It is noteworthy that maximum density of the larvae of V. zaikai (119 ind.-3 was regularly measured in the oxycline overlying the hydrogen sulphide zone (Table 2). The obtained estimate is slightly greater than the early record of 113 ind.m3 , and has been the acknowledged maximum for V. zaikai in the Black Sea as yet.
nectochaetes Protodrilus sp. (fam. Protodrilidae) inhabited the seawater layer of 120 - 145 m. In greater depths (145 - 180 and 150 - 185 m) only the larvae of Phyllodocidae g.sp. no identified to species and Protodrilus sp. were found. M. I. Kisseleva  suggested that the larvae of Protodrilus sp. were a new species. From the typical P. flavocapitatus they differed in that their trunk was 2 - 3 times smaller as usual, the eyes were often absent, the adhesive organs unusually shaped, and eggs smaller and more numerous. Probably, this taxon should be given name in honour of M. I Kisseleva, but this hypothesis requires more detailed study. It is noteworthy that larvae of other polychaetes associated with V. zaikai were mostly solitary and rarely found. Presumably, the larvae of V. zaikai, Protodrilus sp. and Phyllodocidae g. sp., are the polychaetes which have evolved the tolerance for the oxygen-deficient environment.
Professor Vodyanitsky, along with Protodrilus sp. different developmental stages and adults of the benthic form of V. zaikai were found in the narrow depth range (117 - 151 m) in the northwestern Black Sea (. These findings have confirmed the early hypothesis about the occurrence of adult V. zaikai in the interface layer (120 - 150 m) between the aerobic oxic and the anaerobic anoxic (hydrogen sulphide) zones along the continental slope . Maximum density (9140 ind.m-2) of the adults of V. zaikai was registered at the depth of 139 m. It is noteworthy that the polychaetes inhabited the 1 - 2-cm thick uppermost layer above grey silty mud .
Early, a hypothesis was advanced about a benthic transition belt going along the Black Sea and populated by the polychaetes V. zaikai and Protodrilus sp. However, only one location has been detected in the Black Sea as yet that is, probably, because in taking samples for the investigation special box-corer and multicorer were used .
50 - 75 to 200 -250 m  under the temperature of 6.3 - 9.0 °C, the salinity of 18.2 - 21.1%o, in the presence of hydrogen sulphide and low DOC (0.18 - 0.25 and 0.05 - 0.06 ml/l, respectively). Maximum density of the larvae (113 ind.m-3) was measured in the sample taken with a water bottle from the 111 - 96-m layer (st. 4632, the maximum depth of 2062 m) during the expedition of the R/V "Mikhail Lomonosov" in 1985. The proportion between the trochophores at junior developmental stages and the nectochaetes at senior stages was 38 : 75.
Experiments were conducted to learn more about the survival rate, nutrition and oxygen consumption rate in V. zaikai kept under oxygen deficiency and in the presence of hydrogen sulphide [9, 10, 11]. Results of the investigation point out that ubder those conditions V. zaikai can survive for a longer time than other zooplankton, e.g. Copepoda (adults of Oithona nana, juvenile stages of Pseudocalanus elongatus and Calanus euxinus). Larvae of V. zaikai exhibited highest resistance to the adverse environment, staying alive for 5 hours under the concentration of hydrogen sulphide greater than 1.6 ml/l and DOC lesser than 0.3 ml/l.
1. Flint M. V. Vertical distribution of mesoplankton in the lower depths of aerobic zone in relation to the structure of oxygen field. In: The structure and production characteristics of plankton communities of the Black Sea. - M.: Nauka, Publishing House, 1989. - 187 - 222 (in Russian).
2. Kiseleva M. I. The distribution of polychaete larvae in plankton of the Black Sea // Proc. Sevastopol Biol. Station. 1959. - 12: 160 - 167 (in Russian).
3. Kiseleva M. I. A new genus and species of the family Chrysopetalidae (Polychaeta) from the Black Sea // Zool. J. - 1992. - 71, no 11. - P. 128 -132 (in Russian).
4. Kiseleva M. I. On changing the generic name of a polychaete Vigtorniella zaikai (Polychaeta, Chrysopetalidae) // Zool. J. - 1996. - 25, no 7.­1092 (in Russian).
Two opposite opinions have been offered to explain why some mesozooplankton organisms concentrate in the redox zone: Sorokin  suggested that this is owing to the feeding selectivity of the mesozooplankton feeding on thiobacteria, while Vinogradov and Flint  rejected the very possibility that the mesozooplankton might consume the production of thiohaemosynthesis.
The findings of numerous dense populations of mesozooplankton, including copepods and the larvae of polychaetes, in the redox zone have allowed to hypothesise about a "false bottom" situated at a local oxycline in the depth of oxic zone , i.e. in the lower depth limiting the vertical distribution of the dominant mesozooplankton organisms. For example, copepods can migrate through seawater column, ascending from the depth to the layers more enriched with dissolved oxygen [14, 15]. Unlike copepods, the larvae of V. zaikai rare have been found in the subsurface overlying the 50 - 75 m depth, their dense populations are the major component of bathypelagic fauna in redox zone of the Black Sea.
Zool. J. - 1998. - 77, no 5. - P. 533 - 539 (in Russian).
6. Murina V. V. On finding the pelagic polychaete larvae in the Black Sea // Zool. J. - 1986. - 65, no 10. - 1575 - 1580 (in Russian).

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