Feeding Intensity of females and males of the veined rapa whelk Rapana venosa (Valenciennes, 1846) (Gastropoda, Muricidae) in the Black Sea

The veined rapa whelk Rapana venosa, a predatory gastropod living in a wide range of salinities from 15 to 32%o is indigenous to the coastal waters of China, Korea and Japan and is an invasive species in the Black Sea. This invasion has led to structural alterations in bivalve communities, which have changed the species dominance and ratio (Ivanov, 1961; Chukhchin, 1984; Zolotarev, Terentyev, 2012; Skolka, Preda, 2010), and was one of the reasons for the stocks of commercial bivalves - mussels and oysters - to decrease (Chukhchin, 1984; Ivanov, 1968; Chukhchin 1961b; Snigirov et al., 2013). R. venosa feeds on bivalves, and the mussel Mytilus galloprovincialis is its main food. The purpose of this work was to determine the rations of females and males of the gastropod R. venosa throughout the year as a function of the sea water temperature. The animals with the shell height of =60 mm and total weight of =50 g collected in March 2020 at a depth of 9-12 m under a mussel-and-oyster farm (outer roadstead of the Sevastopol Bay, 44°37'13.4"N; 33°30'13.6"E) were kept in a plastic cage at a depth of 2.5 m. The cage was divided into two compartments (for females and males) sheathed with a plastic mesh on top. The sex determination carried out at the beginning of the experiment, when observing the rapa whelk copulation, was confirmed at the end of the experiment by examining gonadal smears under a microscope (See Fig. 1, Fig. 5). The mollusks were amply fed, and the food, which was live mussels grown on the mussel-and-oyster farm, was replenished as needed. To determine the daily rations of R. venosa, correlations between the weight of soft tissues of mussels and the shell length were found in the form of the power-law equations (See Table 1). In the experiment, R. venosa individuals applied several methods of attacking mussels: smothering by pressing the valves with the foot, periodically weakening the pressing force until the valves opened; excreting a biotoxin near the prey, causing paralysis of the mussel adductor muscle; pressing a mussel with the foot to the edge of their shells, breaking off a piece of the mussel valve and violating the mussel shell integrity. The proportion of the damaged mussel shells was about 17% over the study period. The rest of the mussel shells were opened without damage to the shell. Whatever the attack method was performed, the gastropod ate out mussel’s soft tissues after the mussel slightly opened its valves. During the study period, the daily rations of females and males varied, respectively, in the ranges 0-1.0390 and 0-0.9012 g-ind.'1-day'1 (See Fig. 2). Breaks in the food intake were noted in winter, from 7 to 14 days for females, and from 14 to 30 days for males. The feeding intensity rose with the increase of water temperature and reached its highest in the late June till mid-August at a temperature of 23.5-26.8 °C. The maximum feeding intensity of females was noted during the laying of egg capsules, from June to August. The food intake was uneven, being related to the feeding rhythms. The feeding intensities of R. venosa females (See Fig. 3) and males (See Fig. 4) were fitted with third-order polynomials as a function of water temperature in the sea. The mean values of the linear or weight characteristics of females and males at the beginning and end of the experiment did not differ significantly. It was established that the average daily rations of females are significantly higher than those of males. The calculated average daily ration of females with a shell height of 63.9 mm and a total weight of 45.97 g was 0.3784 g-ind/'-day'1, or 0.82% of their total weight. For males with a shell height of 61.2 mm and a total weight of 47.1 g, the calculated average daily ration was 0.2750 g-ind/'-day'1, or 0.67% of the total weight. One R. venosa female consumed 100.28 g of mussel soft tissues per year, or 45 commercial-size mussels (L = 50 mm; mean soft tissue weight = 2.21 g), which is 2.0 times the total weight of the gastropod female. One R. venosa male consumed 72.88 g of soft tissues of mussels (or about 33 specimens) per year, which is 1.5 times its total weight. The data obtained are necessary to study the population structure of R. venosa and assess the impact of this predatory gastropod on the local populations of the mussel M. galloprovincialis in the Black Sea. The analysis of the feeding intensity data shows that ecological damage from the predator will be noticeable for a mussel population with the density of 500 g-m'2 (or 50 ind.-m'2; mean shell length = 50 mm) if the rapa whelk density in this area is 25 g-m-2. The mussel biomass will then have decreased by 50% over the year. The mussel settlement density can be restored within 2-3 years. It is known that the largest number of larvae enters the pelagial from the mussel biocenosis because M. galloprovincialis has very high fecundity and its spawning periods are extended in time (Kiseleva, 1981). The soft-bottom (silty) habitat of this mussel is limited by the depth range from 30-40 m to 50-60 m and its rocky shore habitat extends to a depth of 30 m (Zaika, 1998). R. venosa inhabits mainly sandy-shell and shelly seabeds to a depth of 26-30 m (Chukhchin, 1984; Bondarev, 2016; Danilov et al., 2018). Consequently, the range of the mussel M. galloprovincialis only partially overlaps with the range of R. venosa in contrast to the range of the oyster Os-trea edulis which exhibits full overlap with that of the rapa whelk (Chukhchin, 1961b). Mussel farms in the Black Sea are also an important source of larvae. In the last decade, the annual production of mussels in Russia amounted to about 200 tons (Kholodov et al., 2017). On farm collectors, mussels grow to a commercial size within 1.5-2 years. During this period, they can spawn 3-4 times since they become sexually mature in the first year of life. Hence, the experimental results on the feeding intensity of R. venosa and the analysis of the benthos state monitoring results do not confirm the hypothesis about ecological threat to the population of mussels in the Black Sea. The paper contains 5 Figures, 1 Table, and 35 References. The authors are grateful to Vladislav D. Shenyavsky, Director of Research Organization LLC Marikultura, for the opportunity to conduct the experiment. The Authors declare no conflict of interest.

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