экология/6. экологический мониторинг

К.б.н. Габаев Д.Д.

A.V. Zhirmunsky Institute of Marine Biology, Far East Branch of the Russian Academy of Sciences, Vladivostok, Russia

35-YEAR STUDIES OF reproduction at marine

invertebrates

The studies of fouling of pioneer artificial substrates, conducted for the past 35 years near the coast of Primorsky Territory, Sea of Japan, Russia, have revealed an asynchronism in reproduction of invertebrates. Juveniles of Japanese scallop Patinopecten yessoensis and other widely distributed bivalves of boreal origin (Mytilus trossulus and Hiatella arctica) were, as a rule, abundant (over 250 scallops per a square meter of artificial substrate) in the odd-numbered years from 1977 to 1985. This is related mainly to the duration of ice season in previous winter, amount of precipitation, surface water salinity, and wind speed in June. The even-numbered years 1986 to 2008 were most favorable for recruits of P. yessoensis and the accompanying species. This asynchronism can be explained by the beginning of another 22-year solar cycle, which results in the opposite trend in response of atmospheric circulation to the solar activity. The asynchronism in the abundance dynamics between the low boreal scallops Chlamys farreri, Swiftopecten swifti and the widely distributed boreal Pacific mussel m. trossulus, is caused by the differences in biochemical compositions of their eggs. Each species needs certain duration of the period of optimum temperatures for their eggs to mature. Therefore, species of different biogeographical origin manifest the opposite reproduction response to conditions during a year.

Keywords: reproduction at marine invertebrates. Environmental factors. Solar cycle

Introduction

The studies of inhabitants of ocean and development of mariculture technologies are highly demanded in countries having seacoasts. Bivalve mollusks are most attractive for mariculture, as they quickly grow and do not require feeding. The Japanese scallop P. yessoensis is a commercially important bivalve, cultivated not only in the Far East but also in North America [1] and Europe [2]. according to FAO, it is the 12^th most harvested (1.4 million tons) and valued ($2.0 billion) species in the world [3]. Mariculture of this scallop mainly depends on larvae collected from the wild [4,5]. For other scallop species, collecting “wild” larvae on net bag collectors after recruitment of the parental stock also becomes reasonable [6–9]. Observations over reproduction of P. yessoensis and its competitors and predators showed that the former is unstable. At a scallop farm in Minonosok Inlet, Posyet Bay, Sea of Japan, the abundance of P. yessoensis juveniles on collectors in the most productive year was 47 times as high as that in the lean year [10]; in Saroma Lake, Hokkaido, Japan, the difference reaches 388 times [11]. variability of reproduction was noted also in other invertebrates and fishes [12]. fluctuations in abundance of P. yessoensis recruits on collectors reduce the value of the extensive cultivation technology and increase the cost of production. To create the method for stabilization of recruits’ abundance, it is necessary to know for sure what it depends on.

The goal of this study is to determine the factors and mechanism of their influence on the reproduction process in P. yessoensis and accompanying invertebrates. It can help improve the method of cultivation of P. yessoensis and other scallop species.

Materials and methods

Determination of beginning of spawning

The work was carried out in Minonosok Inlet in 1977-2011, at Cape Nizmeny (Posyet Bay; 42°36’N, 130°50’E) and in Kit Inlet (Sea of Japan; 42°31’N, 134°10’E) in 1985-1988 (Figure 1). Permanent mariculture facilities, situated in these areas, facilitate long-term observations of reproduction in marine invertebrates. A total of 25 to 30 individuals of P. yessoensis were caught by divers in Minonosok Inlet every ten days from middle May to late June of 1977-1990, and in Kit Inlet from late June to July of 1985-1987. The total weight of scallops, the weight of their soft tissues, muscle, and gonads were measured to an accuracy of ± 0.02 g. The sex of caught mollusks was determined by color of gonads. The gonadal index of P. yessoensis was calculated using the method of Ito et al. [13]. The time of beginning of spawning period was defined as a rapid 9-12% decline of the gonadal index in females. The difference between the maximum and minimum values of the gonadal index in females was considered as spawned volume of gonad. For constructing the graphs with an ordinate axis: time of beginning spawning the values have obtained by addition the real days to the earliest period.

Study of plankton

Plankton was studied in order to define the optimum time of exposition of collectors. Plankton samples were taken from the horizon 0-10 m in June and July, a week after spawning, every two-three days at 1 or 3 stations in Minonosok Inlet during 14 years and at 5 stations in Kit Inlet during 3 years. the mesh size of the sieve was 100 μm. The samples were fixed in 3% formaldehyde. Live (with body) and dead (without body) larvae of P. yessoensis, M. trossulus, and S. swifti were counted and measured under a microscope, and the abundance of larvae was expressed in terms of individuals per 1 m³.

Study of artificial substrates

The abundance dynamics of the young of marine invertebrates were studied using net bag collectors of Japanese design, placed at a mariculture installation within the horizon 8-12 m, in Minonosok Inlet in 1977-2011 and in Kit Inlet in 1985-1988. The casing of bag collectors was manufactured from nylon net with the mesh size of 5 mm, in which a polyethylene net with the mesh size of 10 mm was inserted. The total area of the collector was 1.44 м². The collectors were installed in the sea after scallop larvae reached the shell length of 250 μm (before settling). After three or four months of exposition, 10 collectors were hauled out. All bivalves and sea stars were retrieved from the collectors, and live and dead individuals counted. The objects studied were cold-water species such as the Japanese scallop P. yessoensis, the wrinkled rock borer H. arctica, the bay mussel M. trossulus, and also the warm-water Akazara scallop C. farreri, Swift’s scallop S. swifti, and the predatory North Pacific seastar Asterias amurensis. Live and dead individuals were summed up. The obtained results were expressed in terms of individuals per 1 m² of the substrate. The statistical processing of the materials was carried out in STATISTICA 6 (StatSoft Inc., Tulsa, Oklahoma, USA). Before analysis, each version was tested for normality using Shapiro-Wilk test [14]. The values were tested at α = 0.05.

Results

Determination of beginning of spawning

The studies of dynamics of gonadal index in P. yessoensis during 14 years showed that in spring the gonadal index varied from the maximum of 33% in 1977 up to the minimum of 7.2% in 1989. The maximum gonadal index in P. yessoensis in 1977-1990 oscillated from 33.0% to 17.5%. In highly productive years (1977-1979, 1981, 1986-1988), the gonadal index of P. yessoensis breeders exceeded 25%. In the non-productive years (1980, 1982, and 1989), it did not reach 20%. In years of the highest gonadal index, the mean surface water temperature in Minonosok Inlet in June was lower. An analysis of our materials revealed that since the second half of the 1970s to the early 1990s the beginning of spawning season shifted to a later time (in 1977, spawning started on May 20; in 1988, on June 17). The water surface temperature during the beginning of spawning increased from 14.7°C in 1977 to 16°C in 1990.

Study of plankton

The time of appearance of P. yessoensis larvae in the plankton of Minonosok Inlet varied depending on the time of beginning of spawning season. In the years of low surface water temperature in June, larvae of P. yessoensis appeared later. The maximum abundance of P. yessoensis larvae in Minonosok Inlet during productive years was also recorded later (usually from late June to early July) than that in lean years (usually in early June). The number youngs of P. yessoensis in the greater degree depends not on mean and integrated number of larva’s, not on spawned volume of gonads, but from time of a spawning and time of maximal number of larva’s.

Study of artificial substrates

A delay in beginning of spawning up to middle June favored reproduction of P. yessoensis in Minonosok Inlet. High abundance values of P. yessoensis recruits were observed on collectors, as a rule, in the years of later spawning. For some of invertebrate species, the modern climatic conditions appear to be favorable for reproduction, and dynamics of abundance of their young tend to grow. Species of similar biogeographic origin respond to changes in environmental conditions in the same way. Such boreal species as P. yessoensis, H. arctica, and M. trossulus in Minonosok Inlet manifest a similar reaction to climate changes. In the second group of molluscs, the low boreal scallops S. swifti and C. farreri had a significant positive correlation of dynamics of recruits’ abundance in Minonosok Inlet. In boreal P. yessoensis, H. arctica, and A. amurensis, dynamics of abundance of recruits negatively correlated with those of low boreal C. farreri (Table 1). It causes animals of boreal and low boreal origin to manifest opposite dynamics of their abundance. quasi-biennial climate fluctuations are observed in Minonosok Inlet. Owing to these oscillations, the opposite biogeographic groups of invertebrates alternately get advantages in reproduction. The analysis of dynamics of P. yessoensis juveniles’ abundance in Minonosok Inlet showed that since the beginning of our observations (1977) and till 1985, the odd-numbered years were, as a rule, favorable for high abundance of P. yessoensis juveniles in Minonosok Inlet (over 250 ind./m² of collector). After 1985, mostly the even-numbered years became favorable for abundance of these juveniles (Figure 2). The 22-year solar cycle started in 1986 and finished in 2008, and since 2009 the odd-numbered years again became productive for recruits of P. yessoensis and other species, having a similar thermopathy, in Minonosok Inlet (Figure 2). The solar activity is a global factor exerting the influence all over Earth. However, the asynchronism observed between the dynamics of abundance of P. yessoensis recruits in Kit Inlet and those in the other studied region indicates also a significant effect of regional climatic features on reproduction of invertebrates.

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figure caption for the manuscript D.D.Gabaev " 35-YEAR STUDIES OF

reproduction at marine invertebrates".

Figure 1: Study area: Posyet Bay with Minonosok Inlet and cape Nizmeny, and as well Kit Inlet. 1- Minonosok Inlet, 2 - cape Nizmeny.

Figure 2: Abundance of youngs of marine invertebrate and logarithmic trends. a - P. yessoensis, b – M. trossulus, c – C. farreri, d – S. swifti, e – H. arctica, f – A. amurensis. All figures are obtained for one interim, therefore years are represented only for lower.

Table 1. Pearson’s correlation coefficients (r) between by abundance of juveniles invertebrate. n = 35 for each correlation. Bold type indicates a significant relationship, P < 0.05.

Species	P. yess.	A. amuren.	H. arctica	S. swifti	M. tross.
A. amuren.	0.122
H. arctica	0.550	0.126
S. swifti	0.132	-0.070	0.325
M. tross.	0.209	-0.380	0.044	-0.078
Ch. nippon.	-0.137	-0.163	-0.157	0.682	-0.115