Study On Biological Control Of Larval Stage Of Giant Jellyfish

Jellyfish blooms have aroused public attention worldwide and have caused serious problems to tourism,fisheries,coastal industries,and the marine ecosystem.Jellyfish bloom is affected both by physical and chemical factors and biological factors.Comprehensive researches have been conducted on physical and chemical factors in many previous studies.However,there are still some deficiencies in study of biological factors.It is well known that the life cycle of scyphozoan jellyfish is composed of a sexual pelagic medusa stage and a asexual benthic polyp stage,and the ephyrae stage is the transition stage of asexual reproduction and sexual reproduction.The polyp stage and ephyrae stage are collectively referred to as larval stage.Asexual reproduction and strobilation of the polyps determines directly the number of pelagic medusae;thus,the larval stage is the key to understand the population dynamics of medusae.In order to comprehensively evaluate the biological control factors of jellyfish bloom,it is necessary to study the interaction of different species of jellyfish at different life cycle stages and the influence of other organisms on jellyfish at different life cycle stages,including the following aspects: the interspecific predator-prey relationship of different species of jellyfish at different life cycle stages,competition and predation relationship between other benthos and benthic polyps,competition and predation relationship between other organisms and ephyrae,and competition and predation relationship between other organisms and planktonic jellyfish.In this study,the interspecific predator-prey relationship was explored,main predators of polyps of giant jellyfish in Chinese coastal waters were identified and their predation rates were determined,the predation of other organisms on the ephyrae was preliminarily discovered.The details were as follows:Nemopilema nomurai,Aurelia coerulea,and Rhopilema esculentum are three scyphozoan jellyfish commonly appeared in Chinese coastal waters.Polyps of Aurelia coerulea had been found along coastal waters wildly,while the natural habitats of polyps of N.nomurai and R.esculentum had not been confirmed.This problem is also a common problem in international jellyfish research,or a worldwide problem.Polyps of many jellyfish species are difficult to find in natural environment.The small size of the polyps and severe contraction when disturbed should be the main reason.The distribution characteristic of the medusa indicated that the polyps of all three species may occur together.To explore intraguild predation by the polyps of these three species and further evaluate the distribution pattern of polyps of the three species,a laboratory experiment considering the attachment sequence and size relationship of calyx diameter of the polyps by artificial breeding was conducted.The results showed that the polyps of Aurelia coerulea could prey on polyps of N.nomurai in all treatments,polyps of A.coerulea could prey on polyps of R.esculentum which were smaller than polyps of A.coerulea.Polyps of R.esculentum could prey on the polyps of N.nomurai only when polyps of R.esculentum attached first and were bigger than those of N.nomurai.Consequently,we conjectured that colonies of N.nomurai polyps were hard to form and expand the population size in places inhabited by A.coerulea polyps.In addition,it was well known that polyps of A.coerulea inhabited at depths of less than 20 m in coastal sea,thus,we speculate that colonies of N.nomurai polyps might occur at depths of more than 20 m.Future studies should search for the polyps of N.nomurai at greater depths and more advanced underwater detection equipment needs to be introduced.Our finding that polyps of Aurelia coerulea aggressively preyed on polyps of other species may help understand other such systems of jellyfish bloom in the world,which might biological control bloom of other jellyfish species.The number of polyps of jellyfish was affected by predation of other organisms,and in situ simulation experiments showed that the number of polyps of different species had shown different trends when exposed to the same benthic community.A total of 39 species of benthos were collected by different sampling methods from the coastal waters in China and predation experiments were carried out on the polyps of Nemopilema nomurai,Aurelia coerulea,and Rhopilema esculentum.The results showed that 7 species of macrobenthos could directly prey on or feed on polyps,including 3 species of sea slug Okenia plana,Pleurbranchaea novaezealandiae Cheeseman,Chromodoris tinctoria and 4 species of anemones Paracalliactis sinica,Calliactis japonica,Anthopleura incerta,Authopleura midori.These species of macrobenthos could prey on polyps of Nemopilema nomurai,Aurelia coerulea,and Rhopilema esculentum.However,the predation rate on the polyps differred among polyps of different species,the predation rate was positively correlated with the body length of the predators.The polyps might be rarely found in the sea areas dominated by sea slug and anemones,it was more likely to exist in the sea areas dominated by other species,such as crustaceans,echinoderms and gastropods.This might provide theoretical basis for understanding the relationship between the jellyfish bloom and the species and quantity changes of benthos,and for the formulation of prevention and control strategies for jellyfish bloom.The population size of polyps was affected by the biology control of the benthos community,and also depended on the specific community structure characteristics of benthos in different sea areas.The stage of ephyrae is the transition stage of asexual reproduction and sexual reproduction.The newly released ephyrae always sinked to the bottom of the ocean and contact with the benthic community preferentially,and this phenomenon was confirmed by indoor observation experiments.On the other hand,previous studies showed that the mortality of newly released ephyrae was extremely high,and the reason was not clear.In this paper,a preliminary study was conducted to explore the predation of macrobenthos on the ephyrea.The results showed that Charybdis bimaculata,Jonas distincta,Erisphex pottii and Paracalliactis sinica were able to prey on the ephyrae of Aurelia coerulea,and the predation rate was positively correlated with the body length of the predators.The downward biological control of benthos on jellyfish ephyrae might be an important biological factor leading to high mortality of jellyfish ephyrae.In recent years,the zoobenthos community in the coastal waters of China has been declining,which might reduce the downward biological control of the jellyfish ephyrae,and therefor provide favorable conditions for the outbreak of jellyfish.The gradual restoration of zoobenthos community structure may be very important in controlling the outbreak of jellyfish.The number of jellyfish was affected by the competition and predation of fishery organisms.To evaluate the relationship between jellyfish numbers and fishery organisms,it was necessary to analyze the variation characteristics of jellyfish population and the community structure characteristics of fishery organisms.Take the Yangtze Estuary as an example,The data of fishery resources species collected from bottom trawl surveys from the Yangtze River estuary and its adjacent waters in June,2015 was used to analyze the composition,dominant species,community diversity characteristics and community structure.Previous researchs found that silvery pomfret,Navodon septentrionalis,Stephanolepis cirrhifer,pagrus sp.,and salmon were important predators of jellyfish.Further research should focus on exploring the relationship between the changes of species,population size,temporal and spatial distribution of these key fish and the population dynamics of jellyfish.