A Study On The Behavior Characteristics Of The Early Developmental Stages Of Rapana Venosa

Rapana venosa is an important economic shellfish in China,but it is an invasive species in the Black Sea,Adriatic Sea and Aegean Sea,and seriously destroys the local bivalve shellfish.In the life cycle,zooplankton larvae need to undergo settlement and metamorphosis into juvenile snails,and juvenile snails become adult snails after growth and development.However,in the process of settlement and metamorphosis of zooplankton larvae,the larvae show high mortality rate and low rate of metamorphosis,as well as the high mortality rate in the growth process of juvenile snails.These problems are directly related to the resource change of shellfish population.The artificial breeding process of R.venosa was seriously restricted.There are many behaviors in larval settlement and metamorphosis and early development of juvenile snail,which is closely related to the success of settlement and metamorphosis and the high mortality rate of juvenile snail.The behavior of the snail refers to the adaptive response of the snail to the changes of the external environment or its own physiological condition at the individual level.Most of the behaviors are dynamic,and it is of great significance to the survival of the snail.The object of the study is the different behaviors and the laws behind the survival of the snail.In this study,the settlement and metamorphosis behavior,cannibalistic behavior,perceptual behavior and burrowing behavior of juvenile snails,as well as the radula related to these behaviors,were studied.The effects of light,oyster polysaccharide and different shellfish on the settlement and metamorphosis of the snail were studied in this paper and the generation period and morphological characteristics of the radula were studied.The problem of low rate of metamorphosis in the breeding process was solved;the effects of cannibalistic mode,behavior characteristics,feed,size and density of juvenile snail on its cannibalistic behavior were studied,and the methods to solve the cannibalism were found;the perceptual ability of juvenile snail to food in static water and different water flow environment was studied;the effects of temperature,dissolved oxygen,particle size of substrate and size of juvenile snail on the burrowing behavior of juvenile snail were studied,and the survival rate of juvenile snail was significantly increased.The study on the early development stages of R.venosa has important theoretical and practical significance for elucidating the population change and the recovery of resources and improving the efficiency of seedling raising.The main findings are as follows:1.The settlement and metamorphosis behaviorThe behavior characteristics of a settlement and metamorphosis of zooplankton larvae of R.venosa were observed,and the effects of attachment bases,light,food and oyster polysaccharide on the settlement and metamorphosis of larvae were studied.The development and morphological structure of radula were studied.The results showed that scallop shells with oyster juveniles could significantly increase the rate of larval settlement and metamorphosis in the middle and late stage of the middle and late 4 spiral whorls,and the dark environment was beneficial to larval metamorphosis.Shellfish and their exudates can significantly induce larval settlement and metamorphosis,and suitable concentration of oyster polysaccharide can induce larval settlement and metamorphosis.Radula originated in the stage of 4 spiral whorls,in preparation for drilling holes to feed shellfish when larvae settlement and metamorphosis.Each transverse row of the radula band has three pieces of tongue,including one central tooth(tridentate,central tip protruding);one pair of lateral teeth(sickle-shaped);no marginal tooth,teeth type is 1:1:1.2.Cannibalistic behaviorThe characteristics of cannibalistic behavior,the influencing factors and the methods to solve cannibalistic behavior of juvenile snail were studied.Two ways of cannibalism of juvenile snails were found: 1.5-5mm through hole-drilling,and 2.5-10 mm through hole-drilling(86.4%)and enveloping(13.6%).Finded out the relationship between cannibalistic methods and the shell length ratio of prey to predator.It was found that there were four different drilling locations in juvenile snails.The ratio of shell length between predator and prey affected the selection of drilling locations.Different drilling patterns,such as complete hole-drilling,incomplete hole-drilling and multiple hole-drilling,were found.Small size,starvation and high density of juvenile snails significantly increased the cannibalism rate of juvenile snails.We found effective ways to solve the cannibalism: timely feeding of bait and controlling the cultivation density are important ways to reduce cannibalism.The shell of juvenile clams such as Ruditapes philippinarum and Meretrix vulgaris are thin and suitable for bait.The small size of juvenile snails are prone to cannibalism,so they should be fed timely after metamorphosis.3.Perceptual behaviorThe perception of prey of the juvenile snail in static water and flowing water was studied.It was found that in still water environment,distance significantly reduced the perception ability of juveniles to prey.In upstream prey experiment,it was found that flow could significantly enhance the perception ability of juveniles,and the effect of water flow eliminated the influence of distance.In downstream prey experiment,it was found that juveniles were difficult to perceive prey.Results indicate that juveniles use chemical cues released by prey to perceive and locate their prey.At the same time,the time of hunting for prey by juveniles is also affected by water flow.The relationship between the time of hunting is as follows: upstream prey experiment < still water < downstream prey experiment.Different water velocity did not significantly affect the perception behavior of juveniles,but the smaller size of juveniles had higher perception rate and shorter perception time.4.Burrowing behaviorJuvenile snails capture food and avoid enemies through burrowing sand.The effects of temperature,dissolved oxygen,sediment size and size of juvenile snails on burrowing behavior were studied.The results showed that burrowing rate of juvenile snails increased significantly with the decrease of temperature;dissolved oxygen significantly affected burrowing behavior of juveniles.When dissolved oxygen was higher(>6mg/L),the juveniles had obvious burrowing behavior,the dissolved oxygen decreased,the juveniles would reduce burrowing behavior.Substrate size also affects the burrowing behavior of juveniles,and the shortest burrowing time of juveniles is in the small size sediment.The burrowing behavior of juvenile snails with different sizes is different,and large-size juveniles have strong burrowing capacity.It was found that small-sized snails(shell height < 2cm)exhibited obvious shelling behavior when water temperature was below 15 C.Small-sized snails could reduce shelling rate by submerging in sand.