Exploration Of The Over-summering Mechanism Of Calanus Sinicus In The Yellow Sea: Effects Of Temperature And Food

Calanus sinicus is the dominant species of zooplankton in the Yellow Sea and East China Sea,which plays a key role in marine ecosystems of the Yellow Sea and East China Sea.The population dynamics of C.sinicus could affect the structure and function of marine ecosystem.In summer,C.sinicus over-summers inside the Yellow Sea Cold Water Mass(YSCWM)to conserve population abundance.However,the mechanisms of initiation and termination of over-summering are still unknown.Here,we conducted laboratory controlled experiments and field studies to explore the mechanism of over-summering and the effects of temperature and food.The lipid reserve in oil sacs is the main energy resource for over-summering C.sinicus to survive.The lipid accumulation of C5 copepodids(C5s)would affect the success of the over-summering.Given the temperature and food conditions in the YSCWM in late spring,we systematically studied the effects of temperature(e.g.constant temperature and temperature difference),food concentration and food type(diatom and natural diets)on the lipid accumulation of C.sinicus.The lipid was accumulated most in C5 s,and then was consumed as gonad developed.At low temperatures,C.sinicus stored more lipid than at high temperatures by reducing the energy cost at colder temperatures.Larger diurnal temperature differences(10–16 and 10–19 °C)could promote the lipid accumulation,and the oil sac proportions(OSV%)of C5 s were 1.8–6.1 time of those at 16 or 19 °C.C5 s accumulated more lipid as the food concentration increased.Diatoms were the main food resource for lipid accumulation in C5 s.We compared C.sinicus captured insisde and outside the YSCWM in late spring,2014.In late spring,the Chl a concentration was lower inside the YSCWM.However,the C5 s inside the YSCWM had 32% lower metabolic rates and 1.4 time larger oil sacs than those outside.All the C5 s inside the YSCWM were in pre-apolysis,indicating inactive molting development.Therefore,the C5 s inside the YSCWM might suppress molting,reduce metabolic activity,and assign most energy to lipid accumulation other than molting.In addition,the OSV% of C5 s in the bottom of YSCWM were 1.9—4.2 times of those in the surface layers.Thus,wih enough lipid accumulation,C.sinicus preferred to stay inside the bottom of the YSCWM.In this study,we compared the metabolic rates,morphological characteristics and relative gene expressions of C.sinicus captured both inside and outside the YSCWM in Aug,2013.With large oil sacs,low metabolic rates and suppressed molting development,the C5 s inside the YSCWM were probably quiescent.When quiescent,the gene associated with molting(ecdysteroid receptor,EcR)was down-regulated,while genes associated with stress tolerance(ferritin)were up-regulated.Females inside the YSCWM were active for higher metabolic rates.C5 s at the margin of the YSCWM up-regulated EcR expression,which could be a response to the elevated Chl a concentration,suggesting that improved food condition may serve as a trigger that terminated the over-summering of C.sinicus in the Yellow Sea.We examined the effects of temperatures and food on metabolic rates,morphological characteristics,and relative gene expressions of six genes involved in molting,gonad development,lipid catabolism and stress tolerance processes of C.sinicus during the termination of over-summering and subsequent development.Both elevated temperature and external food supply rapidly ended over-summering of C.sinicus,accompanied by up-regulation of EcR expression on the 1st day and increased metabolic rates on the 3rd day.The improved temperature or food conditions could ensure the success of molting.Ferritin was sensitive to the food condition and was down-regulated only when copepods fed.During the subsequent development,the lipid reserved in oil sacs could support molting and early gonad development.When fed with food,C.sinicus up-regulated VgR(vitellogenin receptor)expression,and the gonad got mature thereafter.Thus,the environmental changes would induce the termination of over-summering.In addition,food played an indispensable role in population recruitment after the termination of over-summering,whereas the elevated temperature just accelerated these physiological processes.In autumn,the metabolic rates of C5 s were similar to those in spring inside the YSCWM,while the OSV% reduced a lot.Meanwhile,the proportion of adults inside the YSCWM increased,and ~6.7% of C5 s were in post-apolysis,suggesting the termination of over-summering and beginning of ecdysis.Both the improved environmental conditions and lipid consumption would trigger the termination of oversummering.