Feeding and growth of the tonguefish Cynoglossus semilaevis larvae

Experiments were performed in the Shandong Laizhou Mingbo Fish Farm from 8 th October to 1 st November,2003.Adult tonguefish Cynoglossus semilaevis,caught in the Laizhou Bay of the Bohai Sea,were domesticated in indoor concrete ponds.After artificial inducement by controlling such environmental factors as temperature and illumination,gonads of the adult fish reached full development.At 11 p.m., 8th October,the female ovulated,while the male ejaculated in the same pond and completed fertilization.Until midday(11 a.m.to 01 p.m.)on 9th October,160 thousand fertilized eggs were collected and transferred to another pond for incubation.The hatchery pond was maintained at temperatures of 23.6-24.0℃,salinity 32.00and pH 8.0.At 8 a.m.on 10th October,all the collected eggs had hatched.At 3 p.m.on 11th October,about 140 thousand larvae were obtained for both seedling rearing and starvation tests.;The newly-hatched larvae were collected,placed and cultivated in a round concrete pond with a bottom area of 10 m2 and 1.5 m in depth.Water exchange volume was daily maintained in proportion of 20% to 30% of the total volume.The larvae were fed rotifers Brachionus plicatilis which were nourished with unicellular algae(Chlorella sp.)and rationed at a food density from 5 ind./ml(in the third day after hatched-out)to 10 ind./ml(fourth day after hatching)according to feeding status.The 15-day-old post-larvae were fed nauplii of brine shrimp Artemia salina at a food density of 2-3 ind./ml,while,the density of unicellular algae was maintained at 10×104 cell/ml.Since the 15th day after hatching,the density of unicellular algae was gradually decreased to 5×104 cell/ml.Seawater came from the natural seawaters after settlement and filtration.Normally,changing or adding water was conducted in the morning.Foods were supplemented twice a day,i.e.,one after changing water every morning and at 5 p.m.;Some 800 larvae were randomly dredged and placed in a blue,round plastic container with a volume of 25 L for a starvation test.During the test,the larvae were withheld until all the tested animals died of starvation.Filtered seawater was re-filtered through absorbent cotton to use in the starvation test.During the experiment,mortality was observed and one-third seawater was changed at 8 p.m daily.As usual,gentle aeration was continuously supplied.On cultivating unfed fish,the water temperatures were kept at 23.4-24.0℃.A salinity of 32.00 and the pH value at 8.0 were controlled.Dissolved oxygen was maintained at 8.80-12.00 mg/L,while luminance was controlled at 10-200 lux for daytime and less than one lux at night.; The initially feeding rate of larvae was observed by the feeding test.From the fourth day after hatching,20 starved larvae were randomly selected and transferred to a 400 ml beaker at 8 p.m.daily.These starved larvae were fed rotifers(with a food density of 20 ind./ml)and kept in the cultivated temperature for two hours.Afterwards,the tested larvae were picked up one by one.After anaesthesia with benzocaine(200 μg/ml),each of the anesthetized animals was observed under a stereo microscope to check its feeding status and morphologic characteristics and to measure its body length.Fish containing rotifers in their intestines were regarded as feeding individuals.The initially feeding rate was calculated as the ratio of the numbers of feeding individuals to the total numbers of observed individuals.; The point of no return(PNR)is defined as a critical time at which the initially preying larvae can tolerate starvation.The PNR was measured as follows:the initially feeding rate of larvae was daily measured and the maximum feeding rate was estimated.It was PNR when the initially feeding rate decreases to half of the maximum feeding rate,namely,50% of the starved larvae were too weak to re-gain taking food.Therefore,PNR was represented as the number of days(day-old)after hatched out.;The volume of the larva's yolk sac was calculated as:4/3π·R/2·(r/2)2,where,R is the long diameter of yolk sac,r is short diameter of yolk sac.The structural characteristics of pectoral angle was observed under stereo microscope and photographed by using digit camera.; As observed,the total length and body length of the newly hatched tonguefish larvae cultivated under such conditions as water temperature ranged from 23.4 to 24.0℃,at 33.00 salinity and pH valued 7.78-8.02 were 1.31±0.049 mm and 0.94±0.066 mm,respectively.The long and short diameters of the yolk sac in newly hatched larvae were 1.31±0.049 mm and 0.94±0.066 mm with a capacity of some 0.606 mm3,respectively.2-day-old larvae developed a cruising swimming mode.2.5-day-old larvae started exogenous feeding.3-day-old larvae established exogenous nutrition relationships.The feeding rate of 4-day-old post-larva reached 100%.Here,post-larvae completed the conversion from endogenous to exogenous nutrition.The yolk sac of 5-day-old post-larvae was almost consumed and become a small oil globule with a volume of 0.002 mm3.Therefore,it can be estimated that the period of the mixed nutrition(endogenous and exogenous nutrition)lasted for 2.5 days.Hereafter,the feeding intensity of post-larvae gradually increased while the volume of the residual oil globules dwindled and vanished.But,there still were 40% of 21-day-old juveniles containing some tiny residual oil globules with an average volume of 0.0000005±0.000003 mm3.The total length and body length of 21-day-old juveniles were 12.96±0.611 mm and 12.65±0.591 mm,respectively.Length increments averaged 0.45 mm/d.Here,the relationships between increment in length(TL)and age in days(D)was described as:TL(mm)=0.0026D3-0.0704D2+0.7993D+3.55(R2=0.9811,n=324).; In the starvation tests,negative growth of the starved fish was not obvious.The 10th day after hatched is the critical time for a 9-day-old larva to resist starving.The preying capabilities of the starved larvae could stand for 6 days after feed was withheld.The period of no return from re-gaining food was about three days.; Long-term existence of oil globules in the early life stages of the tonguefish,to some extent,acts as parts of endogenous nutrition to supplement exogenous feeding,which may play an important role in decreasing larva mortality.Pectoral angle is one of the obvious morphological characteristics in the early life stages of the tonguefish C.semilaevis.The pectoral angles of the starved larvae and those larvae at PNR were much more obvious and keen-edged.Therefore,pectoral angles cannot be taken as the exclusive feature to identify the difference between the healthy and starved C.semilaevis larvae.Further studies are required on how to discriminate ill-fed and healthy larvae of C.semilaevis.