| 1. Effects of delayed first feeding on larval growth，survival and development of seacucumber （Apostichopus japonicus）The effects of delayed first-feeding by the sea cucumber Apostichopus japonicus larvaewere investigated. Groups of larvae were fed immediately after completion of the digestivetract （fed control） or starved for24,48,72,108, and120h. The growth rates of A. japonicuslarvae were the same as those of the control group when first feeding was delayed for lessthan72h. When the period of starvation was beyond72h, growth rates were reduced. The fedcontrol group had the highest percentage of late auricularia larvae （70.0%） on day11,followed by the groups starved for24,48, and72h respectively, which had35.00%,43.48%,and41.67%of late auricularia larvae. When larvae were starved for more than72h, thepercentage of late auricularia larvae never exceeded12.0%. Most larvae halted indevelopment and were deformed before reaching metamorphosis when starved for108and120h. Prolonged exposure to starvation （more than48h） resulted in a significant decrease inpercentage of larvae reaching settlement. The point-of-no-return （PNR） of A. japonicus larvaewas calculated110.4h. Ratios of stomach length to total length, stomach width to total widthwere more sensitive to starvation than other larval morphometrics. To avoid mortality andobtain adequate growth and development, A. japonicus larvae in commercial culture shouldbe fed within48h of the completion of the digestive tract.2. Effects of temporary starvation on larval growth, survival and development of seacucumber A. japonicusThe larvae of the sea cucumber A. japonicus were exposed to different feeding regimesto evaluate the effects of temporary starvation on growth, survival and development. The totalnumber of days of starvation was2,4and6days, and each had two different timings forstarvation. In all the treatments, the growth rates of larvae were not significantly affected bytemporary starvation up to day4, while they were significantly influenced from day6to14after hatching. For larvae that had been starved for2and4days, the percentages of late auricularia were23.33%,36.67%,30.00%, and30.00%, which were higher than those oflarvae starved for6days, which were16.67%and6.67%. For the starved group, larvae neverdeveloped beyond the mid-auricularia stage, having43.33%of early auricularia and56.67%of mid-auricularia. The survival rate of starved larvae decreased steadily and100%mortalitywas observed up to day14. On day14, significantly lower survival rates of7.78%and28.89%were observed when larvae were starved for6days. To achieve high growth rates andfast development of auricularia, and maximize larval survival in hatcheries, starvation of6days or more should be avoided at the early stage of development of the sea cucumber A.japonicus.3．The effect of food availability on phenotypic plasticity in larvae of sea cucumber A.japonicusThree food treatments （low, medium and high） were used aiming to investigate potentialfor phenotypic plasticity in the sea cucumber A. japonicus. On day5and7after first feeding,the significant delayed development of larvae occurred in2,000cells mL-1day-1. Principalcomponent analysis indicated that larvae in the low food group differed morphologicallyfrom others mainly due to long lateral maximum and depth on day5, but no morphologicallydistinct on day10. The results presumably indicated that early-stage larvae responded toscarce food by development of undulating profiles and long sinuous bands, and thusincreased the surface area of ciliated band for feeding at the expense of the delayeddevelopment of larvae. No significant difference in larval stages was observed by day10, but60%of total larvae in15,000cells mL-1day-1had greater than two hyaline spheres comparedwith20%and30%in2,000and30,000cells mL-1day-1. As the presence of hyaline spheres isindicative of metamorphic competence for sea cucumber, it is suggested that A. japonicuslarvae in commercial culture should be fed15,000cells mL-1per day.4. Effect of chemical cues on settlement of larvae of the sea cucumber A. japonicusThe effects of four ions and eight neuroactive compounds on inducing larval settlementof A. japonicus were assessed in the present study. There were significant inductive effects ofK+（10mM）, NH4+（0.1mM）, GABA (10-3M), acetylcholine (10-5M), L-DOPA (10-5M),norepinephrine (10-5M) and dopamine (10-7M and10-5M) on larval settlement of the seacucumber. L-DOPA and dopamine are the most efficient chemical cues to induce A. japonicuslarvae to settle. The highest percentage of larval settlement was induced by10-5M L-DOPA and dopamine （33%and40%） compared to the control （7%）. However, Ca2+, Mg2+, choline,serotonin, and epinephrine were less effective on larval settlement in all tested concentrations.This study evaluates the stability and feasibility of chemical stimulus for settlement indifferent culture systems, which can be applied to improve the hatchery production of thisvaluable species.5. Comparative mitochondrial genomics within sea cucumber （A. japonicus）: Providenew insights into relationships among color variantsTo investigate genetic relationships among three color variants （Red, Green and Black） ofsea cucumber A. japonicus, complete mitochondrial （mt） genomes of two Red and one Greenindividuals of A. japonicus were sequenced and compared with those of other eightindividuals （two Red, two Black and four Green） from GenBank. All the eleven A. japonicusmitogenomes share an identical gene complement, consisting of13protein-coding genes,22tRNA genes and2rRNA genes, arranged in the same order. Except for ND6and5tRNAgenes （tRNA-Ser （S2）, tRNA-Gln, tRNA-Ala, tRNA-Val and tRNA-Asp） encoded on theL-strand, all genes are encoded on the H-strand. Comparative genomic analyses reveal thatnon-coding intergenic regions have the highest mutation rate （7.36%of sites were variable）,while tRNA genes have the lowest mutation rates （0.99%）. Three hundred and eighty-sixsingle-nucleotide polymorphism sites are observed in protein-coding genes across the elevenmt sequences. The overall mean genetic distance for the complete mtDNA of the elevenindividuals is0.76%（p-distance）. For all protein-coding genes, the pairwise p-distancesamong the eleven A. japonicus range from0.04%to1.55%. As the consequences of extremelylow levels of divergence, they strongly support that the assertion that the three color variantsof sea cucumber belong to a single species A. japonicus.6. Embryonic, larval development and juvenile growth in hybrid between Chinese （♂）and Japanese red （♀） sea cucumber, A. japonicusIntraspecific hybridization was carried out between Chinese sea cucumber （♂） andJapanese red sea cucumber （♀）, A. japonicus. The embryonic development of hybrids wasobserved and the morphological characteristics of each period were described detailedly.Moreover, the growth rates and survival between hybrid juveniles and inbred Chinesejuveniles were compared. At20-22°C, the hybrid larvae required24h38min to develop toearly auricularia,72h8min to develop to mid-auricularia, and167h8min to develop to lateauricularia. Furthermore, the hybrid larvae required9days to develop to doliolaria. During the late stage of doliolaria, with the loss of cilia band, the larvae sank and became to settle onthe bottom. From38to51days after hatching, with the average water temperature of25.17°C,specific growth rate for weight （SGR） in the hybrid juveniles was2.58%, while inbredjuveniles grew at slightly higher SGR （3.03%）. When the average water temperature rose to27.17°C from51to80days, despite the growth rate of hybrid juveniles slowed down withSGR decreased by0.66%, the average weight increased to0.34±0.035g on80d afterhatching. However, with the decreased food intake of inbred juveniles, the SGR droppedsharply to a extremely low value （0.08%）, with the average weight of0.19±0.034g on80d.There was no significant difference in survival between hybrid and inbred juveniles （82.2%and73.9%）. This indicated that the advantages of growth characteristics for the hybridjuveniles in the higher water temperature.7. Effects of salinity and stocking density on growth, survival and settlement of thehybrid sea cucumber larvaeThe effects of different salinities （15,20,25,30,35and40‰） and stocking densities （0.1,0.2,0.5,0.8and1.0ind/ml） on growth, survival and settlement of the hybrid larvae betweenChinese sea cucumber and Japanese red sea cucumber （A. japonicus） were investigated in thisstudy. The salinity treatments show that the highest larval survival rates （46.7%and40.0%）occurred in salinities of30and35‰on day5after hatching. From day2to5, the highestgrowth rate was observed in a salinity of30‰, reaching the maximum body length of672.18μm on day5. The highest settlement rate （17.8%） was observed in a salinity of30‰, whilethere was no settled juvenile in salinities of15and20‰. For stocking density treatments, thehighest survival rate （66.7%） occurred in a density of0.1ind/ml on day5, while nosignificant difference in survival rate was detected among the different stocking densities.Larvae in a density of0.5ind/ml grew at the highest rate, reaching the maximum body lengthof801.38μm on day10. The highest settlement rate （19.1%） was detected in0.5ind/ml group,in which the maximum number of juveniles was produced. To maximize the seed productionin the commercial industry, a salinity of30‰and a stocking density of0.5ind/ml are optimalfor culturing the hybrid larvae. |
PDF Full Text Request