Effect Of Diet And Salinity On The Reproduction And Resting Egg Hatching Of Rotifer Brachionus Plicatilis

Brachionus plicatilis is a major group of monogonta rotifer and plays an important role in the development of marine aquculture.Because of characteristics like strong adaptability,short generation time,rich nutrition,easy cultivability and slow swimming speed,rotifer(B.plicatilis)are widly used as first feed for most fish larvae,shrimp,and crab.The quality(nutrient value)and quantity of rotifers directly affect the survival and growth of aqucultural larvae.Therefore,culturing high-quality and high-density B.plicatilis has great significance to the flourishing development of aquatic industry.Rotifer B.plicatilis have a heterogenic life cycle including sexual(mictic)and asexual(amictic)reproduction.Resting eggs are the final product of sexual reproduction caused by harsh environment.The efficient utilization of rotifer also related to the higher quality and hatching of resting egg.Diet and salinity are the most important external fators that affect the population growth and sexual reproduction of rotifer.The nutritional value of B.plicatilis is commonly influenced by their diet.Different salinity promotes or inhibits the hatching of rotifer resting egg,but the molecular mechanism remains unclear in the current.Hence,this article focused on three parts: Firstly,Comparing the effects of different diets on the growth and reproduction of B.plicatilis species complex.Secondly,the influence of nutrient-enriched diet on the sexual reproduction efficiency of rotifers was studied in order to find high-quality bait for rotifers mass culture.Thirdly,exploring the effects of salinity on the hatching of resting eggs and the underlying molecular mechanism.1.Effects of two kinds of food species on the reproduction and hatching of resting egg of rotifer Brachionus plicatilis species complexTetraselmis tetrathele is an optimal diet to induce sexual reproduction of euryhaline rotifers.However,in terms of economic cost-effectiveness and deficiency nutrition,T.tetrathele is not a proper diet for the mass culture of high-quality rotifer.Hence,a commercial diet which has high nutrition and can increase the rotifer reproduction efficiency is required for the flourishing development of aquatic industry.This study investigated the efficiency of commercial Chlorella vulgaris in the mass culture of rotifer by comparing their reproduction and resting egg hatchability with T.tetrathele.The result shows that when the Japanese strain,Brachionus plicatilis sensu stricto fed with two different diets,the produced resting eggs showed higher hatching rate with C.vulgaris(70.0%±11.1%,p<0.05)than T.tetrathele(24.0%±10.8%).For the Australia strain,the hatchability of resting eggs is no significant difference between the two diets,while,Brachionus manjavacas showed higher population growth rate(r)with C.vulgaris(0.28±0.02,p<0.05)than with T.tetrathele(0.23±0.00).Hence,the commercial selenium enriched chlorella is beneficial to the population growth and sexual reproduction of Brachionus plicatilis species complex.Furthermore,the body length and width of rotifers fed with C.vulgaris diet were lower than with T.tetrathele,this result confirmed that the population density of rotifer influence its body size.Based on these results,this study indicates that the Chlorella diet is considered as an efficient candidate for T.tetrathele to improve the sexual reproduction efficiency of rotifer in the mass culture of B.plicatilis species complex.2.Usability of Se-fortified Chlorella diet for resting egg mass production and hatching in the rotifer Brachionus plicatilis sensu strictoThis study further tested hatchability of resting eggs produced by C.vulgaris.Through successive batch cultures of three generations,resting egg production,hatching as well as population growth of rotifers fed with different diet were compared.The result shows that the diet of C.vulgaris induced active asexual reproduction(r=0.28~0.31)in second and third generations originated from first generation cultured with Nannochloropsis oculata.In the sexual reproduction,the resting egg production was promoted with T.tetrathele for the second generation,while no differences were observed in third generations between two diets i.e.,T.tetrathele and C.vulgaris.The hatchability of the produced resting eggs was higher with C.vulgaris(74.0~81.0%)regardless of generations.These results prove the efficiency of C.vulgaris for mass production of resting eggs with high hatchability.3.Comparison of resting egg gene expression related to different salinity variations in marine rotifer Brachionus manjavacasSalinity is a significant factor affecting resting egg hatching in the euryhaline rotifers.In order to clarify the effects of salinity on resting egg hatchability,this study investigated gene expressions of resting eggs related to two different incubation salinities(17 and 33 ppt)with B.manjavacas.The resting eggs showed higher hatching rate at 17 ppt of incubation salinity,compared to those at 33 ppt.Related to these circumstance,the resting eggs incubated at 17 ppt expressed genes involved in cellular differentiation and embryonic development i.e.Late embryogenesis abundant protein(LEAs),α-amylase and deaminase.The resting eggs incubated at 33 ppt,expressed the transcripts related to energy metabolism,i.e.,ABC transporter permease,NAD+ synthase,Copper-translocating P-type ATPase,Enoyl-Co A hydratase,3-oxoacid Co A-transferase and Transcription factor with AP2 domain.It is speculated that these genes are highly expressed at 33 ppt,because the resting eggs may need a greater energy(ATP)to endure saline stress during incubation.The obtained results indicated that the resting eggs regulate their hatching with the mechanisms of energy allocation between embryo development and self-defense against environmental conditions like salinity stresses.