Aidanosagitta crassa(Tokioka,1938) is the only dominant chaetognatha species in Laizhou Bay,Bohai Sea.This gelatinous species plays an important role in the ecosystem of Laizhou Bay,acting as the major carnivorous predator of small invertebrate and fish larva.In addition,it has different morphs responding to variations in environmental factors.To determine the population structure in Laizhou Bay,the detailed body size and abundance of A.crassa were analyzed monthly from 147 vertical net samples collected from May 2011 to April 2012(excluding December 2011 and January and February 2012,because of the presence of sea ice).Results showed that A.crassa occurred continuously and was distributed extensively throughout the survey seasons and area in the bay.Both the abundance and biomass of the species presented two peaks,a considerable peak in May,and a second peak during August to September.Individuals with a body length of about 4 and 15 mm were the major contributors to the total abundance and biomass,respectively.The production of the A.crassa population was estimated to be 0.25 mg C m^-3 d^-1,whereas the feeding rate was 0.88 mg C m^-3 d^-1 during the survey months.The body size of A.crassa fluctuated seasonally,with the largest sizes in March and April (mean body length > 10 mm),and the smallest in May and November (mean body length < 5 mm).The mean body length of the species showed an inverse correlation with both surface water temperature (SWT) and surface water salinity (SWS) of the corresponding period.Based on length-frequency distributions,different types of population pyramids occurred,namely,the expanding type in May to November except in June,stationary type in March to April,and constrictive type in June.The growing season for the A.crassa population in Laizhou Bay was presumed to be from May to August,because of the presence of abundant small individuals.Moreover,from the monthly length-frequency distributions,three common normalized cohorts (I,Ⅱ,Ⅲ) were detected with mean lengths of <5,5-10,and >10 mm,respectively.Regarding their spatial distributions,individuals of cohorts Ⅲ and I were the most abundant at sites with an SWT of 15-17℃ and SWS of 29-32 during spring.However,cohort Ⅲ showed a considerable negative correlation between its abundance and water temperature,with a limited abundance at warm sites with an SWT > 24℃.Additionally,cohort I showed a positive correlation between its abundance and water temperature,and showed a limited abundance at cold sites with an SWT < 11℃.Cohort Ⅱ was most abundant at sites with an SWT of 26-27℃ and SWS of 30-32 during autumn.From above the results,it was assumed that the water temperature threshold was lower than 15-17℃ for the transformation between the two typical size morphs,i.e.,the large winter and small summer forms of A.crassa.