한국 근해 난바다곤쟁이의 분포와 Euphausia pacifica의 개체군 구조 및 유전학적 분석

Abstract

The distribution and abundance of euphausiids were investigated in Korean waters in 2016. Genetic analysis was conducted at three stations in the middle of the water masses which are clustered by temperature and salinity, plus two stations at the boundaries of water masses. Euphausiids were collected using a Bongo net (80 cm mouth diameter, 330 ㎛ mesh size). A Bongo net was obliquely towed at a speed of about 2 knots, both day and night. When the water depth was shallower than 200 m, a Bongo net was towed from near bottom to surface, while the water depth was deeper than 200 m, it was towed from approximate 200 m depths to the surface. Water temperature and salinity were measured during the sampling using a CTD. Water masses were grouped based on the water column averaged temperature and salinity. Water masses were divided into three groups according to the analysis of the Nb cluster function. The group W includes the Yellow Sea. The group S contains the East China Sea, and the eastern part of the northern East China Sea. The group E comprises the East Sea. Mean water temperature ranged 8.14-15.32ºC in the group W, highest in October and lowest in March. Mean water salinity ranged 32.22-32.76 psu in the group W, highest in March and lowest in November. The group W had a tendency of low salinity and formed a thermocline in summer. Mean water temperature ranged 13.45-20.27 ºC in the group S, highest in September and lowest in February. Mean water salinity ranged 33.77-34.23 psu in the group W, highest in March and lowest in October. The group S showed high temperature and salinity. Mean water temperature ranged 5.23-10.86 ºC in the group E, highest in November and lowest in February. Mean water salinity had higher temperature and lower salinity than other groups. Euphausiid species of the group W were Euphausia nana, E. pacifica, Pseudeuphausia latifrons and P. sinica. Among euphausiid species, E. pacifica was the most dominant species with the highest density in September. E. pacifica density was negatively correlated with mean water temperature. E. pacifica was distributed at the bottom cold water in the group W during summer when a thermocline was formed. In the group S, Euphausia gibboides, E. nana, E. pacifica, P. latifrons, P. sinica, and Stylocheiron affine occurred. E. pacifica and E. nana dominated in the group S. Although E. nana density was not correlated with mean water temperature and mean water salinity, E. nana was found in the group S where the warm and saline Tsushima Warm Current, a branch of Kuroshio Current, is influential. E. pacifica, P. latifrons, S. affine, Stylocheiron cariatum, and T. longipes occurred in the group E. E. pacifica and T. longipes were dominated in the group E. E. pacifia and T. longipes were distributed in the deep and cold group E, because they prefer low water temperature and perform vertical migration. The geographic distribution of euphausiids is determined by characteristics of water masses such as temperature and salinity. There were differences among euphausiid populations of the water mass groups. E. pacifica was dominant species in all water mass groups. Both female and male occurred throughout the sampling periods. Female length ranged 7.1-20.5 mm. Male length ranged 7.4-19.3 mm. Females were larger than males in three groups. The average of sex ratio of females to males was 2.14: 1 in the group W and 1. 41: 1 in the group S. Sex ratio was 1.84:1 in the group E. Ratio of female was higher than male in every group. Eggs and females with spermatophores, which are considered to be index of recent spawning, appeared in the groups W and S from February to March, and September to November. Three cohorts occurred in February and March, while one or two for the other months. This result suggests that E. pacifica of the groups W and S spawns once a year. Three modes occurred except in February and June in the group E. Eggs and females with spermatophores occurred in the group E throughout the sampling periods. E. pacifica of the group E spawns year-round but main spawning peaks were assumed to be twice a year. First spawning was from February to March and second spawning was from September to November. A phylogenetic analysis of E. pacifica using mtDNA COI sequences was conducted in five sampling stations. The sequences were amplified to 566bp. Sequences were compared with euphausiid species of five outgroups. Sequences were classified by the groups, and a clade was formed in the group E. Genetic distance was close to E. pacifica in Bering Sea, while E. pacifica in Yellow Sea near China was distant. In genetic analysis, seven haplotypes were formed. Among haplotypes, Hap-1 and Hap-2 were shared in five groups, while Hap-3 was shared in the groups W and WS. Four independent haplotypes appeared only in the group E. Diversity of haplotype and nucleotide were highest in the group E and lowest the group S. The Fst distances between the group E and other the groups were highest, but distances among other the groups were low. The distribution of Euphausiids is affected to the water masses. Also E. pacifica, which is distributed along the water mass, has a different population. E. pacifica, which evenly distributed in Korea waters, was divided into two populations according to ecological and genetic analysis. One population was in the groups W and S, and the other was in the group E. Therefore, this paper provides basic data for ecological information and population structure of E. pacifica in the Korea waters.