This paper considers features of the external morphology and internal structure of the genital system of adult males and females of Metridia longa Lubbock from the White Sea, as well as related with them features of reproduction biology of this mass Arctic copepod. Males of M. longa are dimorphically asymmetric. In the population, there are two morphotypes, which are mirror images of each other. Asymmetry manifests in them as the presence of a right (males “righties”) or left (males “lefties”) geniculate antenna, structure of the fifth pair of swimming legs, and location of an unpaired gonad and genital opening. The relative position of these struc? tures is linked together. In females, there are two widely spaced copulator pores that lead to the two spermathe? cae not connected to each other. The latter, in turn, are connected to two gonopores, through which shedding of eggs occurs. The vast majority of the females in the studied population M. longa in all seasons of the year had only one filled spermatheca, either the right or the left. The females with bilateral insemination also occurred, but did not exceed 2.7% of all specimens. The ratio of females with the right and left filled spermathecae and “right?handed” and “left?handed” males suggests that the male morphotype determines in which spermathecae male gonads are most likely to get into. The presence of females with two filled spermathecae implicates insem? ination by two males of different morphotypes. Since each spermatheca is connected to only one of the two ovi? ducts, it is assumed that a half of the eggs produced by unilateral inseminated females remains not fertilized. The morphology of genital structures and literature data on the egg production of M. longa indicate that almost a half of eggs produced by females is not viable and thus wasted.
