The Gonad Maturation and Size Structure of the Population of Abundant Planktonic Predator Eukrohnia hamata (Mobius, 1875) (Chaetognatha) in the Eurasian Basin of the Arctic Ocean in Summer

The size structure of the population, morphology of the reproductive system, and gonad maturation of the chaetognath Eukrohnia hamata (Mobius, 1875) (Chaetognatha) were studied for the first time from zooplankton samples collected in Fram Strait and the deep Eurasian basin of the Arctic Ocean north of the Kara and Laptev seas in the summers of 1995 and 1997. We describe the gonad morphology and the progress of gonad maturation in male and female specimens and propose a system for classification of gonad maturity stages. During the Arctic summer, the population was represented by individuals ranging in size from 2 to 37 mm. The newly hatched young (2.0-3.5 mm long) and juveniles (4.0-12.5 mm long) dominated numerically (30 and 50% of the population, respectively). Our results indicate that E. hamata successfully matures and reproduces in the Arctic basin, but mature individuals make up less than 1% of the population in the summer against the background of juvenile abundance.