Population structure, vertical distribution and fecundity of Eukrohnia hamata (Chaetognatha) in the Arctic Ocean during summer

Abstract The population size structure, vertical distribution, gonad maturation, individual fecundity and embryonic development of the chaetognath Eukrohnia hamata were studied in the northernmost part of its distribution area, the Arctic Ocean. Analysis was based on stratified zooplankton samples collected in the Canada Basin during July 2005 that covered the depth range from the surface down to the bottom (or 3000 m). E. hamata abundance averaged 1087 ind. m-2 (0.8% of zooplankton community), while their biomass averaged 379 mg DW m-2 (10% of community biomass). Individual length of E. hamata ranged from 2 to 41 mm. Younger juvenile specimens 20 mm contributed only 5%. Size of the mature specimens, carrying 2 ovoid egg-sacs within a marsupium, varied between 24 and 38 mm. E. hamata was encountered down to 1000 m depth, but was absent from deeper layers. Specimens with eggs or developing embryos in the marsupium, as well as the smallest young specimens of 2–3 mm, were found predominately between 300 and 750 m depths. Juveniles 4–6 mm and 7–12 mm long showed a clear preference for the depth layer between 100-500 m and 50–500 m, respectively, suggesting an upward ontogenetic migration of the growing specimens to the strata with richer prey densities. The peak of fully mature male, female and specimens with ruptured marsupial sacs within the lowermost portion of the species vertical range (500–750 m) indicates that the ascent of growing individuals is followed by the downward migration of mature ones for reproduction. The number of eggs and embryos in marsupial egg sacs of 63 mature specimens collected on six expeditions to the deep Arctic Ocean (1993–2016) averaged to 153 with a maximum of 266 ind-1. For the first time, embryonic development of E. hamata from the egg until young hatching from marsupial sacs is documented. Our data provide novel insights into E. hamata ecology and distribution in the deep Arctic Ocean.