Gamete ultrastructure in the arctic holothurian Molpadia borealis M. Sars, 1859 (Holothuroidea: Molpadiidae)

This study examines the gonad condition and gamete morphology of the Arctic deep-sea holothurian Molpadia borealis M. Sars, 1859 (Molpadiidae) collected from the Kara Sea in September 2006. The intensive process of gametogenesis observed in the holothurian gonads was close to completion, suggesting upcoming spawning. The sperm ultrastructure in M. borealis is similar to that in most Holothuroidea. This species has classic flagellated sperm, echinosperm, which are typical of animals with external insemination. The sperm head has a 2.5-??m spherical nucleus with a proximal acrosome consisting of a spherical acrosomal vesicle surrounded by periacrosomal material. A single circular mitochondrion located in the sperm mid-piece surrounds the proximal and distal centrioles, which are arranged at an obtuse angle to each other. The eggs of M. borealis are approximately 300 ??m in diameter. This suggests indirect development with a planktotrophic larva. This type of development of M. borealis is probably related to life in high latitudes in the deep sea.