Photoreceptor ultrastructure in the Antarctic mussel shrimp Acetabulastoma(Crustacea : Ostracoda), a parasite of Glyptonotus antarcticus (Crustacea : Isopoda)

The ultrastructure of the nauplius eye of the tiny Antarctic ostracode Acet abulastoma sp. is described and conclusions about its possible function are drawn. Each of the three eye-cups measures approximately 20 mu m in diamet er and is optically isolated from its neighbour by screening pigments, whic h are contained in pigment cells behind a tapetum of concentrically arrange d, ca. 1-mu m-long and 0.1-mu m-thick, crystals. Three and sometimes four s eparate rhabdoms with microvilli measuring 50-60 nm in diameter project fro m the concave side of the tapetum up to 5 mu m deep into the eyecup interio r, which is filled by the retinula cell bodies with their spherical nuclei and various organelles. Desmosomes and microtubules are seen and light-indu ced cell or membrane damage was minimal. The observations suggest that the Acetabulastoma eye has photoreceptors that can tolerate an exposure to brig ht light and it may be used to inform its owner of the approach of danger, the depth of water, and/or the season.