Development of the brittle star Ophiothrix exigua Lyman, 1874 a species that bypasses early unique and typical planktotrophic ophiopluteus stages

Abstract

The development of Ophiothrix exigua from spawning through metamorphosis was studied by light and scanning electron microscopy. When some ripe males and females were kept in the same container and subjected to cycles of temperature and light shock, males released sperm, and then, females spawned eggs. After release, spermatozoa survived for approximately 28 h at 24 °C. After fertilization, the egg surface changed from smooth to wrinkled, and then, the fertilized eggs produced polar bodies. After the wrinkles disappeared, the eggs started to cleave holoblastically and equally. The arrangement of blastomeres was irregular after the third cleavage. The embryonic surface developed a thick hyaline layer, and blastomeres were connected to each other by projections and filopodia. After the wrinkled blastula stage, embryos hatched and primary mesenchyme cell ingression occurred. At the initiation of gastrulation, embryos formed tetraradiate spicules and then became typical planktotrophic ophioplutei with four pairs of larval arms. The larvae developed coelomic pouches on their left and right sides, and the pouches divided into anterior and posterior parts. After the left anterior pouch formed a hydropolic lobe, the larvae resorbed all inner larval arms except for the posterolateral arms and then formed an adult rudiment. Finally, juveniles released the pair of posterolateral arms and completed metamorphosis. Our findings indicate that larval development (typical planktotrophic ophioplutei until metamorphosis) of O. exigua is similar to that of other Ophiothrix species, but early development is modified involving the formation of the wrinkled eggs and blastulae.