Brooding strategies in solitary ascidians: Corella species from north and south temperate waters

Abstract

Corella inflata from the northwest Pacific coast of North America and Corella eumyota from southern New Zealand are both ovoviviparous solitary ascidians with very different methods of brood retention. Corella inflata eggs are covered with large follicle cells that are responsible for flotation. The floating eggs are trapped in the upwardly situated atrial chamber, where development ensues. Corella eumyota eggs have small follicle cells and do not float; the follicle cells produce a viscous glue that causes the eggs and embryos to stick to each other and to the right atrium. The follicle cells are not sticky when removed from the ovary but become so in about 15 min. In C. inflata, development to hatching requires 24–26 h at 13 °C and metamorphosis begins 17 h later. Corella eumyota tadpoles hatch at about 25 h at 15 °C and begin metamorphosis 11 h later. Both species retain the tadpoles well beyond hatching and release tadpoles fully competent to metamorphose. In C. inflata the tadpoles swim down and out of the atrium; C. eumyota tadpoles somehow break free of the glue-encrusted vitelline coat and swim out the atrial siphon. Tadpoles of both species swim vigorously when released and then rapidly metamorphose. Most tadpoles metamorphose in 15–20 min. Since both species spawn around dawn, tadpoles are released during the night.