Complete development of the northeast Pacific arminacean nudibranch Janolus fuscus.

Early life-history stages of the nudibranch Janolus fuscus (superfamily Arminoidea) were reared in the laboratory and collected from the field to document embryology, larval morphology, and pre-and post-metamorphic growth. This species produces Type B egg masses containing an average of 23,063 viable embryos. The spiral holoblastic cleavage pattern resembled that of other nudibranchs, with early cell divisions occurring roughly at 4-h intervals at 11-13 °C. Embryos progress through a gastrula stage with cellular extensions covering the blastopore and a trochophore-like stage before hatching as veliger larvae after 10-18 days. Veligers ranged in size from 125-153.8 μm at hatching, with size being positively correlated with the duration of encapsulated development. After a 36-41-day larval period, some veligers ceased growing at 266 μm and showed signs of approaching competence. Four larvae settled and two metamorphosed into 280-μm juveniles on the bryozoan prey of the adults, Bugula pacifica, 46 and 54 days post-hatching. Average growth of pre-ovipositional (<19 mm) J. fuscus was more rapid (8.79% per day) than growth of ovipositional individuals (3.52% per day). Growth continued to a maximum of 57 mm in the laboratory, with an estimated total lifespan of about 5 months. These data, which agree with concurrent field observations, suggest a subannual life cycle.