Hamel et al. (2008) reported that veliger larvae of the nudibranch gastropod Palio dubia settled 1 to 3 days after hatching and that metamorphosis commenced shortly thereafter. This is an anomalously short larval period for a nudibranch described as having planktotrophic development. I examined the embryonic development and hatching larvae of P. dubia collected intertidally from Maine, USA. Veliger larvae with shells 120 μm long and lacking eyespots and propodia developed in 7 days at 20°C from eggs averaging 69 μm in diameter. Their size and morphology were typical of planktotrophic nudibranchs known to have minimum larval periods of weeks to months, and the available evidence does not suggest any other mode of development exists in P. dubia. The larval period of P. dubia is likely an order of magnitude longer than reported by Hamel et al. (2008).

Egg masses, juveniles and adults of the gastropod Palio dubia were found in shallow rocky habitats of eastern Canada dominated by the bryozoan Eucratea loricata. Multiple-choice experiments and direct field observations revealed that P. dubia prefers to feed on E. loricata. Courtship, copulation and egg-laying as well as hatching of P. dubia were closely related to the lunar cycle. Reproduction was preceded by increased pairing and aggregative behaviour. The duration of embryonic development in the capsules was 10–15 d. After hatching, most veligers were retained within the bryozoan branches during their pelagic phase (1–3 d). In multiple-choice experiments, settlement occurred preferentially on the bryozoan E. loricata. In the absence of the preferred substratum, the larvae continued to swim and died after a period that never exceeded 8 d. Juveniles remained associated with the bryozoans on which they settled and reached the adult size in ~3 months.

Little is known about spawning aggregations in subtidal populations of the nudibranch Onchidoris bilamellata. We provide photographic evidence of the spawning aggregations and associated spawning migrations or mass movements whose occurrence was debated.