Organogenesis and histogenesis in the planktotrophic veliger of Doridella steinbergae (Opisthobranchia: Nudibranchia)

Development of the planktotrophic veliger of the dorid nudibranch Doridella steinbergae (Lance) was studied by histological examination of 4, arbitrarily defined larval stages. Following an embryonic period of 7 1/2 to 8 days (12° to 15°C), the newly hatched veligers possess a functional digestive tract, a pair of nephrocysts, a secondary kidney, a pair of cerebral ganglia, a larval shell consisting of a two-thirds whorl, and the metapodial component of the foot. Development during Stage I mainly involves growth of the larval shell and the visceral organs. Stage II is marked by the retraction of the mantle fold from the shell aperture and the appearance of the eyespots, gonadal rudiment, larval heart, and the optic, pedal, and pleural ganglia. At Stage III the radular sac rudiment evaginates from the esophageal wall, the buccal ganglia differentiate, and the propodial rudiment begins to develop on the ventral surface of the metapodium. Stage IV veligers, which are competent to metamorphose, possess 6 pairs of radular teeth, lipid deposits in the left digestive gland, rudiments of the adult kidney and the oral lip glands, an hypertrophied mantle fold, a propodium, and densely packed cilia over the entire ventral surface of the foot. The length of the obligatory larval period, from hatching of the veliger until the attainment of metamorphic competence, is 25 to 26 days under laboratory culture conditions and the larval shell grows from 142 to 168 m in length. The sequence of morphogenetic events and the structure of the competent veliger of D. steinbergae is compared to that of other opisthobranch veligers. It is suggested that the relatively small maximal shell size attained by D. steinbergae results from precocious retraction of the mantle fold. It is further suggested that interspecific differences in the kinds of structures that develop during the veliger phase of opisthobranchs may relate to variations in the requirements of the juvenile phase. The functional adaptations of the gut of planktotrophic veligers are discussed and compared to those of lecithotrophic veligers.