Ametamorphic direct development in Dendrodoris behrensi (Nudibranchia: Dendrodorididae), with a review of developmental mode in the family
Dendrodoris behrensi Millen and Bertsch, 2005 has ametamorphic direct development. The embryos passed through a vestigial veliger stage lacking a shell, operculum, larval retractor muscle and pedal sensory cilia. After an embryonic period of 38 days (16-19 oC) they hatched as juveniles averaging 512 µm in dorsum length. Newly hatched juveniles possessed eyespots and a lattice of spicules on the ventral surface of the dorsum, and after an additional four days, they possessed rudimentary rhinophores. Only three other examples of direct development have been noted from nudibranchs from the northeast Pacific Ocean, all from the Family Dendrodorididae.
Based on a survey of the literature, mode of development was determined for 26 species and forms of dendrodoridids worldwide. Fifty-four percent of these taxa have planktotrophic development, 4% lecithotrophic development, and 42% direct development. Direct development is significantly more prevalent in the Dendrodorididae than reported for opisthobranchs worldwide by Hadfield and Miller (1987). Three hypotheses are presented to explain this: (1) direct development is adaptive in overcoming size constraints on post-metamorphic, juvenile dendrodoridids stemming from their lack of a radula and suctorial mode of feeding on sponges. (2) Direct development is prevalent because small adult size, which is generally correlated with direct development in marine invertebrates, has been selected for in many dendrodoridids. (3) Direct development is an adaptation against high larval mortality in some regions. Limited evidence tends to support hypotheses 2 and 3, but with some interesting developmental exceptions, not hypothesis 1.