Meeting Abstract
Shell-less gastropods are known to use multiple defensive mechanisms, including internally generated or externally obtained biochemically active compounds and structures. Within Nudipleura, a group of nudibranchs called Cladobranchia possess such a defense: the ability to sequester cnidarian nematocysts – small venom-filled capsules that can be discharged into the tissues of other organisms. This ability is distributed across ~600 species within Cladobranchia, and many questions still remain in regards to the comparative morphology and evolution of the structure that houses the nematocysts, called the cnidosac. In this research, we use histological techniques to describe the cnidosac morphology across all groups in Cladobranchia in which it has been identified, and discuss this variation in a phylogenetic context. Overall, we find that the length, size and structure of the opening to the cnidosac can vary more than expected based on previous work, as can the structure of the exit and the musculature surrounding the cnidosac. There are also few clear evolutionary patterns in relation to this variation. The sequestration of nematocysts has originated twice within Cladobranchia based on the phylogeny presented here (within Hancockiidae and Aeolidida), but low support for long branches at the base of Aeolidida results in low confidence in this reconstruction. Additionally, the presence of a sac at the distal end of the digestive gland may have originated prior to that of the sequestration of nematocysts. This study provides a more complete picture of the variation in, and evolution of, morphological characters among nematocyst sequestering taxa in Cladobranchia.
