Data from: Comparative morphology and evolution of the cnidosac in Cladobranchia (Gastropoda: Heterobranchia: Nudibranchia)

Background: A number of shelled and shell-less gastropods are known to use
multiple defensive mechanisms, including internally generated or
externally obtained biochemically active compounds and structures. Within
Nudipleura, nudibranchs within Cladobranchia possess such a special
defense: the ability to sequester cnidarian nematocysts – small capsules
that can inject venom into the tissues of other organisms. This ability is
distributed across roughly 600 species within Cladobranchia, and many
questions still remain in regard to the comparative morphology and
evolution of the cnidosac – the structure that houses sequestered
nematocysts (called kleptocnides). In this paper, we describe cnidosac
morphology across the main groups of Cladobranchia in which it occurs, and
place variation in its structure in a phylogenetic context to better
understand the evolution of nematocyst sequestration. Results: Overall, we
find that the length, size and structure of the entrance to the cnidosac
varies more than expected based on previous work, as does the structure of
the exit, the musculature surrounding the cnidosac, and the position and
orientation of the kleptocnides. The sequestration of nematocysts has
originated at least twice within Cladobranchia based on the phylogeny
presented here using 94 taxa and 409 genes. Conclusions: The cnidosac is
not homologous to cnidosac-like structures found in Hancockiidae.
Additionally, the presence of a sac at the distal end of the digestive
gland may have originated prior to the sequestration of nematocysts. This
study provides a more complete picture of variation in, and evolution of,
morphological characters associated with nematocyst sequestration in
Cladobranchia.