Distribution of granuloside in the Antarctic nudibranch Charcotia granulosa (Gastropoda: Heterobranchia: Charcotiidae)

The loss of the shell in nudibranch gastropods has been related to the acquisition of chemical defensive strategies during evolution, such as the use of natural products to deter predation. In the present study, we investigated the origin, location, and putative role of granuloside (1), a homosesterterpene lactone, recently isolated from the Antarctic nudibranch Charcotia granulosa Vayssisre, 1906. Several adults, egg masses, and its bryozoan prey, Beania erecta Waters, 1904, were chemically analyzed by chromatographic and spectroscopic techniques. Light-and transmission electron microscopy of the mantle revealed complex glandular structures, which might be associated with the storage of defensive compounds in analogy to mantle dermal formations described in other nudibranchs. Although preliminary in situ repellence bioassays with live specimens of the nudibranch showed avoidance against the Antarctic generalist sea star predator Odontaster validus, the specific role of the terpene granuloside requires further investigation. The egg masses do not present granuloside, and the glandular structures are absent in the trochophore larvae. Our results suggest that C. granulosa synthesizes granuloside de novo in early stages of its ontogeny, instead of obtaining it from the prey. Considering the wide geographic area inhabited by this slug, this may be advantageous, because natural products produced by the slug will not be affected by fluctuant food availability. Overall, the Antarctic sea slug C. granulosa seems to possess defensive strategies that are similar, in terms of production and storage, to nudibranchs from other regions of the world. This species is one of the few cladobranchs investigated so far that present de novo biosynthesis of a defensive compound.