Abstract
Nudibranchs (Mollusca, Gastropoda) feeding on tentacles and polyps of Cnidaria thereby ingest both latent and discharged nematocysts (NCs) along with the food mass. In eolid nudibranchs (Opisthobranchia, Aeolidacea), many of the undischarged NCs are transported to terminal cnidosacs in their body appendages (cerata) and incorporated as kleptocnidae for defense. In the present report, the occurrence and fate of NCs in the digestive tracts of eolids is compared with hydrozoan-feeding dendronotacean nudibranchs (Opisthobranchia, Dendronotacea), which may show more basic stages in the evolution of cnidosacs. Tomographic reconstructions of the distal tips of cerata were composed from series of semithin light microscopic sections, utilizing 3D-surface rendering software. Doto acuta (Dendronotacea, Dotidae) does not have cnidosacs; transmission electron micrographs show that the NCs are digested in lysosomes of digestive gland cells. In contrast, species of the genus Hancockia (Dendronotacea, Hancockiidae) have several small cnidosacs in each ceras; they accumulate NCs in the digestive cells, as well as in the cnidosacs. Many of the cnidosacs were found open to the exterior with NCs in the process of expulsion. These and other structural details suggest assigning a function of expelling the NCs to the Hancockia spp. cnidosacs. It is proposed that cnidosacs similar to those of Hancockia spp. provide a clue to understanding how the defensive function of eolid cnidosacs may have evolved.
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Acknowledgments
Yuri Hooker (Lima) helped to collect specimens of Hancockia schoeferti. The fieldwork in Peru was supported by the GeoBioCenter (LMU) and the Deutsche Forschungsgemeinschaft (DFG SCHR 667/4 to MS). Claus Valentin provided excellent working conditions at the Institute of Marine Biology at Giglio. We would like to thank Prof. E. Koenig (Buffalo) for critically reading and revising the manuscript.
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Communicated by J.P. Grassle.
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Phylogenetic relationships of cnidosac bearing nudibranch gastropods
. The morphological data set (47 characters; one hypothetical plesiomorphic group plus 2 pleurobranchoidean taxa as outgroups, and 29 nudibranch taxa) used to reveal the phylogeny of the Nudibranchia by Wägele & Willan (2000) herein was supplemented by Hancockia and reanalyzed (PAUP). All characters were treated as unweighted and unordered; the tree was unrooted. The strict consensus tree of 120 equally parsimonious trees (102 steps) is shown; numbers refer to bootstrap values (<50 not indicated), obtained by a separate analysis (1000 replications, PAUP) with the same settings; a clade of Protaeolidiella and Phyllodesmium (marked in grey) received some bootstrap support but is not represented in the strict consensus tree. Species possessing cnidosacs with kleptocnidae are in bold face; Protaeolidiella has cnidosacs but without containing kleptocnidae. While the monophyly or e.g. Nudibranchia, Anthobranchia, and Doridacea is supported by high bootstrap values (>90), none of the traditional major dexiarchian taxa Arminacea, Dendronotacea and Aeolidacea results monophyletic. The Arminidae from the sister clade to a polytomy composed of members all three groups. The dendronotacean Hancockia (with cnidosacs) and Doto (without cnidosacs) group together, nestling within cnidosac-possessing eolid taxa in the strict consensus tree; however, these relationships are not statistically supported. From this, due to limited taxon and character sampling, very preliminary phylogenetic point of view, the systematic position of Hancockia and the evolution of cnidosacs remain unresolved
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Martin, R., Heß, M., Schrödl, M. et al. Cnidosac morphology in dendronotacean and aeolidacean nudibranch molluscs: from expulsion of nematocysts to use in defense?. Mar Biol 156, 261–268 (2009). https://doi.org/10.1007/s00227-008-1080-2
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DOI: https://doi.org/10.1007/s00227-008-1080-2