RESUMO
Aeolid nudibranchs are well-known for their ability to incorporate cnidarian nematocysts and use them for defense; this process is tightly linked with the feeding preferences of molluscs. As many nudibranch groups show signs of ecology-based adaptive radiation, studies of prey-based defensive mechanisms can provide valuable insight into details of nudibranch evolutionary history. The main goal of this study is to test the correlation of ecological traits, feeding mechanisms, and prey preferences with cnidosac fine morphology and to pinpoint the phylogenetic value of these traits. We study the cnidosac morphology in thirteen species-representatives of the main lineages within the family Fionidae s.l. The morphological analysis includes histological sections, transmission electron microscopy, confocal laser scanning microscopy, and scanning electron microscopy. For phylogenetic study, available molecular data from public repositories were used, and phylogenetic trees were produced based on Bayesian Inference and Maximum likelihood analysis for a concatenated dataset of three molecular markers (COI, 16S, H3). In general, fionid cnidosacs fit the common aeolid pattern, but among different species we detected a high variation in type of obtained nematocysts, their arrangement within cnidophages, and in number of cell types within cnidosacs. We report on presence of cellules speciale in the haemocoel of all studied species, and for the first time, we report on cells with chitinous spindles in the haemocoel of all fionids except Eubranchus. The function of both these cell types remains unknown. The loss of functional cnidosacs occurred at least three times within Fionidae, and in case of the genera Phestilla, Calma, and Fiona, this loss is linked to their non-cnidarian diet. The diversity of cnidosac fine structure within Fionidae s.l. correlates with that of the radular morphology and feeding preferences of each species. Prey shifts between cnidarian and non-cnidarian prey (both through evolutionary shifts and individual variation) rarely occur within Fionidae s.l.; however, microevolutionary shifts between different hydrozoan species within a single genus are more common. Cnidosac morphology demonstrates considerable resulting changes even when switching between similar hydrozoan species, or changing the feeding site on same prey species. These data indicate that cnidosac morphology likely follows microevolutionary prey shifts-in other words, it is affected by switches in prey species and changes in feeding sites with a single prey species. Thus, the cnidosac morphology may be a useful indicator when studying ecological features of particular species.
RESUMO
Onchidoris muricata is a widespread shell-less species of nudibranch molluscs, which has unique for Gastropoda skeletal elements - subepidermal calcite spicules. The general and fine morphology of the spicules, as well as their maturation process in ontogenesis, have been studied in detail by authors. The uniqueness of spicules lies in their intracellular formation and location under the ectodermal epithelium, which is more typical for deuterostomes. We present O. muricata as a potentially new model species for studying calcification of intracellular protein structure. A total of 96 individuals were collected in the Kandalaksha Bay of the White Sea, both manually and by scuba diving. All individuals were divided into three groups based on morphological characteristics such as specimens' size, spicule condition etc. This division suggests the existence of three stages in postembryonic ontogenesis of O. muricata reflecting the maturation of the spicule complex. Total RNA samples were isolated from three size groups of molluscs in three biological replicates. Libraries were prepared from the polyadenylated RNA fraction and sequenced at NovaSeq6000 (Illumina), yielding a total of 112.8 Gb of 150 bp paired-end reads, corresponding to almost 1,000-fold coverage of the transcriptome. Representative transcriptome assembled de novo with Trinity. In addition to obtaining the transcriptome sequences of O. muricata, differential expression analysis was also performed for these three size groups. This allows us to trace the dynamics of molecular and biological processes during the life of a mollusc. The obtained data can then be used as a reference transcriptome for closely related species, to study specific expressed genes, to identify various unique sequences, including protein-coding ones, to understand biological processes, including biomineralization and much more.
RESUMO
The drilling mode of feeding is known from two clades of Gastropoda: Caenogastropoda and Heterobranchia. However, the level of convergence and parallelism or homology among these two lineages is unclear. The morphology of the buccal complex is well studied for drilling caenogastropods, but poorly known for drilling nudibranchs. It is also unclear whether the drilling feeding mechanism is similar between inside gastropods. Accordingly, a comparison between the feeding mechanisms of drilling nudibranchs and caenogastropods can help to understand the evolutional trends inside gastropods. In this study, we redescribe the morphology of the buccal complex of drilling dorid nudibranch Vayssierea cf. elegans, and compare it to that of previous investigations on this species and closely related dorid species. We describe the feeding mechanism of this species based on the obtained morphological and literature data and compare it to the feeding mechanisms described for drilling caenogastropods. The feeding apparatus of Vayssierea cf. elegans corresponds to the general morphology of the dorid buccal complex; that is, it has a similar arrangement of the buccal musculature and pattern of radular morphology. However, there are also adaptations to the drilling feeding mode similar to those found in Caenogastropoda: that is, specialized dissolving glands and lateral teeth with elongated pointed cusps; and even Sacoglossa: the specialized muscle for sucking. The feeding process of Vayssierea cf. elegans includes the same two stages as those described for drilling caenogastropods: (a) the boring stage, which is provided by mechanical and chemical activity, and (b) the swallowing stage.