Functional ultrastructure and cytochemistry of vitellogenesis stages of Rohdella amazonica (Aspidogastrea, Aspidogastridae, Rohdellinae), a parasite of the Amazoninan banded puffer fish Colomesus psittacus

Morphology of the buccal apparatus of Dendronotus frondosus (Gastropoda: Nudibranchia)

Nudibranchia is the order of Gastropoda, which is known for its high diversity of dietary specializations and a diversified morphology of the buccal armature. The characters of the buccal armature are adaptive to the feeding mechanisms and diet preferences, but they also are phylogenetically informative. The morphological differences in the buccal armature imply different mechanisms of the synthesis and morphology of the formation zones. Here, we report new data on the morphology of the buccal complex in a cladobranch species with broad radula, Dendronotus frondosus, using light microscopy, transmission and scanning elctron microscopy and computer-based three- dimensional reconstruction and compare them with those on other nudibranchs. We report the presence of protective chitinous spindles in the epithelia of the oral tube and esophagus, which suggests the presence of Cnidaria in the diet of the studied mollusc species. Similar to other Cladobranchia, the jaw plate of D. frondosus is synthesized by the layer of gnathoblasts located posteriorly in the epithelial fold. Near the zone of jaw synthesis, the gnathoblasts with concentric granular endoplasmic reticulum structures were found. This may indicate the turnover of gnathoblasts during the life of a specimen. Despite that D. frondosus has a broad radula, the zone of its synthesis does not split into two parts, unlike other molluscs with broad radulae. A single radular tooth is formed by a few cells, such as in other Nudibranchia. The number of odontoblasts per tooth in D. frondosus is not defined by a tooth size. At the same time, the linear dimensions of the cells secreting radular teeth depend on the size of the teeth synthesized by them.

ULTRASTRUCTURE OF EGG ENVELOPES AND EARLY EMBRYOS OF ROHDELLA AMAZONICA (TREMATODA: ASPIDOGASTREA) PARASITIC IN BANDED PUFFER FISH, COLOMESUS PSITTACUS

Egg structure and early embryonic development of the aspidogastrean, Rohdella amazonica, a basal trematode, were studied by transmission electron microscopy (TEM) to gain insight into functional, developmental, and phylogenetic characteristics. Gravid worms were removed from the intestine of naturally infected banded puffer fish Colomesus psittacus, collected from the Bay of Marajó, Paracauari River (Pará, Brazil) and processed by standard TEM methods. By the time of pronuclear fusion, the fertilized zygote was already enclosed in a thick, electron-dense pre-operculate eggshell and an underlying layer of vitellocytes that fused into a vitelline syncytium as they were still secreting their shell granules. When cleavage commenced, a small number of macromeres moved to the area just underneath the eggshell, where they fused to form a single syncytial embryonic envelope. Simultaneously, the smaller blastomeres continued to divide as they maintained contact with each other, but remained separate from the vitelline syncytium. Concurrent with these cellular changes, a thickened knob expanded at one pole of the eggshell and began to form an opercular suture. By the time the operculum was fully formed, the vitelline syncytium had mostly degenerated, while the smaller blastomeres had become cohesive as a single mass that preceded the differentiation and morphogenesis of the cotylocidium larva. The general pattern of cleavage and eggshell formation resembles that of other trematodes and polylecithal cestodes, but the single embryonic envelope has been reported only in a few basal taxa. The only other aspidogastrean studied in detail to date is very similar, indicating close phylogenetic affinity and conservatism within this basal neodermatan and neoophoran group.

ULTRASTRUCTURE AND CYTOCHEMISTRY OF LATE EMBRYOS AND COTYLOCIDIUM LARVAE OF ROHDELLA AMAZONICA (TREMATODA: ASPIDOGASTREA), FROM THE TROPICAL ESTUARINE FISH, COLOMESUS PSITTACUS

Developmental ultrastructure of late embryos and cotylocidium larval morphogenesis of Rohdella amazonica, an aspidogastrean parasite of fish, were studied to reveal the functional aspects of larvigenesis within the egg as well as phylogenetically relevant characteristics of the embryos and larvae in this basal trematode group. Gravid worms were removed from the intestine of naturally infected banded puffer fish Colomesus psittacus, collected from the Bay of Marajó, Paracauari River (Pará, Brazil) and processed by standard methods of transmission electron microscopy (TEM) and cytochemistry. During late cleavage and rearrangement of the blastomeres, the vitelline syncytium that plays a role in eggshell formation and nutrient provision to the embryo completes its apoptotic degeneration as the embryonic mass grows substantially. Early larval morphogenesis involves cellular positioning that defines the anteroposterior polarity of the differentiating larva. Progressing through larvigenesis, the anterior end forms a muscular oral sucker surrounding the mouth, which leads inward into the pharynx and expanding digestive cavity. At the posterior end, a large disc forms as a precursor to the eventual ventral disc. The fully formed cotylocidium, still within the eggshell, is flexed ventrally, bringing the 2 poles into near juxtaposition. The neodermatan tegument with outwardly projecting small microvilli becomes fully formed, as myocytons, a protonephridial system, and 2 glandular regions occupy the body's interior. The ultrastructural features described here are very similar to those reported for Aspidogaster limacoides from fish and somewhat similar to those reported for Cotylogaster occidentalis from molluscs, but differ from the more diverse larvae of neodermatan taxa that have been studied more extensively.

The unusual cytoarchitecture of "vitelline follicles" in freshwater blood flukes of the genus Sanguinicola (Digenea, Aporocotylidae)

This is the first study assessing the cytoarchitecture of the vitellarium of members of the freshwater, teleost-infecting lineage of blood-flukes (Aporocotylidae). The vitelline cytoarchitecture of two innominate species of Sanguinicola from freshwater fishes in Russia showed that vitelline cells at different stages of maturation are widely distributed throughout much of the body and are mixed with other cell types. The latter feature indicates that use of the term "follicular vitellarium" is inappropriate for species of this genus. An additional characteristic of the vitelline cells in these Sanguinicola spp. is their ability to form long, pseudopodia-like extensions of the peripheral cytoplasm that contact neighbouring vitelline cells and sarcoplasmic extensions, forming both heterologous and homologous intercellular junctions. Within the vitelline duct lumen, the cytoplasm of mature vitelline cells is filled with regular clusters (0.5-1.0 μm in diameter), comprising 10-30 vitelline globules, which have heterogeneous contents and electron-lucent lipid droplets (1.1-1.7 μm in diameter), but no apparent modifications of vitelline globules occur within the vitelline duct. The flattened, ciliated, epithelial lining of the common vitelline duct contains intra-epithelial nuclei, its luminal surface bears shallow lamellae and adjacent cells are adjoined by apical septate junctions. All of these observations, when compared to the marine Aporocotyle simplex, likely represent additional characteristics supporting the divergent evolutionary lineages of marine and freshwater aporocotylids.