A multiple comparative study of putative endosymbionts in three coexisting apple snail species

Pathogenesis of an experimental mycobacteriosis in an apple snail

In this work, we aimed at investigating cell and tissue responses of the apple snail Pomacea canaliculata, following the inoculation of the zoonotic pathogen Mycobacterium marinum. Different doses were tested (10, 20, 65, and 100 M CFU) and the mortality rate was negligible. The histopathogenesis was followed at 4, 9, and 28 days after inoculation. Overt histopathological lesions were consistently observed after the two largest doses only. In the lung, marked hemocyte aggregations, including intravascular nodule formation, were observed within the large blood veins that run along the floor and roof of this organ. Hemocyte aggregations were found occluding many of the radial sinuses supplying the respiratory lamina. Acid-fast bacilli were contained in the different hemocyte aggregations. In addition, hemocytes were observed infiltrating the storage tissue, which makes up most of the lung wall, and the connective tissue of the mantle edge. Additionally, signs of degradation in the storage tissue were observed in the lung wall on day 28. In the kidney, nodules were formed associated with the constitutive hemocyte islets and with the subpallial hemocoelic space, in whose hemocytes the acid-fast structures were found. Electron microscopy analysis revealed the presence of bacteria-containing phagosomes within hemocytes located in the surface zone of the islets. Additionally, electron-dense spheroidal structures, which are likely remnants of digested mycobacteria, were observed in close proximity to the hemocytes' nuclei. The size attained by the hemocyte nodules varied during the observation period, but there was no clear dependence on dose or time after inoculation. Nodules were also formed subpallially. Some of these nodules showed 2-3 layers with different cellular composition, suggesting they may also form through successive waves of circulating cells reaching them. Nodular cores, including those formed intravascularly in the lung, would exhibit signs of hemocyte dedifferentiation, possibly proliferation, and death. Hemocyte congestion was observed in the hemocoelic spaces surrounding the pallial ends of the renal crypts, and the renal crypts themselves showed de-epithelization, particularly on day 28. The diverse cellular responses of P. canaliculata to M. marinum inoculation and the high resilience of this snail to the pathogen make it a suitable species for studying mycobacterial infections and their effects on cellular and physiological processes.

First morphological and molecular identification of the cercaria of Stomylotrema vicarium from the endemic apple snail Pomacea americanista

The adult fluke Stomylotrema vicarium (Stomylotrematidae, Microphalloidea) was described for the first time in Theristicus caerulescens in 1901, but the complete life cycle has remained unknown to date. Here, we found a stomylotrematid trematode in the digestive gland of the endemic apple snail Pomacea americanista. The digestive gland's tubuloacini were compressed by the trematode larvae placed on connective tissues and haemocoel spaces. Non-virgulate, stylet-bearing cercariae showed three pairs of penetration glands with a body, oral sucker and stylet morphometrically similar to those of stylet-bearing, unencysted young metacercariae of S. vicarium found in the aquatic coleopteran Megadytes glaucus, and at a lesser extent with cercariae of S. gratiosus found in the apple snail Pomacea maculata. The larvae molecular phylogeny was inferred using the markers rRNA 28S and ITS1, being these sequences grouped with the sequences of S. vicarium obtained from adult flukes. Together, these findings indicate that the life cycle of S. vicarium begins in P. americanista, thus supporting the hypothesis that the ampullariid snails act as a first intermediate host.

Composition and diversity of gut microbiota in Pomacea canaliculata in sexes and between developmental stages

Background

The apple snail, Pomacea canaliculata, is one of the world’s 100 worst invasive alien species and vector of some pathogens relevant to human health.

Methods

On account of the importance of gut microbiota to the host animals, we compared the communities of the intestinal microbiota from P. canaliculata collected at different developmental stages (juvenile and adult) and different sexes by using high-throughput sequencing.

Results

The core bacteria phyla of P. canaliculata gut microbiota included Tenericutes (at an average relative abundance of 45.7 %), Firmicutes (27.85 %), Proteobacteria (11.86 %), Actinobacteria (4.45 %), and Cyanobacteria (3.61 %). The female group possessed the highest richness values, whereas the male group possessed the lowest bacterial richness and diversity compared with the female and juvenile group. Both the developmental stages and sexes had important effects on the composition of the intestinal microbiota of P. canaliculata. By LEfSe analysis, microbes from the phyla Proteobacteria and Actinobacteria were enriched in the female group, phylum Bacteroidetes was enriched in the male group, family Mycoplasmataceae and genus Leuconostoc were enriched in the juvenile group. PICRUSt analysis predicted twenty-four metabolic functions in all samples, including general function prediction, amino acid transport and metabolism, transcription, replication, recombination and repair, carbohydrate transport and metabolism, etc.

Conclusions

This study provided a general understanding of the diversity characteristics of intestinal microbial communities of P. canaliculata, and indicated that developmental stage and gender could both influence the intestinal microbes of P. canaliculata. Further study may focus on the interaction between the gut microbiota and their host.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02259-2.

A freshwater symbiosis as sensitive bioindicator of cadmium

The vulnerability of aquatic ecosystems due to the entry of cadmium (Cd) is a concern of public and environmental health. This work explores the ability of tissues and symbiotic corpuscles of Pomacea canaliculata to concentrate and depurate Cd. From hatching to adulthood (4 months), snails were cultured in reconstituted water, which was a saline solution in ASTM Type I water. Then, adult snails were exposed for 8 weeks (exposure phase) to Cd (5 μg/L) and then returned to reconstituted water for other 8 weeks (depuration phase). Cadmium concentration in the digestive gland, kidney, head/foot and viscera (remaining of the snail body), symbiotic corpuscles, and particulate excreta was determined by electrothermal atomic absorption spectrometry. After exposure, the digestive gland showed the highest concentration of Cd (BCF = 5335). Symbiotic corpuscles bioaccumulated Cd at a concentration higher than that present in the water (BCF = 231 for C symbiotic corpuscles, BCF = 8 for K symbiotic corpuscles). No tissues or symbiotic corpuscles showed a significant change in the Cd levels at different time points of the depuration phase (weeks 8, 9, 10, 12, and 16). The symbiotic depuration through particulate excreta was faster between weeks 8 and 10, and then slower after on. Our findings show that epithelial cells of the digestive gland of P. canaliculata and their symbiotic C corpuscles are sensitive places for the bioindication of Cd in freshwater bodies.