Pathology and infectious agents of unionid mussels: A primer for pathologists in disease surveillance and investigation of mortality events

Freshwater mussels are one of the most imperiled groups of organisms in the world, and more than 30 species have gone extinct in the last century. While habitat alteration and destruction have contributed to the declines, the role of disease in mortality events is unclear. In an effort to involve veterinary pathologists in disease surveillance and the investigation of freshwater mussel mortality events, we provide information on the conservation status of unionids, sample collection and processing techniques, and unique and confounding anatomical and physiological differences. We review the published accounts of pathology and infectious agents described in freshwater mussels including neoplasms, viruses, bacteria, fungi, fungal-like agents, ciliated protists, Aspidogastrea, Digenea, Nematoda, Acari, Diptera, and Odonata. Of the identified infectious agents, a single viral disease, Hyriopsis cumingii plague disease, that occurs only in cultured mussels is known to cause high mortality. Parasites including ciliates, trematodes, nematodes, mites, and insects may decrease host fitness, but are not known to cause mortality. Many of the published reports identify infectious agents at the light or ultrastructural microscopy level with no lesion or molecular characterization. Although metagenomic analyses provide sequence information for infectious agents, studies often fail to link the agents to tissue changes at the light or ultrastructural level or confirm their role in disease. Pathologists can bridge this gap between identification of infectious agents and confirmation of disease, participate in disease surveillance to ensure successful propagation programs necessary to restore decimated populations, and investigate mussel mortality events to document pathology and identify causality.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

Impairment of swimming performance in Tritia reticulata (L.) veligers under projected ocean acidification and warming scenarios

Tritia reticulata (L.) is a neogastropod ubiquitous in the coastal communities of the NE Atlantic. Its life cycle relies on the swimming performance of planktonic early life stages, whose sensitivity to the climate conditions projected for the near future, namely of ocean acidification (OA) and warming (W), is, to our best knowledge, unknown. To examine the resilience of larval stages to future environmental conditions, this work investigates the effect of OA-W on the swimming performance of T. reticulata veligers under a range of experimental conditions, based on the end-of-century projections of the Intergovernmental Panel on Climate Change. Veligers were exposed to six experimental scenarios for 14 days, employing a full factorial design with three temperatures (T°C: 18, 20 and 22 °C) and two pH levels (pH_target: 8.1 and 7.8). Mortality was assessed throughout the trial, after which swimming behaviour - characterised by the activity, speed and the distance travelled by veligers - was analysed by automated video recordings in a Zebrabox® device. Mortality increased with OA-W and, although more active, larvae travelled shorter distances revealing reduced swimming speed under acidic and warmer conditions, with the interaction of the tested stressors - pH and T°C - being highly significant. Results motivated the morpho-histological analysis of larvae preserved at the end of the trial, to check for the integrity of the organs involved in veligers' motion: statocysts, velum and foot. Statocyst and velar morpho-structure were conserved but histological damage of metapodial epithelia was evident under acidity, namely an apparent hypertrophy and protrusion of the secretory cells, with dispersed pigmented granules and, at 22 °C, less cilia, with potential functional implications. Negative consequences of the OA-W scenarios tested on veligers' competence are unveiled, pointing towards the eminent threat these phenomena constitute to T. reticulata perpetuation in case no mitigation measures are taken, and projections become effective.

Review: Antibacterial components of the Bivalve's immune system and the potential of freshwater bivalves as a source of new antibacterial compounds

Antibacterial research is reaching new heights due to the increasing demand for the discovery of new substances capable of inhibiting bacteria, especially to respond to the appearance of more and more multi-resistant strains. Bivalves show enormous potential for the finding of new antibacterial compounds, although for that to be further explored, more research needs to be made regarding the immune system of these organisms. Beyond their primary cellular component responsible for bacterial recognition and destruction, the haemocytes, bivalves have various other antibacterial units dissolved in the haemolymph that intervene in the defense against bacterial infections, from the recognition factors that detect different bacteria to the effector molecules carrying destructive properties. Moreover, to better comprehend the immune system, it is important to understand the different survival strategies that bacteria possess in order to stay alive from the host's defenses. This work reviews the current literature regarding the components that intervene in a bacterial infection, as well as discussing the enormous potential that freshwater bivalves have in the discovery of new antibacterial compounds.