Disturbance in invasion? Idiopathic necrotizing hepatopancreatitis in the signal crayfish Pacifastacus leniusculus (Dana, 1852) in Croatia

Crayfish pet trade as a pathway for the introduction of known and novel viruses

Expanding international pet trade has emerged as one of the main introduction pathways of aquatic invasive species, with ornamental crayfish species commonly available on the EU and global markets. Besides most frequently studied crayfish pathogens, such as Aphanomyces astaci and white spot syndrome virus (WSSV), ornamental crayfish carry associated microbial communities, which may potentially lead to the emergence of known or even novel diseases following intentional or unintentional release of animals into the wild. This is especially problematic in the case of viruses, which represent an important, yet considerably understudied, group of crayfish pathogens. Here we analyzed viromes of hepatopancreas tissue of four crayfish species acquired in the international pet trade in Europe (Procambarus clarkii, Procambarus alleni, Cherax holthuisi, and Cherax quadricarinatus) using a high throughput sequencing based metagenomic approach. Seven different known viruses were identified, which were previously either directly associated with crayfish (WSSV, Cherax quadricarinatus reovirus, chequa iflavirus, athtab bunya-like virus) or with hosts from subphylum Crustacea or invertebrates associated with freshwater environment (Shahe ispoda virus 5, Dicistroviridae sp.). Additional sequences represented 8 potential novel and divergent RNA viruses, most similar to sequences belonging to members of Picornavirales, Elliovirales, Reovirales, Hepelivirales, Tolivirales and Ghabrivirales orders. We discuss our findings in relation to their phylogenetic relationships, geographical origins, and putative pathogenicity implications. The results highlight the need for further research into the risks related to disease emergence associated with the pet trade.

Host-related traits influence the microbial diversity of the invasive signal crayfish Pacifastacus leniusculus

The microbiome influences a variety of host-environment interactions, and there is mounting evidence of its significant role in biological invasions. During invasion, shifts in microbial diversity and function can occur due to both changing characteristics of the novel environment and physiological condition of the host. The signal crayfish (Pacifastacus leniusculus) is one of the most successful crayfish invaders in Europe. During range expansion, its populations often exhibit differences in many traits along the invasion range, including sex-composition, size-structure and aggressiveness, but to date it was not studied whether crayfish traits can also drive changes in the host microbiome. Thus, we used 16S rRNA gene amplicon sequencing to examine the effects of host-related traits, namely total length (TL), body condition index (FCF), hepatosomatic index (HSI) and sex on the microbial diversity of the signal crayfish. We examined both external (exoskeletal) and internal (intestinal, hepatopancreatic, hemolymph) microbiomes of 110 signal crayfish individuals from four sites along its invasion range in the Korana River, Croatia. While sex did not exhibit a significant effect on the microbial diversity in any of the examined tissues, exoskeletal, intestinal and hemolymph microbial diversity significantly decreased with increasing crayfish size. Additionally, significant effects of signal crayfish condition (FCF, HSI) on microbial diversity were recorded in the hepatopancreas, a main energy storage organ in crayfish that supports reproduction and growth and also regulates immune response. Our findings provide a baseline for evaluating the contribution of microbiome to an invader's overall health, fitness and subsequent invasion success.

Virome Analysis of Signal Crayfish (Pacifastacus leniusculus) along Its Invasion Range Reveals Diverse and Divergent RNA Viruses

Crayfish are a keystone species of freshwater ecosystems and a successful invasive species. However, their pathogens, including viruses, remain understudied. The aim of this study was to analyze the virome of the invasive signal crayfish (Pacifastacus leniusculus) and to elucidate the potential differences in viral composition and abundance along its invasion range in the Korana River, Croatia. By the high-throughput sequencing of ribosomal RNA, depleted total RNA isolated from the crayfish hepatopancreas, and subsequent sequence data analysis, we identified novel and divergent RNA viruses, including signal crayfish-associated reo-like, hepe-like, toti-like, and picorna-like viruses, phylogenetically related to viruses previously associated with crustacean hosts. The patterns of reads abundance and calculated nucleotide diversities of the detected viral sequences varied along the invasion range. This could indicate the possible influence of different factors and processes on signal crayfish virome composition: e.g., the differences in signal crayfish population density, the non-random dispersal of host individuals from the core to the invasion fronts, and the transfer of viruses from the native co-occurring and phylogenetically related crayfish species. The study reveals a high, previously undiscovered diversity of divergent RNA viruses associated with signal crayfish, and sets foundations for understanding the potential risk of virus transmissions as a result of this invader's dispersal.