The muscle dwelling myxozoan, Kudoa inornata, enhances swimming performance in the spotted seatrout, Cynoscion nebulosus

Post-mortem 'soft flesh' in three commercial fish species from off Atlantic Morocco associated with the myxosporean parasites Kudoa thyrsites and K. encrasicoli (Myxozoa)

Small pelagic fishes represent one of the most important food resources off the Northwest coast of Africa. Despite their economic significance, little is known about the infections with flesh invading myxosporean parasites of genus Kudoa (Cnidaria, Myxozoa). Heavy infections in the flesh may be associated with post-mortem myoliquefaction, commonly known as 'soft flesh'. This condition may reduce the quality and marketability of the fish fillet, resulting in both economic losses to the fishing industry and loss of consumer confidence. In this study, we investigated Kudoa-induced 'soft flesh' occurrence in European anchovy Engraulis encrasicolus, European pilchard Sardina pilchardus, and Atlantic chub mackerel Scomber colias caught in 2019 off the Moroccan Atlantic coast. Five hundred specimens of each fish species were examined for 'soft flesh' by texture testing and visual inspection 48 h post-catch. 'Soft flesh' occurred in 0.2 % of the European anchovies, 1.4 % of the European pilchard, and in 4.4 % of the Atlantic chub mackerel. Microscopic examination of muscle samples revealed that 'soft flesh'-affected fish were infected with myxospores of K. thyrsites-like morphotype. Analysis of the kudoid SSU rDNA sequence obtained from European pilchard and the Atlantic chub mackerel identified these as K. thyrsites (100 % identity), whereas analysis of the sequence from European anchovy identified the presence of K. encrasicoli (100 % identity). Even if there are no known human health consequences associated with the ingestion of these Kudoa species, the unsightly appearance of some infected fillets is a food quality issue, that can eventually lead to reduced marketability and value.

Limited accrual of myxospores of Kudoa inornata (Cnidaria: Myxosporea) in their wild fish hosts, Cynoscion nebulosus (Teleostei: Sciaenidae)

Kudoa inornata is a myxosporean that infects the seatrout Cynoscion nebulosus. Increased prevalence of infection as fish age and absence of inflammation against plasmodia led to the hypothesis that seatrout retain and accumulate myxospores throughout their lives. However, opportunistic observations that wild-caught seatrout cleared infection when maintained in aquaculture conditions and evidence of encapsulated infected necrotic myofibers suggested that fish develop an immunity against this parasite, or that myxospores have a limited life span. To evaluate myxospore clearance and to test putative resistance to re-infection, we examined 44 wild-caught seatrout broodstock maintained in parasite-free water for 2-6 yr. Twenty-five fish served as negative controls (time zero of experiment), and 19 were exposed to water-borne K. inornata infective stages for 18 wk. Over 73% of the exposed fish became infected, compared to ~12% of control fish, indicating that fish were susceptible to re-infection by K. inornata. Whether plasmodia degenerate because K. inornata myxospores have a limited life span or seatrout develop an adaptive immunity against these life stages remains unknown. To test for accumulation of myxospores over time, we compared myxospore densities and intensities between sexes and across ages and sizes of wild seatrout. There was no significant difference in myxospore densities with size, age, or sex. However, intensities increased significantly with increasing fish age and size, indicating accrual of myxospores over time. These results combined with evidence of infection clearance suggest that K. inornata myxospores do not persist but nevertheless accrue in wild seatrout due to continuous contact with infective stages.

Increased parasite load is associated with reduced metabolic rates and escape responsiveness in pumpkinseed sunfish

Wild animals have parasites that can compromise their physiological and/or behavioural performance. Yet, the extent to which parasite load is related to intraspecific variation in performance traits within wild populations remains relatively unexplored. We used pumpkinseed sunfish (Lepomis gibbosus) and their endoparasites as a model system to explore the effects of infection load on host aerobic metabolism and escape performance. Metabolic traits (standard and maximum metabolic rates, aerobic scope) and fast-start escape responses following a simulated aerial attack by a predator (responsiveness, response latency, and escape distance) were measured in fish from across a gradient of visible (i.e. trematodes causing black spot disease counted on fish surfaces) and non-visible (i.e. cestodes in fish abdominal cavity counted post-mortem) endoparasite infection. We found that a higher infection load of non-visible endoparasites was related to lower standard and maximum metabolic rates, but not aerobic scope in fish. Non-visible endoparasite infection load was also related to decreased responsiveness of the host to a simulated aerial attack. Visible endoparasites were not related to changes in metabolic traits nor fast-start escape responses. Our results suggest that infection with parasites that are inconspicuous to researchers can result in intraspecific variation in physiological and behavioral performance in wild populations, highlighting the need to more explicitly acknowledge and account for the role played by natural infections in studies of wild animal performance.

Host size influences prevalence and severity of Kudoa thyrsites (Cnidaria: Myxosporea) infection in Atlantic salmon Salmo salar

Kudoa thyrsites is a cosmopolitan myxozoan parasite of marine fish. The infection causes an economically important myoliquefaction in farmed Atlantic salmon in British Columbia, Canada. Laboratory exposure of Atlantic salmon smolts to infectious seawater was used to test the hypothesis that infection with K. thyrsites is more severe in age-matched, smaller salmon. In each of 2 trials approximately 4 mo apart, smolts were graded into small (80 and 68 g), medium (117 and 100 g) and large (142 and 157 g) initial weight groups (IWGs) and concurrently exposed to infectious seawater. The effects of IWG and time on fish size and infection severity were assessed by linear mixed-effects models. The fish were screened for infection by histological examination at intervals following exposure. Increases in mean length and weight were statistically significant in all IWG during both trials. The infection was detected in fish in both trials, and in Trial 2, the prevalence was significantly greater in larger fish 1000 degree-days (DD) after exposure. The severity of infection (plasmodia mm-2 muscle) was significantly higher in larger smolts: between medium and large IWGs at 2500 DD in Trial 1 and between small and medium IWGs at 1500 and 2000 DD in Trial 2. The hypothesis is rejected and possible explanations for the greater occurrence of K. thyrsites in larger smolts are discussed.