Structural integrity and viability of Fredericella sultana statoblasts infected with Tetracapsuloides bryosalmonae (Myxozoa) under diverse treatment conditions

Abstract

Fredericella sultana is an invertebrate host of Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonids. The bryozoan produces seed-like statoblasts to facilitate its persistence during unfavourable conditions. Statoblasts from infected bryozoans can harbor T. bryosalmonae and give rise to infected bryozoan colonies when conditions improve. We aimed in the present study to evaluate the integrity and viability of T. bryosalmonae-infected statoblasts after a range of harsh treatment conditions. We tested if statoblasts could survive ingestion by either brown trout or common carp. After ingestion, the fish faeces was collected at different time points. We also tested physical stressors: statoblasts collected from infected colonies were desiccated at room temperature, or frozen with and without Bryozoan Medium C (BMC). After treatments, statoblasts were assessed for physical integrity before being incubated on BMC to allow them to hatch. After 4 weeks, hatched and unhatched statoblasts were tested by PCR for the presence of the parasite. We found that statoblasts ingested by brown trout and those frozen in BMC were completely broken. In contrast, statoblasts ingested by common carp and those subjected to dry freezing were able to survive and hatch. T. bryosalmonae was detected by PCR in both hatched and unhatched infected statoblasts, but neither from broken nor uninfected statoblasts. Our results confirmed for the first time the ability of infected statoblasts to survive passage through a fish, and freezing. These findings suggest potential pathways for both persistence and spread of T. bryosalmonae-infected statoblasts in natural aquatic systems.

Figure 1

Association of statoblast with a mature sac of Tetracapsuloides bryosalmonae. Zooid from an infected Fredericella sultana colony showing a statoblast (black arrow) in close association with a mature sac of Tetracapsuloides bryosalmonae (white arrow).

Figure 2

Fredericella sultana statoblasts extracted from the faeces of brown trout. A Fragmented statoblast with a remnant of its outer membrane. B Opened statoblast inside a zooid (white arrow).

Figure 3

Fredericella sultana statoblast collected from the faeces of common carp. A Broken statoblast (light brown, arrows) surrounded by fish intestinal mucous. B Intact statoblast inside a zooid (dark brown, arrow), broken statoblast with opened valve (light brown, arrowhead).