Proliferative kidney disease (PKD) is an emerging disease of salmonid fishes. It is provoked by temperature and caused by infective spores of the myxozoan parasite Tetracapsuloides bryosalmonae, which develops in freshwater bryozoans. We investigated the link between PKD and temperature by determining whether temperature influences the proliferation of T. bryosalmonae in the bryozoan host Fredericella sultana. Herein we show that increased temperatures drive the proliferation of T. bryosalmonae in bryozoans by provoking, accelerating and prolonging the production of infective spores from cryptic stages. Based on these results we predict that PKD outbreaks will increase further in magnitude and severity in wild and farmed salmonids as a result of climate-driven enhanced proliferation in invertebrate hosts, and urge for early implementation of management strategies to reduce future salmonid declines.KEY WORDS: Temperature · Climate change · Salmonids · Proliferative kidney disease · Myxozoa · Freshwater bryozoans · Covert infections Resale or republication not permitted without written consent of the publisherDis Aquat Org 70: [227][228][229][230][231][232][233][234][235][236] 2006 through the epithelium. The parasite then multiplies in fishes as extrasporogonic stages, primarily in the kidney and spleen, and fish hosts mount a massive cellmediated immune response (Hedrick et al. 1993). The role of fishes in the life cycle of T. bryosalmonae is unresolved. Most evidence suggests that they are accidental and dead-end hosts (see Henderson & Okamura 2004 and references therein), and transmission from fishes back to bryozoans has so far not been achieved . It is becoming increasingly clear that the maintenance of endemic infections in clonally replicating bryozoans provides a constant source of infection for fishes .PKD outbreaks are highly seasonal, generally occurring after waters have reached 15°C (Hedrick et al. 1993). Although fishes can become infected by Tetracapsuloides bryosalmonae at low temperatures (<10°C), PKD will not develop (Gay et al. 2001). In addition to provoking outbreaks, increased temperatures appear to promote disease severity (Naich et al. 2003). Clines in PKD with regard to temperature are apparent. Brown trout suffer from the disease in lower altitudinal regions of Switzerland (Wahli et al. 2002) and, although PKD has been recorded in northern Europe (Hedrick et al. 1993), it is of greatest concern in salmonid aquaculture further south (e.g. southern England, France and Italy). Given the strong influence of temperature on PKD, the disease is likely to become an increasing problem in wild and farmed fish populations as a consequence of climate change. Indeed, this may already be happening in Switzerland, where significant increases in the maximum annual water temperature have been documented in stretches of Swiss rivers where PKD-impacted brown trout populations have been in decline (Wahli et al. 2002, Borsuk et al. 2006.The relationship between PKD and temperature may be explain...

Buddenbrockia plumatellae, an enigmatic worm-like myxozoan, was observed as continuously writhing free and attached 'worms' and as free mature spores in the coelom of the freshwater bryozoans Plumatella fungosa, Hyalinella punctata, and Fredericella sp. 'Worm' numbers could double every three days. 'Worms' and spores could be expelled from colonies by external pressure. Some mature 'worms' exited actively, entraining release of free spores, and gradually ceased movement outside the host. Bryozoans sealed off infected regions of the colony. Infected colonies grew slowly, produced no statoblasts, and eventually regressed and died. Transmission was not achieved and prevalence was low. Electron microscopy of 'worms' revealed a single layer of mural cells on a fibrous basal lamina overlying four longitudinal muscle blocks and an inner sheet of two types of proliferating cells, an organization indicative of the bilaterian ancestry of the Myxozoa. Primary type A cells were attached directly by striated tubules to mural cells at positions between muscle blocks. Secondary type A cells had a secretory function. Type B cells underwent meiosis and subsequently developed to typical malacosporean myxozoan spores filling the internal cavity of the 'worms'. External tubes were formed during capsulogenesis in 'worms' from Fredericella sp. Tetracapsula bryozoides is synonymised with Buddenbrockia plumatellae and a new genus is proposed for Tetracapsula bryosalmonae.

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