L.G. Willoughby

Viable Saprolegniaceae spores on the epidermis of the salmonid fish Salmo trutta and Salvelinus alpinus.

Techniques are described for monitoring Saprolegniaceae in hatchery pond water and on the epidermis of hatchery-reared brown trout Salmo trutta L. and char Salvelinus alpinus L. The water supply to the hatchery occasionally includes spores of Saprolegnia type I (taxonomically in or near the S. diclina — S. parasitica complex), which are responsible for the outbreaks of Saprolegnia infection that occur in the hatchery. The saprolegniaceae spora in the pond water increased from 200 spores/1 or less to a maximum of 22 200 spores/1 when infected fish were present in the water. This increase was solely due to Saprolegnia type I spores. Experimental applications of high concentrations of secondary zoospores of Saprolegnia type I to trout and char indicate that although the majority of spores are removed or inactivated during the first 24 h of subsequent quarantine in clean water, a small but significant number of viable spores remain on the body surface. These findings are discussed in the light of our knowledge of Saprolegnia infections of fish and in relation to the possible defence mechanisms of fish to pathogenic fungi.

The Ecology of some lower fungi at Esthwaite water

At Esthwaite Water (English Lake District) the distribution of saprophytic Chytridiales, both above and below the lake margin, was studied. Certain species proved to be essentially terrestrial, others essentially aquatic. Species of both categories were obtained from the lake margin, and there was strong evidence that this situation was an exceptionally favourable one for the growth of saprophytic chytrids.

Fine structure of secondary zoospore cyst cases of Saprolegnia isolates from infected fish.

The secondary zoospore cyst cases of Saprolegnia isolates from 34 infected fish were examined by means of the transmission electron microscope. Isolates identified from their sexual characters as S. diclina Humphrey Type 1 produced secondary zoospore cysts each bearing bundles of long, hooked-hairs whereas isolates of S. diclina Type 2 produced secondary zoospore cysts with short, single, hooked-hairs. A typical saprophytic strain of S. diclina Type 3, also produced secondary zoospore cysts with short, single, hooked-hairs. Isolates from infected fish which had hitherto been impossible to identify because of their failure to develop sexual organs produced secondary zoospore cyst cases similar in structure to those of S. diclina Type 1. Within the variation of secondary cyst characteristics for S. diclina Type 1 and the sexually sterile Saprolegnia isolates, significant relationships were found between the length of the hooked-hairs, the number of hooked-hairs/bundle and the distance between the bundles of hooked-hairs on the cyst. The apparent trend towards an increase in length of these hooked-hairs and their aggregation into bundles is discussed in relation to the parasitic mode of life of these fungi.

Experimental studies on uptake and interaction of spores of the Saprolegnia diclina-parasitica complex with external mucus of brown trout (Salmo trutta).

The uptake and survival of spores of the salmonid fish pathogen Saprolegnia parasitica and of the saprophyte S. diclina in the surface mucus of brown trout was investigated. Spores of both the pathogen and saprophyte accumulated to a similar extent in the mucus, although the pathogen attached more rapidly. Spores of both species were rapidly shed from inoculated fish placed in water containing very few background spores, but after 24 h viable propagules of the pathogen were retained on the fish in greater numbers than those of the saprophyte. The natural defence of the fish against Saprolegnia has several components, one of which is the purely physical removal of spores in shed mucus. There is also evidence for the presence of a morphogen in the mucus which results in growth inhibition and abnormal differentiation of colonies after their transfer to the assay media. In addition, colonies with granular lysed contents, which had developed over a 24 h period in situ in the mucus, often had fish cells attached to them, suggesting that a cell-mediated defence mechanism also operates. In water that previously contained fish, secondary zoospores of the pathogen germinated whilst saprophyte spores simply encysted and did not germinate. The possible significance of this, in the challenge experiments on fish, is discussed.

Zoospore germination of Saprolegnia pathogenic to fish.

Secondary zoospores of twenty-two different isolates of Saprolegnia were incubated in sterile lake water at 10°C. On the basis of encystment and growth, isolates could be segregated into two groups. The first contains ten isolates, all of which were obtained from diseased fish and comprises eight S. diclina Type 1 and two Saprolegnia sp., the latter with a secondary zoospore cyst ornamentation which relates them to S. diclina Type 1. All these isolates exhibited ready encystment and growth in the first 18–27 h of incubation. The second group, of twelve isolates, includes the typical freshwater saprophytes S. diclina Type 3, S. ferax and S. hypogyna. Encystment and growth did not occur in sterile lake water in this group and in ten of these twelve isolates zoospore motility persisted for 18–27 h. When tested in natural lake waters there was a similar disparity in performance of the two groups of isolates. When they were tested in fish hatchery effluent water, S. diclina and Saprolegnia sp. comprising the first group of isolates showed enhanced growth which was attributed to the presence of additional nutrients, but isolates of the second group showed no detectable response. When there was germination in water, the percentage occurrence of indirect germination was noted and it was generally related to the total length of mycelium obtained. It was always high in fish hatchery effluent water. Using S. diclina, isolate LIC 10, total growth and growth patterns in fish mucus and in full-strength synthetic nutrient solution were compared and in the former medium there was an outstandingly high occurrence of indirect germination. However, indirect germination does not occur exclusively in waters of natural derivation or in fish mucus and it can be induced in synthetic nutrient solutions when these are manipulated, especially by dilution. The findings are discussed in relation to the infection of fish by Saprolegnia.

The ecology of some lower fungi in the English Lake District

Using methods previously employed at a single site at Esthwaite Water the author studied the occurrence of saprophytic Chytridiales and Blastocladiella britannica in soil and lake-mud collections from various sites in the English Lake District. Once again distinct differences were demonstrated between the chytrid floras of these two environments.

Two new saprophytic chytrids from the Lake District

Entophlyctis lobata sp.nov. and Chytriomyces poculatus sp.nov. are described and illustrated.

Studies on soil chytrids: III. On Karlingia rosea Johanson and a multioperculate chytrid parasitic on Mucor

From a natural population of Karlingia rosea Johanson in which several different types of development from the zoospore were represented, nineteen separate monosporangial cultures were made. Potassium tellurite was found to be an effective bactericide in this work. The mode of development in monocentric eucarpic chytrids is of interest as it has previously been proposed that this should be used as a major criterion in classification. All the cultures proved to be of a single type, in which there was even expansion of the zoospore cyst, and this characteristic remained constant through several generations. The theory is expressed that there may be several distinct lines of descent in this species, each different in regard to initial development. Despite slight differences in regard to pigmentation and morphology, the culture material generally agreed well with Johansons account. Further thalii which could not be cultured were also obtained. These were considered to be specimens of K. rosea, but of a type not previously described, with the sporangium ornamented with short setae. A multi-operculate chytrid parasitic on Mucor is also described.

Chitinophilic chytrids from lake muds

Obelidium megarhizum sp.nov. and Podochytrium chitinophilum sp.nov. are described. Phlyctochytrium aureliae Ajello is a new British record. All three were obtained by baiting lake mud with chitin.

Studies on soil chytrids: IV. A chytrid parasite of rotifer eggs

A chytrid very similar to Rhizophydium zoophthorum (Dangeard) Fischer is described. It occurred as a parasite on the eggs of the rotifer Lecane (Monostyla) arcuata Bryce.