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A study was made on morphological and cytological features of four Rivulariaceae strains from marine, partially saline and freshwater environments using three versions of saline and freshwater media (standard. - Fe. low P). A brackish Anabaena sp. was used for comparison. Subculture to - Fe medium led to marked changes in morphology and cytology. These were (base to apex): a series of new heterocysts apical to the original one. Intra-thylakoldal vacuolation. Lack of cyanophycin granules, presence of polyphosphate granules, pale colour, brown sheath, short hair and low hairiness. With low P medium, the changes were the opposite. Hairs did not form in Calpthrix D253 in saline medium, whether the cultures were in - Fe or low P medium. Downshift from saline to freshwater medium led to the synchronized development of hairs in most trichomes. Whereas the reciprocal upshift led to loss of hairs. The Fe content was studied in three Rivulariaceae and Anabaena in batch culture in high and low P media, which eventually led to Fe- and P- limitation. respectively. The content reached a maximum by 24 h in all strains, decreasing subsequently from 1.80 - 0.06 % dry weight. Siderophore production was tested in four Rivulariaceae and Anabaena in saline and freshwater media. Two Calothrlx strains and Anabaena produced a siderophore in freshwater medium. Influence of Fe status on nitrogenase activity was tested in two Rivulariaceae and Anabaena in saline and freshwater media during batch culture. Peaks of nitrogenase activity in the former came three to five days after the Fe peak, by which stage Fe content had dropped to within the range 0.6 - 0.3 % dry weight. In contrast, peak activity in Anabaena came within a day of the Fe peak. Addition of Fe to Fe-limited cultures led to rapid increase in activity. The significance of these data on morphology, cytology and physiology and possible interactions between Fe- and P-limitation are discussed.

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Diplostomum metacercariae were collected from three sources (a) the lenses of sticklebacks from Culter Compensation Dam. (b) the lenses of rainbow trout from Mill of Cantray trout farm. (c) the humours of rainbow trout from Mill of Cantray trout farm. The metacercariae from the lenses of Culter Dam sticklebacks produced infections lasting 12-15 weeks with an egg production of 6,000-10,000 eggs per day, in gulls. The metacercariae from Mill of Cantray trout however, produced short Infections of only 3-4 days with an egg production of less than 1,000 eggs per day, in the same bird hosts. All metacercariae from all sources appeared morphologically identical at the light microscope level. However, the body length and width dimensions were significantly different between all 3 types of metacercariae. Using the available taxonomic keys, all three sources of metacercariae were identified as D. spathaceum. The life cycle of D. spathaceum was established in the laboratory using metacercariae from Culter Dam sticklebacks and maintained throughout the study. Parasite egg production was recorded from herring gulls four days after infection with metacercariae from Culter Dam sticklebacks. Egg output rose to a maximum of 10,200 eggs per day after 4 weeks and then oscillated between 6,000 and 10,000 eggs per day for the. following 9 weeks, after which egg production rapidly declined. 7. Mlracidia hatched from eggs incubated at 29°C after 8-11 days. 8. At 14°C, 29% of the Initial number o£ cercarlae successfully established In the lenses of exposed rainbow trout. 9. An epidemiological survey of D. spathaceum was carried out on rainbow trout from Mill of Cantray trout farm and 3-spined sticklebacks from Culter Dam over a 30-month period. 10. The Infection period at both sites was normally between May and September each year. Transmission from snail to fish did not occur when temperatures were below 10°C. 11. A low snail prevalence of patent infections in L. pereger (0-8%) was recorded at both sites throughout the survey. 12. During the summer of 1982 rainbow trout in Raceway 3 at Mill of Cantray became infected with 139 (humour) and 70 (lens) metacercariae. In April, 1983 the raceway was cleaned and the entire length treated with copper sulphate. This resulted in a 60% reduction in the numbers of metacercariae infecting trout during the following summer. 13. The prevalence of D. spathaceum metacercariae declined from 100% to less than 20% and the abundance of metacercariae per fish declined from 12 to less than 2 during the survey period. Despite this decline a pattern of seasonal variation in prevalence and abundance was observed in both 1982 and 1983. 14. Significant correlation coefficients between abundance of metacercariae per fish and length and weight of sticklebacks Indicated that abundance Increases with size. 15. Within the confines of a flume In which temperature, flow rate, and cercarlal concentration could be manipulated Independently, It was shown that (a) It is possible to control the Infection rate of fingerllng rainbow trout by the manipulation of flow rate. (b) the relationship between mean abundance of metacercariae per fish and cercarlal concentration is linear, with a regression coefficient of 0.99 (c) that no metacercariae are found in rainbow trout infected and maintained below 10°C. (d) that it is migration of cercariae which is inhibited at low temperature and not penetration or attachment. 16. Rainbow and brown trout do not produce circulating antibody at detectable titres in response to infection with D. spathaceum cercariae. 17. A significant difference occurred in the rate of infection of rainbow trout given weekly infections of D. spathaceum cercariae in winter and summer. 18. Rainbow trout injected with a suspension of dead cercariae acquired significantly fewer metacercariae when exposed to a challenge infection.

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