SynopsisSix Barbus species were sampled monthly in two river systems of SW-Sri Lanka, over a 15 month period . Sex ratio, size at maturity, maximum and modal sizes, distribution of egg sizes in mature ovaries and individual fecundities were measured . Seasonal changes in size distributions, sex ratio, frequency of female gonadal stages and the gonado-somatic index (GSI) were analysed on the basis of the monthly samples . Seasonal cycles in reproductive activity were assessed, using mainly the two last-mentioned parameters and the appearance of young in the populations . It was found that three of the species (B . bimaculatus, B . cumingi, B . vittatus) tend to concentrate their reproductive activities in certain months of the year, roughly coinciding with the seasonal peaks of precipitation . The other three (B . nigrofasciatus, B . dorsalis, B . titteya) spread their reproductive efforts more evenly through the year . Sexual dimorphism with females bigger, relatively early maturation of males, high maximum GSI values, a distinct `final mode' of large mature ova in the eggdiameter distribution, and a comparatively steep increase of fecundity with body weight or gonad weight were found to be more or less consistently associated with seasonal reproduction .

Spatial distribution and seasonal variation in densities of the invertebrates were investigated for a year in three stream pools of a South Indian river. The effects of season, substrate type and water depth on the distribution were analyzed. Substrate type and season influenced the invertebrate distribution the most. Leaf packs harboured most of the organisms followed by macro-algal substrate and sand. The lowest densities were observed on rocky substrates and in the water column. Rocky substrates in shallow water supported higher densities of total invertebrates than deeper areas. Chironomid larvae dominated all benthic substrates throughout the year. Of the 19 invertebrate taxa studied, 6 showed no seasonality in densities, and most of the rest showed their highest densities in the pre-monsoon period and lowest in the SW monsoon or post-monsoon periods. However, in two of the three pools, the densities of total invertebrates were highest during the post-monsoon period with secondary peaks in the pre-monsoon period.

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It is claimed that certain processes of individual behaviour and of interaction between individuals run parallel. Such parallels are seen along three axes: antagonismcoordination, constriction-expansion and neutral-play-like.Characteristics of ritualized behaviour and play are analysed and the two categories of behaviour are compared in detail. They are shown to differ largely in degree of expansion. They also differ along the antagonism~coordination axis. Both are play-like.A theory comprising both ritualized behaviour and play is attempted. The concept of field interaction is introduced. The physical theory of electro-magnetism is used as a conceptual model.Positive and problematic aspects of the theory are discussed: Is the behavioural field polarised or not? The relationship between aggression and play-like bebaviour is considered, both as a motivational problem and in phylogenetic perspective. Some limits to the theory are indicated. Relationships of the theory to some other theories are briefly discussed. I. Parallels between internal and interactive processes: Antagonism, coordination, expansion and constrictionClassical ethology views behaviour of the individual animal ~ as a product of internal and external stimuli. Although internal and external influences are seen to interact, they are considered as in essence separate categories. I think it may add to the understanding of certain kinds of behaviour if the existence of a certain continuity between the internal and external events is acknowledged in the sense that internal processes and interactions with the outer world run parallel.

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