Duphniu (Crustacea: Cladocera) reproduce by cyclical parthenogenesis in which the sex of offspring is environmentally determined. Although numerous studies have demonstrated that factors such as crowding and short-day photoperiod stimulate male production, there is limited information on variation in allocation to male and female offspring for any species of Duphniu. The present study assessed the presence or absence of male production in 96 isofemale lines (clones) from each of eight populations of Duphniu pulex. An average of 37% (range l&51%) of clones failed to produce males under crowded conditions in the laboratory. A subset of 14 of these non-male-producing clones also failed to produce males under short-day photoperiod (8L: 16D). Three male-producing clones were within-clone mated as well as crossed to three non-male-producing clones to study the inheritance of the failure to produce males. The average frequency of non-male-producing F, progeny was significantly higher (58%, N = 486) among the outcrossed progeny than the inbred progeny (5%, N = 86). In addition, when sixteen of the male-producing outcrossed progeny were withinclone mated, only 7% (N = 106) of the resulting F, progeny failed to produce males. These results are consistent with a genetic basis for the absence of male production. Average survival of the progeny from the nine outcrossed matings was more than twice (67%) that of the inbred progeny from the three withinclone matings (30%) suggesting that within-clone mating would result in significant inbreeding depression. We present a model that suggests that even low levels of inbreeding could allow non-male-producing females to be maintained in a population. The co-occurrence of non-male-producing females and females that produce both males and females in Duphniu p&x bears a similarity to the gynodioecious breeding system found in some plant species. 560Innes and Dunbrack

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Filmed observations of the feeding behaviour of juvenile coho salmon (Oncorhynchus kisutch) cruising in a large aquarium were used to determine the three-dimensional shape of their reaction field, and to estimate the cross-sectional area of the effective volume searched for prey (scanning area). Reaction distance was greatest above the horizontal visual plane and ahead of the transverse visual plane, and this corresponds to the feeding behaviour of fish observed in the wild. The scanning area for a given size of prey was obtained by plotting all captures as projections on the transverse plane. This distribution of points was then divided into a series of concentric bands characterized by prey attack probabilities, which were then used as weighting factors in the final calculation of the scanning area. Our results and techniques are discussed in relation to previous studies of fish reaction fields, and a simple method of estimating scanning area for salmonids is suggested.

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