Sexual dimorphism in Bosmina: the role of morphology, drag, and swimming

Some Bosmina water flea species develop morphological antipredatory defenses, such as long antennules and a high carapace, but in Bosmina (Eubosmina) coregoni gibbera these traits are larger and more variable in females than in males. Here we propose that this sexual dimorphism derives from differential costs of hydrodynamic drag and selection for mobility in males. We tested this hypothesis by estimating drag of several Bosmina morphologies by using scale models sinking in glycerin of different concentrations and viscosities. Body forms included males, sexual and asexual females of B. c. gibbera, and males and asexual females of Bosmina (Eubosmina) longispina, a taxon with less variable body shape. For a given body length or body volume, male models had lower drag than models of sexual and asexual females, suggesting that males can swim 14-28% faster with the same energy consumption. Consistent with this conclusion, video recordings showed that males of B. c. gibbera advanced 55-73% farther than females in each swimming stroke. We conclude that hydrodynamic drag may have significant implications for swimming and evolution of sexual dimorphism in water fleas, and we suggest that males lack the defensive structures of females of B. c. gibbera (e.g., high carapaces) because competition over mates favors low drag.     

Towards water security: political determination and human adaptation crucial

Although high-level attention has been drawn to the escalating world water problems, few changes are noticeable in terms of actual management of the water resource. Politicians continue to be misled by the apparent simplicity of water. The environment-oriented community tends to focus only on problems arising from man-induced side-effects, and most people tend to take water for granted. The double challenge of providing adequate water supplies and meeting increasing food requirements of expanding populations, in addition to already existing water management problems, requires special attention. In addition, the intensifying pollution of water systems will have to be dealt with. Considerable human adaptation to physical realities as well as ingenious action are needed.
There is a brief discussion of the risk of hydrocide, a condition of serious water-quality degradation where available water is no longer sufficient, or cannot be used for the purposes needed. However, the main focus of the paper is on water quantity issues, highlighting the confusion arising from different methods of assessing water scarcity. Levels of regional water scarcity predicaments are discussed in terms of distinct regional clusters. Attention is drawn to the limitations of dry climate regions to achieve food self sufficiency, and the emerging need in many areas to import food; related trade and price aspects are also discussed. A distinction is made between efficiency of use and efficiency of allocation.
The need for a global ethic regarding upstream–downstream water-sharing is stressed, especially in cases of consumptive (evaporative) use of water for cultivating crops, which may deplete river flows available to downstream users. In conclusion, four key concerns are highlighted that call for global consensus.