Review of the Organismal Biology of Hill Stream Loaches

Hill stream loaches are a group of fish that inhabit fast flowing shallow freshwater. The family has radiated over Asia. For some species their range is limited to single catchments; they provide an excellent example of biogeographical speciation on multiple scales. Hill stream loaches have a range of adaptations which help them exploit environments where competitors and predators would be washed away. They have streamlined bodies and keeled scales reminiscent of Mako sharks and potentially many other as yet undiscovered drag reducing features. They adhere to rocks, crawl over shallow films of water, glide over hard surfaces using ground effects and launch into currents to attack prey or evade predation. They offer a test of modern approaches to organismal biology and a broad range of biomimetic potential. In this paper we analyse what behaviour is associated with their physical adaptations and how this might relate to their evolution and radiation. Our intent here is to review information that is presently available but also to derive new insight through combination and statistical analysis of existing material. We also use our own observations and critically argue several contradictory, or poorly supported assertions in the existing scientific literature. Our conclusion is that, although extensively studied, there remain major gaps in knowledge about their organismal biology. We summarise the research opportunities in the form of hypotheses that are worth testing.