ABSTRACT
The fi shes are the largest vertebrate group with something over 28,000 described species. The majority (around 16,000 species) are marine but around 12,000 species are found only in the freshwaters found on continental and island landmasses. These freshwaters are found as steams, rivers, lakes and ponds and eventually all freshwater fi nds its way to the oceans in the global hydrological cycle. During their course from deposition as rainwater, to eventual arrival at the oceans, some freshwater penetrates the surface to fl ow through various types, and various depths, of subterranean habitats. These vary from the tiny interstices between sand grains, through caves enterable by man, to vast groundwater bodies at great depth below the surface. Evolution has been at work within these subterranean places and animals which have found themselves in the darkness have adapted and evolved. They have evolved a set of very distinctive characteristics typical of, and found only in, subterranean environments. These are found in all taxa, vertebrate and invertebrate, which are now obligate inhabitants of the subterranean (or hypogean) realm. Most obviously these animals loose their eyes and any melanin and carotenoid pigmentation. They very often appear white in colour, or even transparent. These obvious losses have lead to the oft-used phrase “blind white cave animals”. In addition to these losses are a set a gains which are adaptations to life without light and most often very food-poor conditions. For example, the limbs of Crustacea are elongated, the lateral
line of fi shes is expanded, and the sensory setae of insects are lengthened. There are parallel behavioural, physiological and neurological alterations all aimed at survival in the hypogean. All of these changes are referred to as troglomorphic. The relative degree of troglomorphy is related directly to the time of evolution – the phylogenetic age – of the particular taxon in question (see Porter et al. 2007 and Trajano 2007 for discussion of this point). Out of all 12,000 species of freshwater fi sh only a very few, currently just over 150, have successfully colonised the hypogean. The number known is climbing fast but it is probably not likely to pass 1000 species. The purpose of this opening chapter is to set out, in broad terms, the overall patterns of biodiversity in subterranean fi shes and to examine their distribution at global (biogeographic) scale. It sets the scene for the more detailed chapters making the main body of the book. There is a large literature (of more than 2500 publications) on all aspects of the biology of subterranean fi shes. The most recent synthesis of this literature is Proudlove (2006) which provides detailed accounts for 104 species and includes the complete bibliography to 2004. Wilkens (2005) is a highly readable account covering all aspects of their biology. Similar, though briefer, accounts are provided by Proudlove (2004) and Romero (2004). Romero (2001) is a volume similar to this one with a set of contributed papers covering many aspects. The chapters by Weber (2000) and Weber et al. (1998) are both excellent syntheses. Though now very out of date, and out of print, the book by Thines (1969) is well worth seeing.
