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Freshwater sharks and rays

2015; Elsevier BV; Volume: 25; Issue: 20 Linguagem: Inglês

10.1016/j.cub.2015.06.051

1879-0445

Luis O. Lucifora, Marcelo R. de Carvalho, Peter M. Kyne, William T. White,

Fish Ecology and Management Studies

Hang on, I thought sharks and rays only live in the ocean? That's true for about 95%. However, there are species of elasmobranchs (sharks and rays) that occur regularly at low salinities, often beyond the tidal reaches of the sea. These make up around 5% of living elasmobranchs (roughly 56 out of 1154 described species). Species that are confined to freshwaters are termed obligate freshwater species, and comprise all the freshwater stingrays (family Potamotrygonidae) and several stingrays (Dasyatidae). Species that can tolerate a wide range of salinities, from freshwater to brackish and/or marine waters, are termed 'euryhaline species'. Euryhaline species include sawfishes (Pristidae), several whaler sharks (Carcharhinidae), one skate (Rajidae), and a number of stingrays (Dasyatidae). They range in maximum size from only 20–30 cm disc width in several freshwater stingrays, to at least 6.5 m total length in the Largetooth Sawfish (Pristis pristis). Was the colonization of freshwater a unique event? The invasion of and adaptation to freshwater environments has occurred independently many times in elasmobranch evolution. The mostly late Paleozoic, eel-like xenacanth sharks, for instance, occurred in freshwaters and were perhaps euryhaline, whereas the Eocene Green River stingrays (in present-day Wyoming) were true freshwater species. The modern obligate freshwater stingrays of Africa and Southeast Asia (dasyatids) and South America (potamotrygonids) result from multiple independent colonization events. The potamotrygonids, known from four genera and 28 species (with about 10 known undescribed species), are the only group to have significantly diversified in freshwaters from a common marine ancestor. They represent a separate and more ancient freshwater colonization than dasyatids, dating from at least the early Eocene, some 50 million years ago. How do they cope with freshwater? Elasmobranchs keep their internal environment close to equilibrium with sea water by having a high concentration of urea in their blood and excreting salts through a specialised organ, the rectal gland. When entering freshwater, euryhaline elasmobranchs are able to excrete urea and take salt from the environment through their gills. Obligate freshwater elasmobranchs have lost the capacity to keep high concentrations of urea in their blood, and their rectal gland has atrophied. They cannot concentrate urea in their blood when exposed to increased salinity, as they lose urea through their gills. Where do they live? Euryhaline elasmobranchs can be found in most major tropical and subtropical river basins, even thousands of kilometers upstream (Figure 1). Obligate freshwater elasmobranchs are restricted to the major drainages of South America (e.g. the Amazon, Río de la Plata, Orinoco, Magdalena, Maracaibo, Parnaíba, and the Guianas), Africa (Congo, Niger, Sanaga, and Cross), and southern Asia (Mekong, Irrawaddy, Maekhlong, Chao Phraya, and Bornean, Sumatran and peninsular Malaysian rivers; Figure 1). They inhabit lakes too, but, especially for the larger species, their main habitats are large, floodplain rivers. Euryhaline species move extensively along rivers and between rivers and the sea. There is one known instance of a transcontinental introduction and subsequent invasion by an obligate freshwater elasmobranch, as a by-product of the aquarium trade — the South American Potamotrygon motoro in Singapore. Are they important to humans? People tend to be afraid of freshwater elasmobranchs. Sharks have been involved in some biting incidents in rivers, and stingrays are notorious for accidentally stinging bathers in rivers. In South America, stingrays are killed, maimed or scared away from river beaches where they aggregate. Increasingly in recent years, the larger species have become a popular target of big-game anglers in Asia and South America, representing a source of income for many local guides. Smaller species, given their unusual and colourful appearance, are popular in the international aquarium trade. Artisanal exploitation for human consumption as food occurs in some areas. Freshwater elasmobranchs are important icons in indigenous cultures of Australia, South America and elsewhere, appearing in legends and tales. Why are many freshwater elasmobranchs at risk of extinction? Sharks and rays are well known for their inherent vulnerability to population decline and collapse. This is a consequence of their 'slow' life history — late age at sexual maturity, long lifespan, low fecundity, and low levels of natural mortality; all of these factors result in a susceptibility to population depletion from overexploitation and a limited ability to recover once depleted. In addition, freshwater elasmobranchs suffer from an elevated exposure to threats in their more restricted habitat. Over a third of all obligate freshwater species are threatened with extinction; amongst the euryhaline species, the sawfishes and river sharks (Glyphis) face an extremely high extinction risk (Figure 1). Unregulated fishing, high value products (such as fins), susceptibility to capture (e.g. the sawfish's rostrum is easily tangled in nets) and habitat degradation combine to threaten species. For South America's freshwater stingrays, risk comes from habitat degradation, persecution and the international aquarium trade. What else do we need to know about freshwater elasmobranchs? Occurrence in remote habitats in often poor and under-developed regions and countries has limited research. Therefore, we know very little about these fascinating creatures. Basic biological information, such as lifespan and fecundity, is lacking for the vast majority of species; movement patterns, migrations and critical habitat requirements are largely unknown. Assigning levels of fishing and trade that a species can sustain relies on this basic information; as does assessing the impacts on species from developments, such as dams or increased water extraction. The International Union for Conservation of Nature ruled that a lack of information prohibits assigning an accurate conservation status to over half of all obligate freshwater elasmobranchs. What is the future of freshwater elasmobranchs? Only two species of obligate freshwater elasmobranch can be considered to have a secure status. For threatened species, national protection helps but is often limited to developed countries. Even if implemented in Asia, Africa or South America, enforcement and compliance are ongoing issues. There is also a role for international treaties; the listing of sawfishes on the Convention on International Trade in Endangered Species (CITES) prohibits international trade. Consideration is being given to list South America's freshwater stingrays, which would be a positive step. The task ahead is to first of all try and obtain the knowledge to understand their current status and sustainability and, secondly, to secure populations, is considerable.