The Lisbon arched-mouth nase

Photo by Hugo Gante.


The Lisbon Arched-mouth nase (or the Lisbon nase), Iberochondrostoma olisiponensis, was first discovered in 2006 from two small tributaries of the lower Tagus River basin. The species was formally described in 2007, and can be identified based on the combination of the following traits:

  • terminal mouth, i.e. pointing forward;
  • no barbels in the mouth;
  • snout shorter than eye diameter;
  • pelvic fins long, reaching the anus in females but extending beyond the anal fin insertion in males;
  • scales large and fewer in number compared to other species of arched-mouth nases of the genus Iberochondrostoma;
  • insertion of paired fins (i.e. pectorals and pelvics) and anal fin without an orange/red coloration typical of arched-mouth nases of the genus Achondrostoma.

Despite its recent discovery, phylogenetic studies established it as one of the oldest species of its genus, with an estimated age of 10 million years ago.

The Lisbon nase occurs in floodplain areas, irrigation canals and ditches, as well as in natural rivers. Previous surveys conducted in the lower Tagus tributaries confirmed the presence of the Lisbon Arched-mouth nase in only three small sub-basins, namely rivers Trancão, Maior and Muge. The populations were also found to have low densities and to be highly fragmented, likely with little or no contact among them. This situation is of conservation concern since replenishment from neighboring populations in the event of localized depletion is limited or even impossible. Localized depletion may have already occurred in two other tributaries where the species has not been recently detected but historical records indicate its former occurrence. Currently, a decreasing trend may be occurring in the population of Rio Trancão, where the species’ abundance is known to be declining since the late 1990s (Gante & Santos, unpublished data).

An additional threat to the integrity of the Lisbon arched-mouth nase was detected in 2010, when hybrid individuals were found in Rio Maior. Further hybrids were subsequently found in Rio Trancão. Hybrids have part of their genetic makeup belonging to the Lisbon nase and the remainder part belonging to the Portuguese arched-mouth nase Iberochondrostoma lusitanicum, a related species whose distribution overlaps. The reason behind the presence of hybrids is not yet known but may be associated with the low population densities of the Lisbon nase compared to its more abundant relative. Continued hybridization between the Lisbon nase and Portuguese arched-mouth nase could lead to the genetic dilution of the former species and compromise its future as a distinct species.

But there are several other potential threats to the short-term survival of the Lisbon arched-mouth nase associated with human-induced pressure. Current areas of the species’ occurrence are presently under intensive agricultural practices and continued human development pressure (e.g. industrial, domestic and agricultural pollution, introduction of exotic species, and land reclamation). Such practices may have resulted in profound changes on the species’ natural habitat, which are especially problematic during extreme drought events when overall degradation of habitat quantity and quality is more severe.

The extremely restricted distribution range of the Lisbon nase coupled to the current geographical isolation of its few populations, as well as the several potential threats associated with human practices have raised concern for its survival. In 2012, the International Union for the Conservation of Nature (IUCN) classified the Lisbon arched-mouth nase as Critically Endangered. Thus, urgent conservation efforts are needed.


last updated: January 22 2016


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