There are many more Corydoras out there...

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While we know that there are many species of Corydoras catfishes, it seems that we can expect many more.

The results of a study, published in a recent issue of the journal Nature, by Markos Alexandrou from Bangor University and colleagues from Brazil and Scotland show that communities of Corydoras found in the streams of South America that share an identical colour pattern actually consist of three or more species.

Alexandrou and coauthors carried out a comprehensive study of the molecular phylogeny of the genus Corydoras using five mitochondrial and one nuclear gene fragments (3424 base pairs) of 425 species Utilising this molecular phylogeny as its baseline, the study establishes for the first time that many species are Müllerian mimics of each other. 

In Müllerian mimicry, a number of species that posses a similar anti-predator attribute (toxins in the sharp fin spines in the case of these catfishes) evolve to share the same colour patterns to warn off predators.  The study further found that each community community of similar looking catfish consisted of species with a similar colour pattern could belong to different genetic lineages.  This discovery suggests that the true diversity of Corydoras is much higher than previously recognized.

How can so many similar-looking fish live together without fiercely competing for the same resources?  Previous research has suggested that the benefits accrued from mimicking each other outweigh the negative effects of competing for the same resources when Müllerian mimics occupy the same ecological niche; this allows for many similar-looking species to live in the same habitat. 

However, the study finds that this is not true for the Corydoras communities studied, and that the fishes were able to coexist because they utilised different resources in the same habitat.  The authors analysed the shapes of the fishes (to identify ecologically relevant differences), as well as carried out stable isotope analyses (of carbon and nitrogen) to find out what the catfishes were eating.

The analyses of nitrogen isotopes are more informative in that nitrogen-15 accumulates in the body the higher up the food chain you go.  Thus, a catfish that ate primarily algae would have a lower nitrogen-15:nitrogen-14 ratio than one that ate primarily invertebrates.

The study found that similar-looking Corydoras were able to coexist in communities because they were utilising different ecological niches, thus avoiding competition for food. 

This figure was as large as 92 per cent of the communities studied.

Data from the analyses of the fish shapes told a similar story, with species living together differing mainly in their head shapes. As longer snouted species generally feed lower down in the food chain than shorter snouted ones, this is further evidence that the Corydoras are occupying different ecological niches.

According to coauthor Claudio Oliveira: "Besides the unknown biodiversity and interesting evolutionary system revealed by this study, it reinforces the urgent need to preserve and manage South American environments to avoid the loss of many species yet to be discovered and described."

For more information, see the paper: Alexandrou, MA, C Oliveira, M Maillard, RAR McGill, J Newton, S Creer and MI Taylor (2011) Competition and phylogeny determine community structure in Müllerian co-mimics. Nature 469, pp. 84–88.