Mudminnows in Hudson River hybridising

Mudminnows in the USA's Hudson River appear to be hybridising scientists have said.

The Eastern mudminnow, Umbra pygmaeae and the Central mudminnow, Umbra limi, only occur together in the Hudson River and now scientists have shown that the two fish are breeding with each other, as well as their own species.

Robert Schmidt from Simons Rock College and Robert Daniels from the New York State Museum first became aware of the fish when they studied a collection of mudminnows caught in Manitou Marsh, a fresh to slightly brackish tidal marsh in Putnam County, New York.

There are currently only two known Umbra species in the Eastern United States, but these fish didn't look like either limi or pygmaeae - they were hybrids that fell part way between the two fish species. The scientists reckon that hybridisation must have been occuring in the fish for several decades.

Historically, anyone wanting to tell the difference between Umbra pymaeae and Umbra limi used an identification manual that looked at the place in which the fish were collected and their colour pattern. Umbra pygmaea usually has stripes, while limi has blotches or vertical bars.

However, since there are now hybrids around, and the range of both species appears to have changed since the fish were last studied, identification has become more difficult for ichthyologists. What's more, the original type specimens used to define the characteristics of the species have been lost so Schmidt and Daniels have had to designate new type specimens called neotypes and outline in greater detail how to tell the separate fish apart.

It's illegal to keep either of these Umbra species in the UK without the necessary licence from Defra. For more details on the North American species that are licensed in the UK see "Are your fish illegal?".

For more details on the Mudminnows see the paper: Schmidt, RE and RA Daniels (2006) - Hybridization in Umbridae in the Hudson River, New York, with Designation of Neotypes for Umbra limi and Umbra pygmaea. Zootaxa, 1113: 1-20 (2006).