Corydoras, The Boys and the Girls

Author: Ian Fuller

I suppose the most asked question I get from Corydoras keepers is, 'How do I sex them'. A simple enough question you may think but the answer is never quite as easy as that. There are several factors that need to be looked at, with a lot depending on the species involved.

There are three main areas of difference, Body, Fins and Colour. When buying new stock it can often be a very difficult task to determine the sexes, freshly imported fish can be especially difficult depending on their overall condition on arrival. Newly imported fish are often under fed, having been kept in holding tanks awaiting dispatch, sometimes going for weeks on end without any food at all. In such cases it is a good idea to buy at least six specimens and condition them up. Generally speaking Corydoras are hardy little creatures and it does not usually take very long to get them into reasonably good condition, making it far easier to determine the sexes.

Taking each of the three areas in turn: -

Body:

Firstly before I go into body shape in detail, it is reasonably safe to say that the female of a species grows a little fuller and longer in the body than males. Sometimes the difference can be as much as twenty percent. C. paleatus, C. aeneus C. hastatus and C. pygmaeus are prime examples.

Looking at body shape there are two views to consider, the side profile and the overhead. Starting with the side view a female should have a greater body depth between the dorsal and ventral fins and the belly area its self should also look a little plumper. When viewed from above the widest point of a male's body is at the point immediately behind the insertion of the pectoral fins. The widest point of a females body is at a point slightly forward of the ventral fins. (Tip) When buying new unfamiliar stock, politely ask the shopkeeper to place several specimens in a container so you can view them from above. This way it is far easier to see the true body shape than when they are swimming around in a tank. It would not be such a good idea though if the shop were very busy, then I would select at least three small and three large specimens.

The picture below shows a male C.metae from the side:

In this picture you can clearly see the difference in body shape between the female C.melanistus at the top of the picture and the male below.

Fins:
The second area of difference to look at is the fin shape, there are a few species that fin shape and length are obviously different, two examples of this are C. macropterus and C. barbatus. There are other species where there are virtually no discernable differences in fin shape at all some of the long nosed species tend to fall into this category, with C. coriatae and C. vittatus being two examples.

As general rule it is the ventral fins that will show any variance between the genders, even when all of the other fins look identical. The ventral fins of males tend to be elongated and pointed in varying degrees depending on the species females on the other hand have fins that are more rounded and fan shaped. (Tip). Select three fish that have the most pointed fins and three that have the most rounded, ask for them to be put into separate bags, where they can then be compared. Again it is easier to view the lower fins in a clear plastic bag that in the stock tank.

Colour:

There are a number of species where the sexes can easily be distinguished by colour alone most of these belong to the so-called elegans group, with males usually showing brighter more intense colouring. C. elegans. There are one or two species from this particular group, where males and females have colour patterns that are so different it would be very easy to mistake them for two separate species. C. undulatus (Tip). When buying, if you are in any doubt buy equal numbers of both colour forms and if affordable, five or six of each, this will give the best chance of acquiring both sexes if they do turn out to be separate species.

Secondary dimorphism:

There are a number of species where males show secondary dimorphism, which takes the form of odontodes (bristles) developing on the pectoral fin spines. It is most, if not all the members of what used to be referred to as the 'acutus group' that develop these growths, they are long snouted species such as C. acutus, C. blochi, C. fowleri, C. semiaquilus, C. stenocephalus and C. vittatus to name but a few. These growths are usually most evident and develop to there extreme when the males are in their best breeding condition and are trying to coax females into breeding. At other times of the year, which is usually after the breeding season is over the odontodes (bristles) may still be visible but will be greatly reduced in some species they may disappear altogether. Where there are a group consisting of several males, it will almost certainly be the dominant male that will shows the most prominent growth of bristles. There are times when a male will suddenly loose his bristles, almost over night. Water conditions or a sudden decline in health putting the fish under stress can be the cause the bristles reduce in size and may even disappear altogether, regenerating when the conditions are favorable, or more usually at the onset of the next breeding season.

There are three species that instead of growing odontodes (bristles) on the pectoral fin spines, the males grow cheek bristles on the sides of the head. These are permanent growths and as far as I have observed do not show any exaggerated growth at breeding times. The four species are C. barbatus, C. kronei, and C. macropterus.

One species, C. geoffroy is unique in that it has both permanent cheek bristles and seasonal growths on the pectoral fins. This is a fish originating from Surinam and is seldom seen in the hobby.

Aquarium

Koi & Pond

Reptiles

Dogs

Small Animals

Cats

Birds

Books

Corydoras, The Boys and the Girls

Other articles by Ian Fuller