An Appreciation of Corydoradinae AKA Cory Catfish
Corydoradinae is a subfamily of the family Callichthyidae. This is a diverse family of armored catfish also containing hoplo catfish and Callichthys, some oddballs that employ bubble-nesting as a means of spawning, similar to gourami and betta fish. All of these catfishes come from South American rivers, and will prefer soft, slightly acidic water (though temperature preferences depends on region). The subfamily Corydoradinae contains three accepted genera: Corydoras, Aspidoras, and Scleromystax. Contrary to popular belief, not all taxonomists agree on “Brochis” as a real genus within this group, and “Brochis” is currently synonymous with “Corydoras”. However, Brochis catfishes have more dorsal fin rays (having 10-18 rays as opposed to the usual 8-10 rays that Corydoras have) and tend to get significantly larger than Corydoras, so many still use the genus “Brochis” when referring to certain species. The number of dorsal fin rays has been used in the past to separate Aspidoras (along with a distinctive skull shape) from the original genus, so if taxonomists stop arguing, Brochis may one day be an accepted genus again.
Most Aspidoras barely reach 1” in length, and they have a slimmer (yet rounded) head and body shape compared to the taller bodied, bulky Corydoras. Scleromystax also have a more slender body shape, but grow much longer and have more pointed mouths than either Aspidoras or Corydoras. As mentioned before, Brochis is not an accepted genus, but I have marked a species below as such to show the differences between it and a normal corydoras (see B. sp. “CW136”). They have a tall, curved back (similar to a botia) and grow as long as most Scleromystax species (with 4” being the average maximum size), making them some of the largest species of the subfamily Corydoradinae. Corydoras, being the largest genus, still has a huge amount of diversity (even after being separated into multiple genera), so you’ll often notice that species are given a lineage number (ex. (ln9)) after the genus in some books and websites. These refer to what the species evolved from, and help break up a genera with such a large number of species. Lineages 7 and 9 contain most of the species frequently seen in the hobby (including C. aeneus, sterbai, and panda) and tend to have a classic corydoras shape (2-3” adult sizes, slightly pointed but not extended mouths, moderately tall body, etc.). Some of these lineages even have subclades that further break them down. To avoid bogging this article down with taxonomy, I’ll recommend Planet Catfish and Scot Cat as great sources for more information on specific lineages.
Corydoras are by far one of the most popular catfishes (right beside plecos) in the aquarium hobby, and for good reason. They’re for the most part diurnal, so they’re active during the day (unlike most other catfishes) and can be enjoyed when the lights are fully on. They turn substrate, will finish off uneaten food from other fish, and have a relatively low bioload, so many employ them as “cleaner fish”. However they are not pure scavengers, and do require their own food. They cannot solely live off of “leftovers”, and will rarely touch algae. In the wild, they feed mostly on insects, worms, and tiny crustaceans that they sift out of the substrate, so they do well on foods rich in insects or shrimp (like Bug Bites pellets or shrimp wafers). They’ll also benefit from frozen or live foods such as mysis shrimp and blackworms. Corydoras may also eat algae wafers and blanched vegetables that are given, but again this should not be a large portion (and especially not the bulk) of their diet. I know a lot of you already know this, but it’s a surprisingly common misconception that I’ve seen too many times to not mention.
One other myth that needs to be put to rest is that Corydoras cannot be kept on gravel. While sand is the best option, smooth gravel is also acceptable. Rough gravel can be abrasive to a fish that spends most of its time against the substrate, but it will not wear off their barbels. What it CAN do is cause minor nicks or abrasions on their barbels, which can become infected if the water quality is poor (which can lead to their barbels rotting off). Also, gravel is more likely to have uneaten food and decomposing matter become trapped in it, and can create an unsanitary environment for bottom feeders (which again can lead to barbel infections). However, this can easily be fixed with gravel vacs and regular water changes, as well as not overfeeding. In my opinion, Corydoras are far more active and fun to watch when they’re kept on sand, but keeping them on smooth gravel will not harm them given that the tank is maintained properly.
In the wild, Corydoradinae can be seen in shoals of hundreds, or even thousands. These fish find safety in numbers, and will likely be shy and reclusive if kept alone or in small groups. 6 is the recommended minimum, though striving for 10-20 or more individuals will result in much more interesting behaviors, along with more active, healthy fish. If a large group seems to “separate” in a tank or individuals are seen swimming alone, keep in mind that being a few feet apart is nothing in a large body of water like a river, and they are still benefiting from being near other Corydoradinae. Seeing other fish is an indicator that predators have not scared off/eaten their shoal, and that they are safe. Having lots of plant cover and hiding spots in the tank will increase their confidence as well. Some Corydoradinae will shoal with other species, but this is more out of necessity than preference. Most would prefer to have conspecific shoals, or at least a very similar looking species. An exception to the peaceful, shoaling nature of most Corydoradinae is Scleromystax species, which typically have very territorial males. Unlike Aspidoras and Corydoras, one should try to keep Scleromystax shoals with only 1 male (unless there is enough territory to support more).
Sexual dimorphism in most species of Corydoradinae is slight (with some exceptions) until the fish are sexually mature. The females will generally grow larger and have a more rounded belly than the males, and the two sexes are most easily discerned by viewing the fish from the top down. In some species, males will have a taller dorsal fin or different coloring as well (see C. sp. “CW111” and S. sp. “CW147” below), especially in those from the genus Scleromystax (although some Corydoras and Aspidoras show dimorphism like this as well). Like mentioned above, Scleromystax should be kept in pairs or harems with 1 male and several females, unless the aquarium has a large footprint. For Corydoras and Aspidoras, this is not necessary. In fact, when breeding Corydoras, it is recommended to have two males for every female in the tank so that the females have options (as they will likely breed with different males at different times, increasing the genetic diversity of the shoal).
Males will “present” themselves to the females when ready to breed by laying on their sides in front of the female (perpendicular to her). This behavior is known as “T-posing” (where the male is the top of a capital T and the female is the vertical line). In C. aeneus, it has been observed that, if the female accepts the male, she will latch her mouth to the male’s genital opening, and quickly pass his sperm (along with her eggs) through her body into a pouch formed by her pelvic fins against her stomach. She then goes and deposits the fertilized eggs, sticking them to a surface she deems fit (often on the leaves of plants, but also on driftwood, flat rocks, or the glass/equipment of an aquarium. There have not been scientific studies to definitively prove if this is how all species of Corydoras reproduce, but since T-posing has been observed in most Corydoras, it is assumed that this unique method of insemination is shared across the genus. When breeding, be aware that Corydoras will eat their own eggs and fry, and that eggs should either be removed and put in an egg tumbler (they can be gently scraped off of surfaces with a fingernail or a credit card). If active breeding isn’t the goal, but you still wish to give the eggs a chance (i.e. you want nature to do the work), provide heavy plant cover and mosses to conceal the eggs and fry until they are large enough to join the shoal.
There’s over 180 species of Corydoras alone, many of which have yet to be truly described (which is why many species are given a C or CW number rather than a species name). Some species are mildly toxic/venomous, and can have glands that release potentially harmful secretions at the base of their spines. This is in no way deadly to people, although being pricked by Corydoras spines can hurt quite a bit. However, such species can actually “poison” the water around them with these toxic secretions if under intense stress and in a small environment like a fish bag (which is why Corydoras are sometimes all dead on arrival after being transported). Another thing that many hobbyists may not know is that Corydoras can breathe atmospheric air like a gourami. Even when they are not in a low oxygen tank, they can frequently be seen swimming to the surface for a breath of air. These fish are far more than bottom feeders for a community tank or scavengers to clean up the messes of other fish. Corydoradinae is a fascinating family of fish, and I will never grow tired of learning more about them.