What is the World’s Smallest Fish?
What is the world’s smallest fish? This question has come up in my mind several times since I started fish keeping. The answer has also changed over time as researchers discover new species around the world.
The world’s smallest fish is the Paedocypris progenetica. It is a tiny fish endemic to Indonesian brackish waterways and peat swamps. The largest known specimen of this fish was a mature female measuring only 7.9 mm (0.31 in).
Now although this is a clearly defined fish species, the real answer to the question “what is the world’s smallest fish?” Is a bit more complicated than just that. There are other tiny fish and even certain genders of others that measure even smaller, However calling them a fish is a bit of an ambiguous decision.
The Paedocypris progenetica
The Paedocypris progenetica is the world's smallest fish, and according to the image above, you can fit 70,000 of them in a coffee mug. I find that statistic hard to believe, but that information came from the Kurzgesagt app “Universe in a nutshell.” The publishers of that information are very credible and I have no reason to feel they have ever led me astray.
Paedocyptis progenetica is endemic to the Indonesian islands of Sumatra and Bintan. They live in highly acidic environments like peat swamps and black water rivers. Mainly staying in the deeper shaded areas of the water column. Based on dissection of the digestive tract on these little fish, the primary diet for Paedocyptis progenetica are tiny planktonic rotifers and cladocerans. The discovery and diagnostics of this information were made available by researchers at the US National Library of Medicine.
A female with ripe eggs that only measured 7.9mm in length is the smallest specimen ever found. The smallest mature male ever seen was 8.2mm in length. Breeding these tiny fish has never been done in captivity, and observations of mature specimens indicate that eggs are deposited individually. Although the most amount of eggs seen inside a living specimen numbered near 60, only four were opaque in the ovaries. The eggs measured about .3mm in diameter.
What is the actual smallest fish?
Although Paedocypris progenetica is the smallest species of fish, there are some fish specimens that measure even smaller than the 7.9mm female. An angler fish known as the Photocorynus spiniceps technically has the smallest fish specimens in the world. The mature male of this species will measure about 6.2mm to 7.1mm in length. However, can this still be called a fish?
The Photocorynus spiniceps males’ body consists of mostly testes. These reproductive organs are all the fish really is because upon finding a mate, the male fuses to his much larger female counterpart. Once this is done, the male becomes a parasite and the female is, by all intensive purposes, a hermaphrodite. The female ranges at about two inches in length lives her life normally. Having full control over the swimming and eating.
Being a deep sea creature, it is advantageous to fuse male and female together. However, I find it hard to call this a fish. The male will not even eat in his lifetime. His sole purpose is to find a female and fuse to her. Once this is done he just becomes a partially exogenous form of semen for the female. So is this male specimen still considered a fish? Is it not more like a separate fish organ that needs to find a “female” to complete the reproductive puzzle of the species?
Picture the World’s Smallest Fish
In this image, courtesy of the University of Washington, I have circled in red the parasitic male. Is this relationship parasitic? The male and female relationship is neither parasitic nor commensal. Moreover, it is hardly mutual.
The male fulfills his purpose of being testicles for the female and essentially ends his life. The vascular systems of the two specimens fuse together making the male really not a fish at all anymore. Just a part of the female that was not there upon her birth.
Final Thoughts
The answer to “What is the world's smallest fish?” is not as simple as pointing to one animal. Beyond the Paedocypris progenetica, there are more Paedocypris species that are still very small. Ranging lengths below 12mm in total, but none are as small as the male Photocorynus spiniceps.
However, can the Photocorynus spiniceps male still be called a fish? I suppose that for a short time while not fused to a female, he is a fish. Indeed the world's smallest fish, but for only a short time. Once fused to his female he becomes just another organ on the body of the opposite sex.
