Looking at the two specimens I caught yesterday, both were middling-large sized and both appeared to be gravid. In fact, at this time of the year the fish are spawning, and migrate to the shallows to spawn in reed banks that line lakes and rivers; they even migrate into the little channels and man-made canals that feed rice paddies and lotus root fields. If you catch a mabuna that has reached sexual maturity and is in good physical condition at this time of year, it will almost certainly be gravid, and in fact you will never catch a male mabuna, not in a lifetime of fishing for them. All mabuna are female. Just casually looking up “mabuna” online to find out its scientific name – generally Carassius auratus langsdorfii, or just Carassius langsdorfii – I came across this unusual habit. I had read about the switching of sexes that some fish display – generally all born female then males develop from these in the right numbers for spawning – I think the grouper and certain bass do this, but never the female-only trait.
C. langsdorfii ‘spawn’ generally at the same time (around April-May) as a number of related species: koi carp, the so-called kinbuna (C. buergeri) and nigorobuna (C. buergeri grandoculis) and others, in the same habitat, usually overgrown freshwater shallows. These types of fish spawn in the multiple-male-multiple-female “orgy” method common to many fish species (and certain humans, if the internet is to be believed), and during this process the entirely female race of mabuna join in, for their eggs require a sperm of a related species to activate, but fertilisation – and perfectly normal development – takes place without the sperm nucleus ever fusing with that of the egg. This is not true parthenogenesis since a male sex partner is required, and so is termed instead gynogenesis. Interestingly the mabuna is polyploid, usually tri- or tetraploid, and so its eggs never require chromosomes from what would usually come from the male gamete. It also means that mabuna are all genetic clones. However mabuna are found all over Honshu, and genomic analysis has shown that different water-systems in different locations, such as Biwako or the Kasumigaura lakes, hold different clones. In fact the classification of funa-type species is extremely difficult, and identification – without post mortem dissection – very tricky; I still have trouble sometimes telling the difference between a herabuna and mabuna, or kinbuna (koi carp are easy to distinguish, as they have barbels). There also appears to be a certain apocryphal cross-breeding of funa species, leading to fish that anglers refer to as ai-bera or hanbera, and that in some seasons the mabuna seem to have different physical characteristics, although how this is possible is not known and it has never been tested in a laboratory. How Linnaeus would tear his hair (or powdered wig) after a season of mabuna fishing!
There are several apparent biological paradoxes regarding this species. The first is that even with a little variation caused by natural mutation, a clone race would be very susceptible to a single pathogen or food shortage, yet mabuna are numerous and have not been affected by serious epidemics that I know of. Second, if a kinbuna or nigorobuna sperm is required for mabuna eggs to ‘fertilise’, by definition that particular kin/nigorobuna sperm would not fertilise its intended target egg, and a small imbalance in either male kin/nigorobuna or female mabuna numbers would soon be amplified so that one or other would become numerically dominant, leading to the at least local extinction of the other; yet funa species co-exist in a balanced nature in all habitats they can be found in. There is also the question of evolution, as how such a species developed in the first place, and the various old anglers’ tales of the cross-breeds that are alleged to exist. The very definition of a species by classical taxonomy is challenged by mabuna. Who would have thought such simple fishing could be so intriguing?