Halfmoon (HM) is the term given to the betta fish species that have a perfect semi-circle caudal when flaring. Specifically, a halfmoon betta fish must have a caudal where both ends are exactly 180 degrees apart or more, spreading towards the front. Caudal edges have to be straight or curving outwards, and not bending inwards.
The acronym ?HM? is used for phenotype fish, as in fish that display the halfmoon trait, while ?hm? is used to describe genotype fish, as in fish that came from phenotype halfmoon parents, but do not display the traits themselves.
It has been debated whether halfmoons should be considered as a separate genetic tail type from deltas. However, if it was a separate genetic tail type, with two genotype parents, the resulting spawn should be throwing on average 25% HMs using the Punnett square. This is not so for most of the present strains throwing HM fish. In fact, most of the phenotype HMs are thrown from delta parents. This goes to imply that halfmoons are actually extreme versions of delta tails. Only when HMs become a fixed strain where the offspring are 100% HMs, can they then be classified as a separate genetic tail type.