Characteristics
Glandulocaudines are small, less than 13 centimetres (5 in). Most are between about 5–6 cm SL (about 2 in), but some species can be even smaller, between 11 and 30 millimetres (.4–1 in). The reproductive adaptations of glandulocaudines is what sets this group apart from the other Characins. Males have a caudal organ associated with gland tissues. Synapomorphies of this subfamily include insemination, a posterior sperm storage area in the testes, and an elongate sperm nucleus.
All males have some form of modified caudal gland used to release pheromones as part of courtship. The structure of this gland depends on the specific tribe; the organ may consist of modified caudal fin rays; modified caudal fin scales, a derived hypural fan, or modified caudal fin musculature, or combinations of the above. Modified scales may act as bellows in releasing chemicals into the water. A caudal fin ray pheromone pump, unique to tribe Glandulocaudini, consists of glandular tissue associated with modified scales and fin rays. The tribe Diapomini is the only tribe in which the caudal gland is equally developed in both the males and females.
Females of all glandulocaudine species are inseminated. The females produce fewer eggs per unit body weight than externally fertilizing species; this is possibly because insemination increases the efficiency of fertilization, so fewer eggs are necessary. This insemination is preceded by courtship in all species. After insemination, the female may retain the live sperm for many months in her ovaries. This allows the eggs to be laid when environmental conditions are favorable. However, there is no evidence of an intromittent organ, and the exact mechanism of insemination is unknown. Hooks on the anal fin of males may play a role, although these are also found in characins that exhibit external fertilization. It was originally believed that internal fertilization occurs in glandulocaudines. However, the exact time of fertilization is unknown and no fertilized eggs are found internally; this suggests fertilization occurs when the eggs are being laid or even outside of the body.
Due to insemination, the sperm of glandulocaudines has adapted. In many species, an elongate cytoplasmic collar binds the flagellum to the elongate nucleus at some stage of spermiogenesis. In almost all species, the sperm cell bodies are elongate. In the tribe Diapomini, the genus Planaltina expresses only round sperm (like that of externally fertilizing characins) and the genera Diapoma and Acrobrycon only express slightly elongated sperm; this may indicate a possible plesiomorphy. Some sperm have enlarged regions containing mitochondria, which may help in prolonging the life of the sperm while stored in the ovary. In some genera, sperm clumping and patterns of arrangement are observed in the sperm ducts and storage regions. In the tribes Xenobryconini (in the genera Tyttocharax, Scopaeocharax, and Xenurobrycon) and Glandulocaudini, there is a form of sperm packaging which would allow for a higher sperm density during transfer from the male to the female. These packets are called spermatozeugmata, and the sperm are packaged parallel to each other; this packaging is further increased by the elongation of the sperm cells. In Xenobryconini, each spermatozeugma is produced and is released fully formed in the spermatocysts, but in Glandulocaudini, the sperm is released from the spermatocysts and packaged elsewhere. The spermatozeugmata are situated in the posterior end of the testes, which serves as a storage area for sperm.
Many of the genera also have a gland situated in the gill cavity called a "gill gland", a secondary sex characteristic found in sexually mature male glandulocaudines that is apparently suited to release chemical signals. No genus contains species that have glands and other species without glands. This gill gland is derived from anterior gill filaments of the first gill arch. Gland size and degree of gill modification varies with species. Though the true function of the gill glands has yet to be determined, they are probably used to release chemical signals into the gill current.
There are many examples of sexual dimorphism (differences in appearance between the genders). In Corynopoma riisei, the males have extended finnage (giving it the common name "swordtailed characin") as well as paddle-like extensions of the operculum. Many other species also have other secondary sex characteristics believed to be involved in courtship.
Many of these characteristics are also shared with the tribe Compsurini in Cheirodontinae. Though unrelated, this group contains inseminating species with caudal organs. However, the caudal organs and other similar characteristics are structured differently. They also share the elongate cytoplasmic collar binding the flagellum to the elongate nucleus at some stage of spermiogenesis, which was previously assumed to be exclusive to glandulocaudines. These fish also occasionally have gill glands.
Read more about this topic: Glandulocaudinae