Much of what is known about beloniform reproductive biology involves the eggs and larvae. Typically, eggs develop in one to two weeks, and the larvae are immediately able to feed upon hatching. Many pelagic beloniform eggs have filamentous projections that cause them to stick to floating debris. Needlefish eggs have tendrils that are particularly sticky, and they form egg clusters that stick to other objects in the water. Likewise, sauries produce filamentous eggs that float in open water, but they are less adhesive than needlefish eggs. Flyingfishes lay pelagic eggs that may or may not have filaments, and some species attach their eggs to floating seaweed. Marine halfbeaks lay eggs with tendrils that float about in open water, but some freshwater representatives bear live young, namely Dermogenys, Nomorhamphus, and Hemirham-phodon. In these viviparous forms, long genital papillae are used for internal fertilization, and the male anal fin is modified into an andropodium.
The adrianichthyid Horaichthys from India, uniquely among atherinomorphs, produces encapsulated sperm bundles, or spermatophores. In adrianichthyids other than Oryzias, fertilization is apparently internal. The eggs of many species of adrianichthyids are retained externally by the female for various lengths of time. Females of the species Xenopoecilus oophorus, known as the egg-carrying buntingi, carry a cluster of about 30 fertilized eggs attached by filaments in an external concavity near the vent. The pelvic fins cover and protect this egg mass.
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