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Abstract
Male phallostethids have complex asymmetrical modifications of the pelvic girdle and associated structures, illustrated here for Neostethus bicornis and N. lankesteri. This is associated with modifications of the reproductive system in these and another species (Phenacostethus smithi): the atherinomorph-type testis leads into a duct system which is specialized for the secretion of a protein-rich seminal fluid. The spermatozoa are packaged as spermatophores in Neostethus spp., whereas any such organization is but transitory in P. smithi. Females produce relatively small numbers of large, filamented oocytes; large numbers of spermatozoa are present in the ovarian lumen, especially in neostethids. Mating behaviour of the two Neostethus spp. is described briefly on the basis of a small number of video recordings, and reconstructed with anaesthetized specimens: the male grasps the female, presumably using modified pelvic elements (the ctenactinia), this culminating in a bolus of spermatophores in a proteinaceous secretion being deposited in the vicinity of the female's oviducal opening. The sperm must somehow then migrate into the oviduct to fertilize the eggs prior to their being laid.