Development of the Viviparous Brotulid Fish, Dinematichthys ilucoeteoides

Abstract

In a Seychelles Islands population of the brotulid fish, Dinematichthys ilucoeteoides sampled during mid-late October, 90% of the mature females contained developing embryos. This is probably the peak of one reproductive cycle. It coincides with a period of calm between the two annual monsoon seasons. The presence of early and late stage oocytes in gravid females suggests that they may reproduce more than once during their lifetime and possibly more than once a year. No major differences in size exist between the two sexes. Females are sexually mature at 45-50 mm. Developing eggs and embryos are incubated in the ovary. Fertilization and development through the tail bud stage takes place in the ovarian follicle. The embryo hatches from the egg chorion and completes its development within the lumen of the ovary. The embryo neither has morphological adaptations for viviparity, nor does it establish any special morphological relationship with the ovarian epithelium or stroma. The embryo utilizes its own yolk reserves. Development is considered to be ovoviviparous. Development is synchronous. All embryos within a female are of the same stage. Development has been divided into five stages: Stage 1, fertilizatiqn through hatching; Stage 2, late tailbud embryo; Stage 3, finfold embryo; Stage 4, mid-embryo; Stage 5, pre-parturition embryo. Preparturition embryos are similar to the unspecialized pelagic larvae of many fishes. D. ilucoeteoides may pass part of its life as a pelagic larva which resembles Stage 5. Pelagic larvae would serve as the chief means of dispersion for this presumably widely distributed species.