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In present study, a microcosm experiment is carried out to investigate the efficacy of 120 and 250-ppm crude aqueous extract of Azadirachta leaf on oocyte maturation, oviposition, embryonic development and hatching of the eggs of a fish ectoparasite Argulus bengalensis. Relative abundance of different maturing oocyte stages in the ovary of the parasite from different age groups was enumerated, and marked variations were obtained. Significant depletion in the abundance of pre-vitollogenic, vitellogenic and post-vitellogenic oocytes was recorded, which indicates impairment in maturation. Chromatin condensation of the oocytes of treated parasite indicates apoptosis of oogenic cells. Strong oviposition deterrence was evident by the elevated oviposition deterrence index of 0.18 and 0.52 at respective toxin levels. The treated parasites invested less number of eggs per oviposition, and hatching percentage of the eggs reduced markedly. In vitro treatment of eggs within 70 min of incubation exhibited coagulation of yolk material and subsequent reduction in hatching percentage. However, treatment applied after this critical period, hatching was not significantly altered. In vitro treatment of eggs at 80 min of incubation resulted in normal development. It signifies that azadirachtin affects the early developmental events but not the later. Presumably, azadirachtin either affects early gene expression of the embryo or antagonizes any of the substances of the zygote required for sustaining early developmental process. The result of the present experiment suggests that azadirachtin could be a promising agent to control argulosis through inhibition of the reproductive maturity of the parasite as well as through interference of its embryonic development.

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This study reports on the anatomy and structural details of the feeding apparatus and associated glands in a tropical fish ectoparasite, Argulus siamensis Wilson, 1926. The results reveal that anatomically the feeding apparatus of the species consists of a proximal buccal proboscis and a retractile pre-oral spine surrounded by a sheath. The mouth is located at the terminal end of the buccal proboscis. Two types of glands, the spinal gland and the proboscis gland, are associated with the feeding apparatus. The sac-like spinal gland, consisting of four polygonal cells, is located at the base of the spine. It is continuous with the median duct of the spine but does not show any connection with the buccal cavity. Numerous secretory vesicles are present in the cytoplasm. The proboscis glands are located parallel to the spinal sheath, one on both sides, and open directly into the buccal proboscis. Each proboscis gland houses two giant cells, the cytoplasm of which is largely occupied by secretory vesicles. The present study, aimed at understanding the functional relation of the glandular secretions with the feeding mechanism, revealed that the secretion of the spinal gland is neither cytotoxic nor haemorrhagic, but may have anaesthetic or vasodilatory activity. Indications are available to show that the secretion of the proboscis gland is anticoagulant in nature.