Eyestalk histology and the effects of eyestalk ablation on the gonads of the shrimp, Macrobrachium rosenbergii (De man)

Abstract

A histological study of the eyestalk of the freshwater prawn, Macrobrachium rosenbergii, was conducted to characterize its neurosecretory elements. Neurosecretory cell groupings (X-organs) were associated with each of the three optic peduncle ganglia--the medulla externa, the medulla interna and the medulla terminalis. Six neurosecretory cell types were described based on appearance, size and staining characteristics. The sensory pore X-organ/Organ of Bellonci complex was located on the ventral proximal surface of the medulla terminalis. A large, single neurosecretory cell was found in the optic nerve tract, well separated from the medulla terminalis. This feature has not been identified in previous studies. Experiments were conducted to determine the effects of ablation of the eyestalk, the source of the putative gonad-inhibiting hormone (GIH), and replacement therapy on the gonads of adult male and female prawns. There was no significant difference in the mean gonadosomatic index of ablated vs. non-ablated males; ablated females exhibited a statistically greater mean gonadosomatic index, relative to controls. Gonadal responses to replacement therapy were varied and inconsistent. Replacement therapy failed to support the hypothesis that GIH is produced in neurosecretory cells of the medulla terminalis X-organ-I, or -II. It is possible that replacement extracts and/or implants were prepared from prawns that were in a reproductively preparatory phase when the endogenous titres of GIH are naturally low. Prawn hemolymph analyses indicated that protein and reducing sugar concentrations were depressed around the time of molt. SDS electrophoresis of hemolymph samples demonstrated a major hemolymph protein ((TURN) 80,000 daltons) common to male and female prawns throughout the molt cycle. Two protein bands ((TURN) 85,000-90,000 daltons) were observed in females with developing ovaries, but these were not considered to be the female-specific proteins (vitellogenins) because their relatively low molecular weights were not consistent with the molecular weight ((TURN) 300,000 daltons) of vitellogenins in other crustaceans.