Neuroendocrine modulation of osmoregulatory parameters in the freshwater shrimp Macrobrachium olfersii (Wiegmann) (Crustacea, Decapoda)

Abstract

Freshwater crustaceans typically exhibit considerable differences in the osmolality and ionic concentrations of their hemolymph compared to the surrounding medium. This steady state equilibrium is maintained by powerful mechanisms of ion uptake and reabsorption, and the excretion of a dilute urine. When acutely exposed to saline media, a new equilibrium is established through readjustment of ion capture and reabsorption rates, and ionic permeability, apparently regulated by neurosecretory mechanisms. This concept is investigated in the euryhaline, freshwater shrimp Macrobrachium olfersii (Wiegmann), using homogenates of the optic ganglia (OG) and ventral nerve cord (VNC). Homogenates prepared from hyperosmoregulating shrimps maintained in freshwater (FW) were injected into the abdominal musculature of test shrimps exposed to FW or sea-water (SW) for 0, 1, 3 or 6 h; changes in hemolymph osmolality and [Cl −], and in body water content and heart rate were measured. Control shrimps received saline alone. Both neurohomogenates reduced hemolymph osmolality and [Cl −], and increased heart rate in SW-exposed shrimps. Body water increased in OG homogenate-injected shrimps in FW while heart rate decreased; the converse occurred in VNC homogenate-injected shrimps in SW. These data show clear effects of putative neurosecretory factors on important osmoregulatory parameters and are interpreted to demonstrate the modulation of osmoregulatory capability by neurosecretory mechanisms.