NEUROENDOCRINE MODULATION OF OSMOREGULATORY PARAMETERS IN THE FRESH-WATER SHRIMP MACROBRACHIUM-OLFERSII (WIEGMANN) (CRUSTACEA, DECAPODA)

Freshwater crustaceans typically exhibit considerable differences in t he 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 equili brium is established through readjustment of ion capture and reabsorpt ion rates, and ionic permeability, apparently regulated by neurosecret ory mechanisms. This concept is investigated in the euryhaline, freshw ater shrimp Macrobrachium olfersii (Wiegmann), using homogenates of th e optic ganglia (OG) and ventral nerve cord (VNC). Homogenates prepare d 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. Con trol shrimps received saline alone. Both neurohomogenates reduced hemo lymph osmolality and [Cl-], and increased heart rate in SW-exposed shr imps. Body water increased in OG homogenate-injected shrimps in FW whi le heart rate decreased; the converse occurred in VNC homogenate-injec ted shrimps in SW. These data show clear effects of putative neurosecr etory factors on important osmoregulatory parameters and are interpret ed to demonstrate the modulation of osmoregulatory capability by neuro secretory mechanisms.