Eyestalkless Crabs Research Articles

Mode of action of dopamine in inducing hyperglycemia in the fresh water edible crab, Oziothelphusa senex senex.

The objective of this study was to investigate the mode of action of dopamine in regulating hemolymph sugar level in the fresh water edible crab, Oziothelphusa senex senex. Injection of dopamine produced hyperglycemia in a dose-dependent manner in intact crabs but not in eyestalkless crabs. Administration of dopamine resulted in a significant decrease in total carbohydrates and glycogen levels with a significant increase in glycogen phosphorylase activity levels in hepatopancreas and muscle of intact crabs, indicating dopamine-induced glycogenolysis resulting in hyperglycemia. Bilateral eyestalk ablation resulted in significant increase in the total carbohydrates and glycogen levels with a significant decrease in the activity levels of phosphorylase in the hepatopancreas and muscle of the crabs. Eyestalk ablation resulted in significant decrease in hemolymph hyperglycemic hormone levels. The levels of hyperglycemic hormone in the hemolymph of dopamine injected crabs were significantly higher than in control crabs. However, no significant changes in the levels of hemolymph hyperglycemic hormone and sugar and tissue carbohydrate and phosphorylase activity were observed in dopamine injected eyestalk ablated crabs when compared with eyestalk ablated crabs. These results support an earlier hypothesis in crustaceans that dopamine acts as a neurotransmitter and induces hyperglycemia by triggering the release of hyperglycemic hormone in the crab, O. senex senex.

Neuroendocrine Control of Water Content and Calcium Concentration in the Crab Ocypode macrocera (H. Milne-Edwards 1852) (Brachyura, Ocypodae)

The present study is focused to see the effect of crustacean neuroendocrine organs on the water and calcium metabolism which is very much important for the osmoregulatory functions. Since the experiments were carried out to investigate the control of water contents and calcium concentration in the crab, Ocypode macrocera. The animals were collected from the shore of the Bay of the Bengal near Annan Koil one among the biggest landing centers of south east coast of Tamil Nadu, India. The data revealed that water content in the hepatopancreas and thoracic muscle of the control crab were 70.16 and 79.86%, respectively, whereas in the experimental ones, the values were 80.32 and 87.44% after eyestalk removal and 54.52 and 66.98% after eyestalk extract injection. Calcium concentration in both the hepatopancreas and thoracic muscle of the control crab were 2.16 and 2.14 mg g(-1), respectively, whereas in the experimental animals the values were 2.76 and 3.52 mg g(-1) in the eyestalkless crabs and 1.52 and 1.57 mg g(-1) after eyestalk extract injection, respectively. Hence it was observed the % of water content is more in eyestalk less crabs as compared to that of control and injected. The roles of neurosecretory secretions, which control these parameters, were discussed. The ability for Ocypode macrocera to adapt rapidly and maintain homeostasis in a wide range of abnormality supports the fact that Ocypode macrocera are a suitable species for land-based aquaculture in ponds as well as critical condition where rapid fluctuation in salinity can occur.

Cadmium and mercury-induced hyperglycemia in the fresh water crab, Oziotelphusa senex senex: Involvement of neuroendocrine system

The effect of exposure to sublethal concentrations of cadmium chloride and mercuric chloride on hemolymph glucose levels of the freshwater crab, Oziotelphusa senex senex, was studied. Intact crabs exposed to cadmium or mercury exhibited a significant hyperglycemia compared to controls, but no significant differences in hemolymph glucose level were detected among the eyestalkless crabs after exposure to metals, suggesting that the effect of metals could be on the sinus gland in the eyestalks, increasing secretion of the hyperglycemic hormone. To test this hypothesis, eyestalks were collected from control and metal exposed crabs, and tested for hyperglycemic effect and also for the hyperglycemic hormone levels. The levels of hyperglycemic hormone and the hyperglycemic effect were significantly low in the eyestalks collected from metal exposed crabs when compared with eyestalks from control crabs. These results strongly suggest that metals act, at least in part, by triggering the secretion of hyperglycemic hormone from the eyestalk.

Effects of melatonin in connection with the antioxidant defense system in the gills of the estuarine crab Neohelice granulata

Numerous studies have shown that melatonin exerts some influence on the antioxidant defense system (ADS) in vertebrates, but for crustaceans no such effect has been demonstrated till now. However, earlier reports did show a similar profile of daily variations in the ADS of the gills and the melatonin content of the eyestalk in the crab Neohelice granulata and, thus, the aim of this study was to take a closer look at the effects of melatonin in the gill ADS of N. granulata. Gill ADS is to a minor extent modulated by reactive oxygen species (ROS), because only the nonproteic sulfhydryl (NP-SH) content increases ( p < 0.05) in the presence of hydrogen peroxide (H 2O 2). No significant differences ( p > 0.05) were observed in the melatonin content of the hemolymph between intact and eyestalkless crabs. Gills from intact and eyestalkless crabs injected with physiological saline showed a daily variation in the total peroxyl radical scavenging capacity (TPRSC) ( p < 0.05) with two peaks, one at the photophase and another at the scotophase. However, in the gills of eyestalkless crabs injected with melatonin (2 × 10 −12 mol crab −1), the daily variation in TPRSC values was abolished ( p > 0.05). This molecule did not change the NP-SH content ( p > 0.05) in vitro, but decreased ( p < 0.05) the oxygen consumption in gills when incubated for 120 min. In the in vivo experiments melatonin also decreased ( p < 0.05) the oxygen consumption in eyestalkless crabs after 390 min. The results suggest that melatonin does not act directly on the ADS of the gills of N. granulata, but decreases the aerobic metabolism possibly involved in variations of tissue ADS.

Melatonin does not affect the black pigment migration in the crab Neohelice granulata

N-acetyl-5-methoxytryptamine or melatonin is a multifunctional molecule. The main physiological function, at least in vertebrates, is to transduce to the animal the photoperiodic information and regulate rhythmic parameters. But studies have also observed the action of this molecule on pigment migration in ectothermic vertebrates. Thus the aim of this paper was to investigate in vivo and in vitro the influence of melatonin on the pigment migration in melanophores of the crab Neohelice granulate. Injections of melatonin (2 × 10−9 moles · crab−1) at 07:00 h or 19:00 h did not affect (p > 0.05) the circadian pigment migration of the melanophores in constant darkness. Additionally no significant pigment migration (p > 0.05) was verified in normal and eyestalkless crabs injected with melatonin (10−10–10−7 moles · crab−1) during the day or night. In the in vitro assay, the response of melanophores to the pigment-dispersing hormone in eyestalkless crabs injected with melatonin (2 × 10−9 moles · crab−1) 1 and 12 hours before the observations did not differ (p > 0.05) from the control group (injected with physiological solution). These results suggest that melatonin does not act as a signaling factor for pigment dispersion or aggregation in the melanophores of N. Granulate.

Antioxidant Defenses and DNA Damage Induced by UV‐A and UV‐B Radiation in the Crab Chasmagnathus granulata (Decapoda, Brachyura)¶

ABSTRACTThe photoprotector role of pigment dispersion in the melanophores of the crab, Chasmagnathus granulata, against DNA and oxidative damages caused by UV‐A and UV‐B was investigated. Intact and eyestalkaless crabs were used. In eyestalkless crabs, the dorsal epidermis of the cephalothorax (dispersed melanophores) and the epidermis of pereiopods (aggregated melanophores) were analyzed. Intact crabs showed only dispersed melanophores in the two epidermis. Antioxidant enzymes activity and lipoperoxidation content were analyzed after UV‐A (2.5 J/cm2) or UV‐B (8.6 J/sm2) irradiation. DNA damage was analyzed by single cell electrophoresis (comet) assay, after exposure to UV‐B (8.6 J/cm2). UV‐A radiation increased the glutatione‐S‐transferase activity in the pereiopods epidermis of eyestalkless crabs (P &lt; 0.05). UV‐B radiation induced DNA damage in the dorsal epidermis of eyestalkless crabs (P &lt; 0.005). In pereiopod epidermis of eyestalkless crabs, there was no significant difference between control and UV‐B‐exposed crabs. In the pereiopods epidermis of eyestalkless, the control group showed higher scores of DNA damage and ∼50% of cellular viability. Because in eyestalkless and irradiated crabs the cellular viability was ∼5%, it was not possible to observe nuclei for determination of DNA damage. The findings show that melanophores can play a role in the defense against harmful effects of a momentary exposure to UV radiation.

Antioxidant Defenses and DNA Damage Induced by UV-A and UV-B Radiation in the Crab Chasmagnathus granulata (Decapoda, Brachyura)¶

The photoprotector role of pigment dispersion in the melanophores of the crab, Chasmagnathus granulata, against DNA and oxidative damages caused by UV-A and UV-B was investigated. Intact and eyestalkless crabs were used. In eyestalkless crabs, the dorsal epidermis of the cephalothorax (dispersed melanophores) and the epidermis of pereiopods (aggregated melanophores) were analyzed. Intact crabs showed only dispersed melanophores in the two epidermis. Antioxidant enzymes activity and lipoperoxidation content were analyzed after UV-A (2.5 J/cm2) or UV-B (8.6 J/cm2) irradiation. DNA damage was analyzed by single cell electrophoresis (comet) assay, after exposure to UV-B (8.6 J/cm2). UV-A radiation increased the glutatione-S-transferase activity in the pereiopods epidermis of eyestalkless crabs (P<0.05). UV-B radiation induced DNA damage in the dorsal epidermis of eyestalkless crabs (P<0.05). In pereiopod epidermis of eyestalkless crabs, there was no significant difference between control and UV-B-exposed crabs. In the pereiopods epidermis of eyestalkless, the control group showed higher scores of DNA damage and approximately 50% of cellular viability. Because in eyestalkless and irradiated crabs the cellular viability was approximately 5%, it was not possible to observe nuclei for determination of DNA damage. The findings show that melanophores can play a role in the defense against harmful effects of a momentary exposure to UV radiation.

Ultraviolet radiation induces dose-dependent pigment dispersion in crustacean chromatophores.

Pigment dispersion in chromatophores as a response to UV radiation was investigated in two species of crustaceans, the crab Chasmagnathus granulata and the shrimp Palaemonetes argentinus. Eyestalkless crabs and shrimps maintained on either a black or a white background were irradiated with different UV bands. In eyestalkless crabs the significant minimal effective dose inducing pigment dispersion was 0.42 J/cm(2) for UVA and 2.15 J/cm(2) for UVB. Maximal response was achieved with 10.0 J/cm(2) UVA and 8.6 J/cm(2) UVB. UVA was more effective than UVB in inducing pigment dispersion. Soon after UV exposure, melanophores once again reached the initial stage of pigment aggregation after 45 min. Aggregated erythrophores of shrimps adapted to a white background showed significant pigment dispersion with 2.5 J/cm(2) UVA and 0.29 J/cm(2) UVC. Dispersed erythrophores of shrimps adapted to a black background did not show any significant response to UVA, UVB or UVC radiation. UVB did not induce any significant pigment dispersion in shrimps adapted to either a white or a black background. As opposed to the tanning response, which only protects against future UV exposure, the pigment dispersion response could be an important agent protecting against the harmful effects of UV radiation exposure.

ANTIOXIDANT DEFENSES AND DNA DAMAGE INDUCED BY UVA AND UVB RADIATION IN THE CRAB Chasmagnathus granulata (DECAPODA, BRACHYURA)

The photoprotector role of pigment dispersion in the melanophores of the crab, Chasmagnathus granulata, against DNA and oxidative damages caused by UV-A and UV-B was investigated. Intact and eyestalkless crabs were used. In eyestalkless crabs, the dorsal epidermis of the cephalothorax (dispersed melanophores) and the epidermis of pereiopods (aggregated melanophores) were analyzed. Intact crabs showed only dispersed melanophores in the two epidermis. Antioxidant enzymes activity and lipoperoxidation content were analyzed after UV-A (2.5 J/cm2) or UV-B (8.6 J/cm2) irradiation. DNA damage was analyzed by single cell electrophoresis (comet) assay, after exposure to UV-B (8.6 J/cm2). UV-A radiation increased the glutatione-S-transferase activity in the pereiopods epidermis of eyestalkless crabs (P<0.05). UV-B radiation induced DNA damage in the dorsal epidermis of eyestalkless crabs (P<0.05). In pereiopod epidermis of eyestalkless crabs, there was no significant difference between control and UV-B-exposed crabs. In the pereiopods epidermis of eyestalkless, the control group showed higher scores of DNA damage and approximately 50% of cellular viability. Because in eyestalkless and irradiated crabs the cellular viability was approximately 5%, it was not possible to observe nuclei for determination of DNA damage. The findings show that melanophores can play a role in the defense against harmful effects of a momentary exposure to UV radiation.

Methionine-enkephalin induces hyperglycemia through eyestalk hormones in the estuarine crab Scylla serrata.

The hypothesis is tested that methionine-enkephalin, a hormone produced in and released from eyestalk of crustaceans, produces hyperglycemia indirectly by stimulating the release of hyperglycemic hormone from the eyestalks. Injection of methionine-enkephalin leads to hyperglycemia and hyperglucosemia in the estuarine crab Scylla serrata in a dose-dependent manner. Decreases in total carbohydrate (TCHO) and glycogen levels of hepatopancreas and muscle with an increase in phosphorylase activity were also observed in intact crabs after methionine-enkephalin injection. Eyestalk ablation depressed hemolymph glucose (19%) and TCHO levels (22%), with an elevation of levels of TCHO and glycogen of hepatopancreas and muscle. Tissue phosphorylase activity decreased significantly during bilateral eyestalk ablation. Administration of methionine-enkephalin into eyestalkless crabs caused no significant alterations in these parameters when compared to eyestalk ablated crabs. These results support the hypothesis that methionine-enkephalin produces hyperglycemia in crustaceans by triggering release of hyperglycemic hormone from the eyestalks.

Inhibition of Ovarian Growth by Cadmium in the Fiddler Crab, Uca pugilator (Decapoda, Ocypodidae)

The effect of cadmium chloride (1 mg/L) on oocyte growth of the fiddler crab, Uca pugilator, was studied during the slow vitellogenesis phase of ovarian maturation of this carb. In vivo experiments were done with both intact and eyestalkless crabs. The intact cadmium-exposed crabs exhibited a significantly lower oocyte diameter than the controls at the end of the 2-week exposure period, but no significant differences were detected among the eyestalkless crabs, suggesting that the effect of cadmium could be on the sinus gland in the eyestalks, increasing secretion of the gonad-inhibiting hormone (GIH). To test this hypothesis, in vitro experiments were done, incubating pieces of ovary with and without eyestalk tissue, in the presence of thoracic ganglion, needed for oocyte growing due to the secretion of the gonad-stimulating hormone (GSH). Only when eyestalk tissue was present in the incubation media was oocyte growth inhibited by cadmium compared to the controls. These results strongly suggest that cadmium acts at least in part to increase the secretion of GIH from the sinus gland. GIH could then in turn act by (a) inhibiting secretion of GSH by the thoracic ganglion, (b) directly inhibit the oocytes, or (c) both (a) and (b).

The role of biogenic amines in the control of blood glucose level in the decapod crustacean, Carcinus maenas L.

1. Administration of biogenic amines into intact Carcinus maenas induces dose- and timedependent elevation of hemolymph glucose level. 2. Removal of the neurosecretory centre containing the crustacean hyperglycemic hormone (CHH) by ablation of the eyestalks did not induce hypoglycemia. 3. Injection of dopamine (DA) into eyestalkless crabs showed no hyperglycemic effect, while serotonin (5-HT), epinephrine (E), norepinephrine (NE), and octopamine (OA) elevated glucose levels. 4. The dopaminergic effect was significantly reduced by administration of trifluoperazine (TFP). 5. 5-HT and OA were found to be strong elevators of glucose levels, while the other biogenic amines had moderate effects only. 6. The results indicate, that DA exerts its hyperglycemic effect by stimulating the release of CHH from the eyestalk neurosecretory centre. Elevation of hemolymph glucose level by 5-HT, OA, E, and NE, occurs independently of CHH.

Post-Moult Calcification in the Blue Crab, Callinectes Sapidus: Timing and Mechanism

ABSTRACT A series of experiments was conducted to elucidate the events immediately preceding and following moulting (ecdysis) in the blue crab, Callinectes sapidus Rathbun. The crabs gain weight (mostly water) to a level more than twice the premoult value excluding the shed carapace. The gain begins about 4 h before ecdysis, accelerates rapidly to a maximum rate at about the time of ecdysis, and is essentially complete by 2h after ecdysis. Both calcification and net H+ excretion remain at control (intermoult) levels until 1–2 h post-moult, whereupon a very rapid increase in both begins, with the same time course for both processes. The ratio of internal to external calcium concentration drops from 1·4:1 during intermoult to 0·85:1 by 2 days after the moult, reversing the electrochemical gradient during the post-moult period. Calcification is strongly inhibited by the protein synthesis inhibitors actinomycin D and cycloheximide. Isolated whole gills and gill slices do not show significant changes in rates of calcium uptake related to moult stage, indicating that the uptake of calcium across the gills may be largely passive. A low-affinity Ca2+-activated ATPase is present in both gills and epithelium, but only the epithelial activity shows a substantial (fivefold) increase in the hours after ecdysis. The control mechanisms for stimulation of the transport processes remain elusive. Eyestalkless crabs progress normally through moult and post-moult calcification. Peptide profiles from blood and from suspected neurohormone-producing tissues differ markedly with moult stage, but several different approaches to developing a bioassay using peptides from crab tissues and vertebrate sources have been unsuccessful. The physiological events in the hours surrounding ecdysis suggest a complex and precisely timed series of control signals.

Entrainability by salinity cycles of rhythmic locomotor activity in normal and eyestalk ablatedcarcinus maenas(L).

The temporal patterns of spontaneous locomotor activity were compared in normal and eyestalk ablated crabs. Study of entrainment of such patterns to simulate tidal cycles of salinity change entails separation of the exogenous responses to low salinity and endogenous circatidal rhythmicity which is re‐phased by episodes of normal ambient salinity. A method to transform rhythmic data to emphasise separate rhythmic patterns of behaviour was therefore developed, and patterns of re‐entrainment to ambient salinity cues were established. Having established the normal pattern of entrainment, experiments were then carried out on eyestalk‐ablated crabs. In such animals it was found that the exogenous responses to low salinity episodes of imposed salinity cycles were not affected but the entrainment responses to ambient salinity episodes were lost. In addition, eyestalkless crabs, unlike controls, showed no evidence of free‐running endogenous rhythmicity after entrainment in a salinity cycle.

THE ROLE OF EYESTALK HORMONES IN VITELLOGENESIS DURING THE BREEDING SEASON IN THE CRAB,PARATELPHUSA HYDRODROMOUS(HERBST)

In P. hydrodromous, vitellogenesis is divided into two phases, Vitellogenesis I (V1, subdivided into Stages 1-3) and Vitellogenesis II (V2). The accelerated ovarian growth induced by bilateral eyestalk ablation during the first half of the breeding season apparently proceeded normally until Stage 3 of V1. Subsequently, the yolk showed signs of impoverishment, particularly in its total protein and lipid contents. In both gross morphology and biochemistry, the ovaries of eyestalkless crabs in V2 during the breeding season were closer to V2 ovaries of normal crabs than to V2 ovaries induced to develop in destalked crabs during the prebreeding season. The ovarian abnormality seen when eyestalk ablation occurs during the breeding season is not due to the unpreparedness of the oocyte population to begin vitellogenesis. In P. hydrodromous, the ovary, at all stages of its development except perhaps at late V2, seems to depend on eyestalks for the normal maintenance of vitellogenesis. P. hydrodromous females do not spawn if eggs artificially induced to form are not normal. Influence of the eyestalk on spawning is not directly via the nervous system; an eyestalk hormone controlling oviposition, if present, is released at least one week prior to spawning.

Effects of neuroendocrine tissue and cyclic AMP on ovarian growth in vivo and in vitro in the fiddler crab, Uca pugilator

1. 1. Thoracic ganglion extracts prepared from female Uca pugilator induced precocious ovarian maturation in intact and eyestalkless crabs. 2. 2. The ability of the thoracic ganglion extracts to induce accelerated ovarian growth varied with respect to the phase of the reproductive cycle. 3. 3. Thoracic ganglia, eyestalk neuroendocrine tissue and cyclic AMP inhibited protein synthesis in previtellogenic ovaries of Uca pugilator in vitro. 4. 4. Eyestalk removal, especially during the breeding season, produced accelerated ovarian growth, as expected. 5. 5. These results support the hypothesis that the ovary of decapod crustaceans is regulated by inhibiting and stimulating hormones.

POSTLARVAL GROWTH IN JUVENILE RHITHROPANOPEUS HARRISH.

Eyestalk removal accelerated the molt cycles of megalopal and juvenile (first through fifth crab instars) Rhithropanopeus harrisii. Eyestalkless crabs also demonstrated a greater increase in size at each ecdysis. The growth rate of eyestalkless crabs was approximately twice the rate measured in control crabs. Epidermal cell density measurements showed that the cell density was the same in intermolt fifth instar control and eyestalkless crabs. The results demonstrate that growth in juvenile crabs is under the influence of eyestalk neurosecretory centers and that growth is a result of epidermal cell proliferation and not cell enlargement.

Partial Characterization of Eyestalk Hormones Controlling Molt and Gonadal Development in the Spiny Lobster Panulirus Argus

ABSTRACT The eyestalk neurosecretory system of Crustacea contains several peptide factors affecting various important physiological processes. Gonad inhibiting hormone (GIH) and a molt inhibiting hormone (MIH) have been separated and partially characterized using Sephadex gel chromatography and polyacrylamide gel electrophoresis. The eyestalks of the spiny lobster Panulirus argus were the source for these isolations, and activity was bioassayed in the fiddler crab Uca pugilator. Gonad inhibiting hormone has an apparent molecular weight near 5,000 daltons. Molt inhibiting hormone does not induce gonadal inhibition in fiddler crabs, but it does delay the onset of the molt cycle in eyestalkless crabs. Melanin dispersion in intact Uca pugilator was inversely correlated with gonadal growth.

Neuroendocrine control of Na+ balance in the fiddler crab Uca pugilator.

The hemolymph ionic composition and unidirectional Na+ fluxes were analyzed in fiddler crabs (Uca) acclimated to 500 mosmol seawater. Eyestalkless crabs had lower hemolymph osmotic, Na+, and Cl- concentrations. The Na+ efflux was increased from 13.6 +/- 0.6 in control crabs to 19.6 +/- 1.3 mueq.g-1.h-1 in eyestalkless crabs; this increase was shown to be due to a higher Na+ permeability in eyestalkless crabs. Hemolymph Na+ in eyestalkless crabs was increased by an injection of eyestalk extract. The Na+ influx was increased from 11.1 in intact to 15.8 mueq.g-1.h-1 in eyestalkless animals. A kinetic study showed that 1) the influx was higher at all external Na+ concentrations in eyestalkless and intact-Na+-depleted crabs than in intact controls, and 2) the influx mechanism exhibited allosterism. The elevated Na+ influx in eyestalkless crabs was decreased by an injected eyestalk extract. An injected brain extract caused in increase in the Na+ influx in intact animals. It is concluded that the Na+ uptake mechanism is activated by decreasing the hemolymph Na+ concentration and that a brain hormone mediates the effect.

Effects of Norepinephrine and Norepinephrine Agonists and Antagonists on the Melanophores of the Fiddler Crab Uca Pugilator

Norepinephrine produces dose-related pigment dispersion in the melanophores of intact and eyestalkless fiddler crabs, Uca pugilator, but has no effect on the melanophores in isolated legs. 2. The effects of agonists and antagonists of norepinephrine on the rates of melanin dis- persion and concentration were determined. 3. The results are consistent with the hypothesis that norepinephrine exerted its effect by acting on the central nervous system to trigger the release of melanin-dispersing hormone. Migration of the pigment in the chromatophores of the fiddler crab Uca pugi- lator is hormonally regulated (Carlson, 1935; Sandeen, 1950; Brown, 1950; Fin- german, 1956a). A series of experiments performed with this crab has revealed that release of red pigment-dispersing hormone is triggered by 5-hydroxytrypt- amine (=serotonin) (Rao and Fingerman, 1970b) whereas release of red pigment- concentrating hormone is triggered by dopamine (Fingerman and Fingerman, 1977). However, no one has as yet identified a neurotransmitter substance which appears to be involved in triggering release of melanin-dispersing hormone (MDH), melanin-concentrating hormone (MCH), white pigment-dispersing hor- mone, or white pigment-concentrating hormone in any species of fiddler crab. Abramowitz and Abramowitz (1938) tested both acetylcholine and epinephrine on melanophores of Uca pugilator, and reported that they had no effect in either producing dispersion or aggregation of the pigment in these cells. Although Bau- chau and Mengeot (1966) found that 5-hydroxytryptamine will produce dispersion of the pigment in the melanophores of the shore crab Carcinus maenas, Rao and Fingerman (1970a) observed that this compound has no such effect on the me- lanophores of Uca pugilator. Presented below are the results of a series of ex- periments which led to identification of norepinephrine as a neurotransmitter substance that appears to be involved in triggering release of MDH in Uca pu- gilator. Experiments were also performed to identify a neurotransmitter sub- stance that might be involved in release of white pigment-dispersing hormone, but these were unsuccessful.