Visual shape discrimination in the fiddler crab,Uca pugilator

1. Research into the visual shape discrimination abilities of compound‐eyed animals has almost exclusively been limited to insects, the crustaceans having been virtually ignored. The two groups have many dissimilarities, having primarily adapted in different habitats to different lifestyles. Differences may exist in visual systems and visually mediated behavior.

2. Fiddler crabs (Uca pugilator), without training, differentially approached dissimilar silhouettes presented simultaneously, demonstrating visual discrimination between stationary, geometric shapes of equal‐area. The strength of response was ordered hierarchically: vertical rectangle, horizontal rectangle, triangle, square, circle.

3. Basic geometric shapes were used to facilitate replication and comparison with research findings from other species.

     

RXR isoforms and endogenous retinoids in the fiddler crab, Uca pugilator

The pleiotropic effects of circulating ecdysteroids in the adult fiddler crab, Uca pugilator, during molting, regeneration, and reproduction are mediated by a limited number of receptor proteins. We hypothesize that hormonal effects in vivo may be the result of complex interactions between at least two receptor heterodimer conformations that differentially respond to multiple ecdysteroid/retinoid signals. Two splicing variants of the fiddler crab retinoid-X-receptor (UpRXR) differ from one another by the addition of a 33 amino acid insert in the ligand-binding domain. We show here that the ecdysteroid receptor in the fiddler crab (UpEcR) behaves differently depending upon the UpRXR isoform with which it is partnered. The two UpRXR variant partners for UpEcR confer slightly different responses in the binding of Ponasterone A (PA)—a naturally occurring ecdysteroid in the blood of Uca. UpRXR can bind 9-cis retinoic acid (9cRA) as well as terpenoids. 9cRA and the naturally occurring terpenoid, methyl farnesoate, influence the binding of PA to UpEcR/UpRXR dimers. Endogenous retinoids are found in the blastema of regenerating limbs of Uca and they (plus blood-borne terpenoids) may add additional levels of differential response by target tissues. Thus, the two sets of heterodimers tested here may represent different dynamic complexes whose properties are defined by the specific heterodimeric subunits involved and the specific ligands available.     

Epidermal collagenase activity and its induction by 20-hydroxyecdysone in the fiddler crab Uca pugilator

The epidermal collagenase activity and its induction by 20-hydroxyecdysone in Uca pugilator were investigated.Zymographic electrophoresis showed four bands of collagenae activity,16,19,22 and 29 kDa in molecular weight,with the former two accounting for 60% and 36%,respectively,of the total coUagenase activity. The collagenase activity varies during the molting cycle. Among the molt stages tested,Premoh Stage Do exhibited the highest epidermal coUagenase activity for both the 16 and 19 kDa isoenzymes and,as the molt stage proceeded,the enzymatic activity of these two isoenzymes decreased,with the lowest activity for both found in Premoh Stage D3-4. Injection of 20-hydroxyecdysone significantly induced the activity of the 16 kDa collagenase in the epidermis of Uca pugilator,suggesting that the activity of this isoenzyme is under molting hormone control.Although 20-hydroxyecdysone injection did not result in a statistically significant increase in the activity of the 19 kDa isocnzyme,a tendency of the induction was nonetheless demonstrated. This is the first report on epidermal collagenase activity and its induction by the molting hormone in a crustacean.     

Epidermal collagenase activity and its induction by 20- hydroxyecdysone in the fiddler crab Uca pugilator

The epidermal collagenase activity and its induction by 20-hydroxyecdysone in Uca pugilator were investigated. Zymographic electrophoresis showed four bands of collagenase activity, 16, 19, 22 and 29 kDa in molecular weight, with the former two accounting for 60% and 36% , respectively, of the total collagenase activity. The collagenase activity varies during the molting cycle. Among the molt stages tested, Premolt Stage Do exhibited the highest epidermal collagenase activity for both the 16 and 19 kDa isoenzymes and, as the molt stage proceeded, the enzymatic activity of these two isoenzymes decreased, with the lowest activity for both found in Premolt Stage D3 -4 - Injection of 20-hydroxyecdysone significantly induced the activity of the 16 kDa eollagenase in the epidermis of Uca pugilator, suggesting that the activity of this isoenzyme is under molting hormone control. Although 20-hydroxyeedysone injection did not result in a statistically significant increase in the activity of the 19 kDa isoenzyme, a tendency of the induction was nonetheless demonstrated. This is the first report on epidermal collagenase activity and its induction by the molting hormone in a crustacean .     

A collagenolytic serine protease with trypsin-like specificity from the fiddler crab Uca pugilator

A second collagenolytic serine protease has been isolated from the hepatopancreas of the fiddler crab, Uca pugilator. This enzyme cleaves the native triple helix of collagen under physiological conditions of pH, temperature, and ionic strength. In addition to its collagenolytic activity, the enzyme exhibits endopeptidase activity toward other polypeptides and small molecular weight synthetic substrates. The polypeptide bond specificity of this enzyme is similar to that of bovine trypsin as is its interaction with specific protease inhibitors. The amino-terminal sequence of this enzyme displays significant homology with other serine proteases, most notably with that of crayfish trypsin, and demonstrates that this enzyme is a member of the trypsin family of serine endopeptidases. The relatively unique action of this protease with regard to both collagenous and noncollagenous substrates has important implications concerning the specificity and mechanism of collagen degradation.     

Effect of melatonin on hemolymph glucose and lactate levels in the fiddler crab Uca pugilator

Melatonin was injected into intact and eyestalk-ablated fiddler crabs (Uca pugilator), and its effects on hemolymph glucose and lactate levels were studied. In intact crabs, glucose and lactate levels cycled simultaneously, with peaks occurring during early and late photophase. Melatonin caused a shift in the glucose and lactate cycles, with only one peak occurring closer to mid-photophase. In eyestalk-ablated animals, the glucose rhythmicity was lost; lactate cycled, but levels were significantly lower than in intact animals. Melatonin caused a delayed hyperglycemia in eyestalk-ablated animals, with concurrent but much lower increases in lactate. Overall, melatonin demonstrated delayed hyperglycemic effects that do not appear to be mediated solely via eyestalk factors such as crustacean hyperglycemic hormone (CHH), though involvement of the eyestalks cannot be ruled out. An influence on extra-eyestalk CHH secretion is a potential mechanism of melatonin activity.     

Effect of naphthalene on color changes of the sand fiddler crab, Uca pugilator

The polycyclic aromatic hydrocarbon, naphthalene, inhibited the melanin dispersion, which normally occurs in the fiddler crab, Uca pugilator, during the dark phase of its circadian rhythm of color change. Naphthalene produced this inhibition no matter whether the melanin was fully aggregated or more or less dispersed at the time of initial exposure. Inhibition of the circadian melanin dispersion does not appear to be due to a direct action of naphthalene on the melanophores. This inhibition was concentration-dependent with acute daily exposure. Naphthalene does not, however, inhibit melanin dispersion in response to a black background, indicating thereby that naphthalene-exposed crabs continue to synthesize melanin-dispersing hormone and norepinephrine.     

Degradation of collagen substrates by a trypsin-like serine protease from the fiddler crab Uca pugilator

The collagenolytic properties of a trypsin-like protease from the hepatopancreas of the fiddler crab Uca pugilator have been examined. All collagen types, I-V, were attacked by this enzyme. Types III and IV were degraded much more rapidly than types I, II, and V. Crab protease produced multiple cleavages in the triple helix of each collagen at 25 degrees C; only in the case of type III collagen, however, was a major cleavage observed at a 3/4:1/4 locus that corresponded to the region of collagen susceptibility to mammalian collagenase action. Additionally, both the affinity and the specific activity of the crab protease for native collagen were lower than those which characterize mammalian collagenase. The results of this study, in conjunction with a previous report on the collagenolytic activity of another serine protease from the fiddler crab [Welgus, H. G., Grant, G. A., Jeffrey, J. J., & Eisen, A. Z. (1982) Biochemistry 21, 5183], suggest that the following properties distinguish the action of these invertebrate collagenolytic enzymes from the metalloenzyme collagenases of mammals: (1) broad substrate specificity, including both noncollagenous proteins and collagen types I-V; (2) ability to cleave the native triple helix of collagen at multiple loci; (3) reduced affinity or higher Km for collagen; and (4) lower specific activity on collagen fibrils.     

Effect of cadmium exposure on lactate dehydrogenase activity in the hepatopancreas and abdominal muscle of the fiddler crab,Uca pugilator

1. Lactate dehydrogenase (LDH) activity in the hepatopancreas and abdominal muscle of fiddler crabs, Uca pugilator, was determined after 24 and 48 hr of exposure to 2 ppm cadmium chloride.2. For the cadmium exposed crabs, LDH activity in the hepatopancreas decreased, whereas that in the abdominal muscle increased.3. The increased LDH activity in the abdominal muscle may reflect increased dependence on anaerobic carbohydrate metabolism in fiddler crabs exposed to cadmium in their environment.     

Chitinase activity in the epidermis of the fiddler crab, Uca pugilator, as an in vivo screen for molt-interfering xenobiotics

We describe an in vivo screening assay that uses epidermal chitinase activity as the endpoint following a 7-day exposure of Uca pugilator to test chemicals. Chitinase, a chitinolytic enzyme, is the end product of endocrine cascades of a multi-hormonal system for control of crustacean molting. Wherever a molt-interfering agent adversely impacts the Y-organ-ecdysteroid receptor axis, the effect should be manifested by the activity of chitinase in the epidermis. Therefore, epidermal chitinase activity is an ideal endpoint for molt-interfering effects of xenobiotics. The validity of epidermal chitinase activity being used for such a purpose is supported by our finding that two injections of 20-hydroxyecdysone at 25 microg/g live weight induced a twofold increase in chitinase activity in the epidermis of U. pugilator. A total of nine chemicals were screened for molting hormone and anti-molting activities. o,p'-DDT was found to significantly inhibit epidermal chitinase activity while kepone and methoxychlor exhibited a tendency of inhibition of enzymatic activity. None of the remaining six chemicals, namely, p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT), atrazine, tributyltin (TBT), methoprene, dieldrin and permethrin, had an effect on epidermal chitinase activity.     

Effects of 9-cis- and all-trans-retinoic acids on blood glucose homeostasis in the fiddler crab, Uca pugilator

9-cis-Retinoic acid (9CRA) and all-trans-retinoic acid (ATRA) are known to be involved in the regulation of glucose homeostasis in vertebrates by inducing insulin release and expression of glucose transporter proteins. In view of the fact that both 9CRA and ATRA are endogenous to the fiddler crab, Uca pugilator, that a retinoid X receptor exists in this fiddler crab and that activities of insulin-like and insulin-like growth factor-like peptides have been reported for crustaceans, we investigated whether 9CRA and ATRA also play a role in glucose homeostasis in U. pugilator. Neither 9CRA nor ATRA was found to produce hypoglycemic effects at a dose of 10 microg/g live mass. However, 9CRA, but not ATRA, induced hyperglycemia. Such 9CRA-induced hyperglycemia was apparently mediated by the eyestalk hormone CHH since injection of 9CRA into eyestalk-ablated crabs did not result in hyperglycemia. ATRA was found to have an inhibitory effect on the recovery of blood glucose concentration following ATRA administration. Discussion on the possible mechanisms for the actions of 9CRA and ATRA was presented.     

Roles of 5-hydroxytryptamine and dopamine as neurotransmitters eliciting release of erythrophorotropic hormones in the fiddler crab,Uca pugilator

This article reviews the endocrinological, pharmacological and biochemical evidence ascribing neurotransmitter roles for 5-hydroxytryptamine (5-HT, serotonin) in eliciting the release of red pigment-dispersing hormone (RPDH) and for dopamine (DA) in stimulating the release of red pigment-concentrating hormone (RPCH) in the fiddler crab, $\text{Uca pugilator}$. 5-HT produces red pigment dispersion in intact crabs, but only indirectly. Likewise, DA evokes red pigment concentration $\text{in vivo}$ but it has no effect on red chromatophores (erythrophores) in isolated legs. The data obtained with 5-HT and DA agonists and antagonists on red pigment translocation $\text{in vivo}$ and $\text{in vitro}$, are consistent with their neurotransmitter candidacies in evoking the release of these erythrophorotropic hormones.     

Experimental studies on the foraging behavior of the sand fiddler crab Uca pugilator (Bosc, 1802)

The purpose of this study was to test the hypothesis that foraging sand fiddler crabs. Uca pugilator (Bosc), move through the habitat in response to low substratum food levels even though these movements may take the crabs considerable distances from the safety of the burrow area. Chl a and ATP concentrations were used as measures of food density in foraged and unforaged substratum. Field and laboratory feeding experiments showed that crab foraging intensity in a habitat patch was directly correlated with food density in the patch either in the presence or absence of alternative food patches. Other experiments showed that sand fiddlers can respond to differences in food level on a scale of millimeters and do this by probing the substratum with minor chelae. Food levels in aggregations of non-ingested particles harvested by sand fiddlers, feeding pellets, correspond to low foraging intensities predicted from foraging experiments and crabs exhibit low foraging intensities on substratum patches derived from feeding pellets. Substratum food levels in two distinct areas corresponded to high predicted foraging intensities and there was no consistent trend in the level of food in the burrow vs. the nonburrow microhabitats. These results suggest that the movements of foraging sand fiddlers are to some extent controlled by the reduction in substratum food levels due to feeding during a single foraging episode. Sand fiddlers can extract over 70% of the food from harvested substratum over a broad range of substratum food densities but harvest only 42% of the available substratum.