Adaptive droving behavior observed in the fiddler crab Uca vocans vocans

Droving is conspicuous in Uca vocans vocans in summer. The crabs burrowing on the upper habitat have a higher tendency to wander compared to the ones burrowing on the lower habitat. Most of the wandering crabs captured on the low tide levels are relatively large and male. Larger crabs and males prefer to burrow on the upper zones of U. vocans habitat, but the smaller ones and females prefer the lower habitat. The upper level of the U. vocans habitat has relatively low N-content compared to the water's edge. Therefore droving is advantageous in crabs that have burrows on the upper level. Female and smaller resident crabs have faster feeding motions than male and larger ones, and can satisfy their feeding demands more rapidly. Therefore, for males and larger crabs it is advantageous to move away from the burrow area and forage in areas of higher food content near the low tide level where the number of feeding motions increases.     

Spatial and temporal feeding patterns of copepods from the North Pacific centra gyre

Spatial and temporal feeding patterns (determined from an index of gut fullness) are described for 10 typical species of calanoid copepods collected from the North Pacific central gyre (September 1968 to June 1977), an area where the zooplankton is food limited and there were a-priori reasons to suspect that feeding and competition for food were important in regulating zooplankton community structure. Over 100 samples from 11 cruises to the eastern part of the gyre were examined, and patterns of gut fullness were related to environmental variables and the copepod species structure. The copepods studied all tended to be omnivores and food generalists. Males had lower indices of gut fullness than females but both males and females of a species had similar spatial and temporal feeding patterns. Guts were usually fuller at night than during the day, even in nonmigrating species; however, within nighttime depth distributions, no depths were preferred for feeding. There were also differences between species in mean gut fullness, but different species tended to have similar spatial and temporal feeding patterns. There was considerable spatial variability, and locales could be identified in which most species had higher indices of gut fullness. The copepods were not necessarily more abundant in these locales, nor did these tend to be areas of above average chlorophyll concentration. These patterns were consistent with relatively nonselective feeding, and there was no evidence that these species separate their niches by feeding at differing places or times.     

Hydrocarbon metabolism in the intertidal fiddler crab Uca pugnax

The fiddler crab Uca pugnax was examined for its ability to metabolize foreign hydrocarbons. The microsomal mixed function oxidase system was identified in U. pugnax tissues using Aldrin epoxidation rates as the assay. Rates were slow: 96 pM Dieldrin per mg microsomal protein per hour in the hepatopancreas, 438 pM mg^-1 h^-1 in the gill, and 228 pM mg^-1 h^-1 in claw-muscle microsomes. Using standard methods, no difference in rates could be detected between crabs living in clean areas and those living in environments highly contaminated with foreign hydrocarbons. In vivo rates of naphthalene oxidation were measured and used to calculate a clearance time for U. pugnax body tissues based on the aromatic hydrocarbon content of crabs collected from an oil-polluted salt marsh. Calculated clearance time was beyond the life span of the crab. It is concluded that this minimal ability of U. pugnax to metabolize foreign hydrocarbons partially accounts for its sensitivity to oil pollution in the environment.     

A bacterial infection in the common shore crab Carcinus maenas and the fiddler crab Uca pugilator

A bacterial infection in Carcinus maenas and in Uca pugilator is described. The percentage of infections recorded in the field was between 0 and 10%, depending on season. Infected crabs remain about 3 times longer in Intermolt Stages C3 and C4 than do healthy crabs. In infected specimens of both species, the glucose level in the hemolymph is increased and protein, copper and lipid concentrations in the hemolymph as well as the glycogen content of various tissues are diminished. The coagulation process is disturbed in infected crabs. The hemocytes aggregate, but the formation of fibrin-like fibers, which can be observed in healthy crabs, does not occur.This paper forms part of a doctoral thesis presented at the University of Ulm (FRG).     

Non-independent mate choice in a fiddler crab: a case of stimulus enhancement

Non-independent mate selection occurs when the choice behavior of a female is altered by the interactions between other females and males. In the fiddler crab Uca mjoebergi, males court mate-searching females by waving their one greatly enlarged claw. When a female approaches a male, he initiates high-intensity waving. We conducted one natural mate choice experiment and two mate choice experiments using custom-built robotic crabs. We show that the decision of one female to approach a group of males increases the probability that another female will approach and visit a male from the same group. We suggest that this behavior is best explained by the ‘stimulus enhancement’ hypothesis, where the presence of a female near a group of males makes them more likely to be detected by other females due to an increase in male display rate.     

Pillar building in the fiddler crab Uca beebei: evidence for a condition-dependent ornament

In the fiddler crab (Uca beebei) males build a small mud pillar next to their burrow which increases their attractiveness to females. Three hypotheses were tested to explain inter-male variation in pillar-building. (1) The benefits of pillar-building are density dependent. The experimental addition of vertical structures did not support this hypothesis as there was no change in the level of pillar-building. (2) There are two classes of males (pillar-builders and non-pillar-builders). This could either be due to an alternative mating strategy, or because pillar building is age or size-dependent. There was also no support for this hypothesis. (3) Pillar-building is an honest signal of male quality dependent on body condition. A food supplementation experiment was performed. Addition of food affected several aspects of male behaviour and resulted in a two fold increase in the number of pillars built between control and food treatments (P < 0.001). However, the percentage of males building pillars did not increase significantly. Pillar building in this species has been attributed to sensory exploitation. Our results indicate that a trait which may well have evolved through sensory exploitation also appears to be condition-dependent. We emphasise that showing that an ornament or behaviour is condition-dependent does not necessarily mean that it evolved through good gene processes. However, in terms of its current selective value, pillar building may be maintained through female choice because it acts as a signal of male condition.     

Control of distal retinal pigment migration in the fiddler crab Uca pugilator

Several concentrations of extracts prepared from the eyestalks of a specimen of Uca pugilator were injected into other U. pugilator individuals. The distal pigment of the eyes first became light adapted and then dark adapted, the whole process lasting 6 h. The mean integrated response for light adaptation increased progressively up to the highest tested extract (3 eyestalk equivalents/dose), but with the darkadapting response the maximal effect was produced by the extract containing 2 eyestalk equivalents/dose. Gel filtration of eyestalk extracts in Sephadex G-50 showed that the fractions associated with greatest light adaptation were also associated with greatest pigment dispersion in the melanophores. Almost no light or dark adaptation of the retinal pigment resulted from injections of eyestalk extracts treated with -chymotrypsin which supports the interpretation that these substances are polypeptides of neurosecretory origin.Supported by Grant GB-7595 X from the National Science Foundation.     

Spatial,annual and seasonal patterns in the condition and muscle size of snow crab

The extent of spatial (depth and locality) and temporal (season and year) variabilities in condition and relative muscle size (a direct proxy of growth) were examined in male and female adult and non-adult snow crabs Chionoecetes opilio. Condition, determined from the relative size of the digestive gland and moisture content of the muscle and digestive gland, and muscle size, determined as the ratio of merus muscle mass over merus volume, separated as different processes in a principal component analysis. Snow crabs showed a wide range of condition and muscle size values. Overall, the condition was better in non-adult than in adult crabs, with adult females being in worst condition, and muscle size was larger in males than in females. Condition variability was greater for seasonal compared to annual samples, probably reflecting annual molt cycles. In contrast, the muscle size variability was greater for annual compared to seasonal samples, possibly as a result of changing crab abundance and competition intensity during recruitment pulses. Condition and muscle size increased through summer in males and immature females, although to different extents depending on instar, but did not change in adult females. Both condition and muscle size were highly variable at the investigated spatial scales. Condition and muscle size had a significant effect on gonad size, once the effect of crab size was removed, suggesting a direct link between these two parameters and reproductive capability.     

Larval mortality during export to the sea in the fiddler crab Uca minax

Dense populations of the fiddler crab Uca minax (Le Conte 1855) are common along tidally influenced freshwater rivers and streams >50 km from the sea. Adults do not migrate from inland sites to release larvae, but instead release them directly into an environment where the zoeae cannot survive. Laboratory salinity tolerance experiments were used to determine how long larvae from the inland-most population of U. minax along the Pee Dee River, South Carolina, USA can survive zero salinity compared to larvae from a brackish water population (salinity 5) near the mouth of Winyah Bay in the same estuary. Larvae from the brackish water population were also exposed to a salinity of 5 and their survival tracked. These experiments were conducted from May to August 2004 and 2005. To determine if inland larvae suffered significant mortality in transit due to salinity stress, current profiles were measured in the field and used to model the time taken by a larva using ebb-tide transport to travel to permissive salinities. The laboratory tolerance experiments showed that larvae from the inland freshwater population had LT50’s of 4–5 days at 0 salinity, which were significantly longer than those of the brackish water zoeae (2–3 days). Zoeae from the brackish water population survived for at least one larval molt at a salinity of 5 with LT 50’s of ∼12 days. Estimated travel times to reach permissive salinities from the inland-most population based on current profiles were 3–5 days for larvae using night-time only ebb-tide transport and 1.5–2.5 days for those using ebb-tide transport both day and night. Previously published field data indicate that U. minax larvae do use both day- and night-time ebb-tide transport, and are found in high densities in the water column during the day. These results lead to the conclusion that U. minax stage I zoeae do not undergo significant salinity-induced mortality during their 50+ km trip to the sea.     

Testing function of fiddler crab claw waving by manipulating social context

Many territorial advertisement signals are thought to be dual-function signals, directed to both rival male and receptive female conspecifics. However, few studies have tested this assumption by examining whether in fact both sexes are likely to elicit signaling behavior from territorial males. In this study, I experimentally manipulated the social context of male sand fiddler crabs (Uca pugilator) to investigate the effect of different audiences on the performance of the claw-waving display, a territorial signal that is often presumed to be directed to both males and females. To test whether males perform this signal to both audiences, I measured the frequency of waving behavior by focal males when housed in field enclosures alone, with only males, with only females, or with both males and females. Focal males waved at a low frequency when alone, and the presence of males had no effect on their level of waving. However, in the presence of females, focal males showed a significantly higher level of waving, whether or not males were also present. In addition, there was no association between fighting and waving behavior. This experiment provides evidence that from the perspective of the signaling male, the claw-waving display of U. pugilator is not a dual-function signal but rather is primarily directed to receptive females. Received: 16 December 1999 / Received in revised form: 1 February 2000 / Accepted: 19 February 2000     

Spatial and temporal patterns of predation on seeds of the surfgrass Phyllospadix torreyi

Spatial and temporal patterns of predation on seeds of the seagrass Phyllospadix torreyi S. Watson were quantified at four sites near Santa Barbara, California, USA. Over a period of four flowering seasons during 1995 to 1998, monthly patterns of seed fall and intensity of seed predation were similar among sites, but were temporally quite variable. Abundance of dispersed seeds varied greatly both among seasons and years. Within any one year, seeds were present in the environment every month, but they peaked in abundance during the fall months following the annual flowering period. Seeds were more abundant during the earlier years of the study. The intensity of predation steadily increased throughout the study period, from a low of ≤10% seeds consumed during 1995 to ∼50% consumption by 1997, and it was not correlated with abundance of seeds in the environment. Pre-dispersal seed loss also was estimated in two flowering seasons by counting the numbers of seeds consumed prior to release from the plant, and was relatively low (<15%). No differences were detected among the four study sites in patterns of pre-dispersal loss. Field surveys were done at two sites to identify potential seed predators. Three of the most abundant species identified in these surveys were tested in the laboratory to determine size-specific patterns of feeding activity and to assess which early life stages of P. torreyi (fruits within spadices, dehisced fruits, seedlings) were most vulnerable to predation. All three of the early life stages were consumed. The crabs Pugettiaproducta (Randall) and Pachygrapsus crassipes Randall were much more voracious predators than the isopod Idotea resecata Stimpson. Received: 8 June 1999 / Accepted: 2 February 2000     

Paternity analysis of two male mating tactics in the fiddler crab, Uca mjoebergi

The fiddler crab Uca mjoebergi mates both underground in male-defended burrows and on the surface near female-defended burrows. The reproductive tract of Uca species facilitates last-male precedence, suggesting that males that do not guard-mated females are likely to gain very little paternity if the female re-mates with another male. Here, we test the reproductive success of burrow and surface matings using paternity analysis. We found that 100?% of the females that mated in burrows extruded a clutch of eggs. Furthermore, we show conclusively, for the first time in a fiddler crab species, that last-male sperm precedence results in the majority of the female’s eggs being fertilised by the burrow-mated male. In contrast, surface matings resulted in significantly fewer females extruding eggs (5.6?%). Paternity analysis also revealed that more than half of the clutches from burrow-mated females showed low levels of extra-pair paternity from previous matings. Although multiple matings appear common in U. mjoebergi, burrow-mated males that guard females are guaranteed a successful mating with extremely high rates of assured paternity. Surface matings therefore appear to be an opportunistic tactic that may increase male reproductive success in a highly competitive environment.     

Chitobiase activity in the epidermis and hepatopancreas of the fiddler crab Uca pugilator during the molting cycle

The activity of chitobiase, also known as N-acetyl-β-glucosaminidase, in the epidermis and hepatopancreas of the fiddler crab Uca pugilator (Bosc, 1802), during the molting cycle, was investigated. A pH optimum of 5 to 6 was found for the enzymatic activity in both the epidermis and hepatopancreas. The temperature optimum for epidermal and hepatopancreatic chitobiase activities was 50 to 60 °C. The K _m values for epidermal and hepatopancreatic chitobiase activities at 19 °C were 0.190 ± 0.027 and 0.203 ± 0.016 mM 4-methylumbelliferyl-N-acetyl-β-glucosaminide, respectively. Hepatopancreatic chitobiase activity was significantly higher than epidermal enzymatic activity in all the molt cycle stages tested except Postmolt Stage A-B. Chitobiase activity varied significantly during the molting cycle, with the epidermal enzymatic activity in Premolt Stage D_3–4 significantly higher than in Stage C (intermolt) and Premolt Stage D₀, whereas hepatopancreatic chitobiase activity in Premolt Stage D_3–4 was significantly higher than in all other molt stages tested. The patterns of chitobiase activity in the epidermis and hepatopancreas correlate well with the hemolymph titer of ecdysteroids in U. pugilator during the molting cycle; this suggests that chitobiase activity in both tissues is regulated at least in part by the steroid molting hormones. Received: 6 May 1998 / Accepted: 12 September 1998     

Osmotic and ionic regulation in the fiddler crab Uca rapax acclimated to dilute and hypersaline seawater

Adult male Uca rapax, collected from the central coast of Venezuela in early 1994 were gradually acclimated to salinities ranging from 1.7 to 139S. The hemolymph osmolality (791±15 mOsmol in crabs from 26S) changed less than three-fold over the entire range of concentrations tested. The urine was isosmotic with the hemolymph in crabs exposed to dilutions <26S, and significantly hyperosmotic in those exposed to media >34.8S. The hemolymph levels of Na⁺, Cl^–, K⁺, Ca²⁺ and Mg²⁺ (320±13, 405±17, 7.8±0.7, 7.2±0.1 and 31±2.2 mmol l^–1, respectively, in crabs acclimated to 26S) were maintained fairly constant over the range from 8.7 to 99S, decreasing by 15% in the more dilute media or increasing sharply to about twice those values in crabs from 139S. The excretory organs contributed to the osmoionic regulation of the hemolymph in crabs exposed to <3.5 or to >34.8S, by means of the partial reabsorption or excretion, respectively, of salts from or into the urine. The results described place U. rapax among the most powerful hypo/hyper-regulating crustaceans known.     

Rhythmic activity responses of the fiddler crab Uca crenulata to artificial tides and artificial light

The fiddler crab Uca crenulata, from California (USA), was exposed to artificial tides in order to differentiate between the influence of a tidal cycle from that of a light-dark cycle on its locomotor activity. Most crabs could be well synchronized by tides, but the activity patterns of other crabs was merely exogenously reinforced. Under constant conditions, after tidal treatment, crabs showed bimodal or unimodal activity patterns. When a 24 h light-dark cycle and a 12.35 h tidal cycle acted as concurrent stimuli, their effectiveness in synchronizing the crabs' activity was variable, depending on the undividual. It is likely that crabs which tend to exhibit a unimodal activity pattern are more sensitive to a light stimulus, whereas crabs with a bimodal activity pattern respond preferably to the tides. Thus, responses of endogenous activity of U. crenulata to tides and light-dark cycles are similar, suggesting that the basic oscillarory mechanism for circadian and tidal activity may be the same.     

Rhythmic motor activity responses of the California fiddler crab Uca crenulata to artificial light conditions

Uca crenulata, the California species of fiddler crab, was exposed to artificial light conditions to separate the influence of the light cycle from that of the tidal input on its rhythmic motor activity. Under both constant light and light-dark cycles, rhythmic activity was demonstrated in only 50% of the experimental crabs; the activity of the remaining 50% was random. Individuals exposed to constant light conditions after having been subjected to 24 h light-dark cycles demonstrated no significant difference in period length of their rhythmic activity from crabs investigated in constant light immediately after field collection. The mean period did not differ significantly from the tidal period of 24.8 h, but the variation was considerable. In artificial light-dark cycles, the activity rhythms were usually masked but, in some cases, synchronized. The results indicate that U. crenulata has an endogenous rhythm with a period close to the tidal cycle which may be synchronized by light as well as by tidal cues. The display of this endogenous rhythm, however, is poor.     

The assessment of female reproductive state during courtship and scramble competition in the fiddler crab, Uca paradussumieri

Male fiddler crabs, Uca paradussumieri, mate underground during a 4- to 7-day period each full and new moon. As soon as the tide recedes, males enter the burrows of females that will ovulate the following day ('pre-ovigerous' females). Males copulate with and guard these females until they ovulate. When interrupted by an intruding male, the first male to reach the female is usually able to defend her and successfully mate with her. In fiddler crabs, females mate multiply and there is last male sperm precedence. Before each semi-lunar mating period, male U. paradussumieri were more likely to court females with whom they would later mate than other nearby females with whom they did not mate. This suggests that males collect information on female reproductive state prior to the females becoming ovigerous. In this species, aggression was common between males that courted the same female. When previously courted females were approached by other males, the initial courter attempted to forcefully disrupt the courtship. This behavior may allow males the exclusive use of information on female reproductive condition. It also suggests a type of scramble competition between males over females. Furthermore, it indicates that males are able to locate receptive females prior to their becoming ovigerous. The shorter guarding period observed in this species, as compared with other fiddler crabs, is caused by females rejecting longer guarding periods. Male ability to assess female reproductive status may therefore be advantageous because it increases male mating success within a scramble type of competitive polygyny.