ECOLOGY AND EVOLUTION OF MATING SYSTEMS OF FIDDLER CRABS (GENUS UCA)

1. General accounts of the natural history and behaviour of fiddler crabs suggest there exist two broad mating patterns in the genus. Most western and Indo-Pacific species mate on the surface of intertidal substrates near burrows females defend. The sexes associate only briefly during courtship and mating. In contrast, males of many American species court from and defend burrows to which females come for mating. Copulation occurs underground in burrows plugged at the surface; the sexes usually remain together for at least several hours. Here we summarize and contrast recent detailed field studies of the mating systems of U. pugilator, an American species, and U. vocans, a species widely distributed in the western and Indo-Pacific. We indicate how differences in the breeding ecology of these two species may account for basic differences in modes of sexual selection leading to the two broad mating patterns in the genus. 2. U. pugilator burrows in protected sandy substrates in the upper intertidal and supratidal zone. During ebb tide, nonbreeding crabs leave burrows they occupy during high tide to forage on food-rich substrates in the lower intertidal zone. Reproductively active males remain in the burrow zone where they fight for and defend burrows from which they court. Large males win most fights for burrows and tend to defend burrows high on the elevation gradient, especially during periods with relatively high tides. Females usually approach and descend the burrows of several males before choosing their mates by remaining in males' burrows. Males remain underground with their mates for 1–3 days until after they oviposit their eggs. Some males then emerge and leave their burrows while others sequester their mates in the chambers where mating and oviposition has occurred, dig new chambers and resume courtship, perhaps attracting additional females. In either case, females remain underground for approximately 2 weeks, finally emerging to release their planktonic larvae. Burrows that do not collapse due to tidal inundation or flooding by groundwater are best for breeding and usually are located relatively high on the elevation gradient. Females choose mates indirectly by preferring to breed in burrows that will remain intact while they oviposit and incubate their eggs. Large males mate more often than small males because they are better able to defend burrows at locations females prefer to breed. The mating system of U. pugilator may be classified as resource-defence polygyny. 3. U. vocans burrows in open muddy substrates in the mid- to lower intertidal zone. At a site near Chunda Bay, Australia, where the reproductive behaviour of this species has been studied in depth, both sexes feed near burrows they defend. Females tend to occupy their burrows for longer periods and move shorter distances than do males. Mating occurs on the surface near the burrows that females defend. Females accept both resident and wandering males as mates. They show no preference for mating with larger males. Female choice may be based on other male morphological or behavioural characteristics. Females oviposit their eggs either while on the surface or in their burrows. They produce relatively small clutches and are active on the surface throughout their breeding periods. Males fight both their neighbours and wandering males. Large males tend to win fights and defend burrows in areas where large females, which produce relatively many eggs, are most dense. Such areas may offer greater protection from predators than areas occupied by smaller females. Small males mate about as often as large males but may father fewer larvae. The mating system of U. vocans is resource-free and promiscuous. 4. The mating systems of U. pugilator and U. vocans differ fundamentally in that female U. pugilator require access to a specific microenvironment to breed successfully, while female U. vocans do not. We suggest this difference occurs because of contrasts in clutch sizes and the mobility and movement patterns of feeding females. Female U. pugilator produce relatively large clutches and probably experience more intense selection from factors that can cause egg loss and mortality than do U. oocans, which produce clutches of sufficiently small volume to be protected by their abdominal flaps. Hence, the range of suitable breeding environments for U. pugilator is small compared to that for U. vocans. In addition, U. pugilator burrows in areas that are relatively food-poor, leading to daily migrations to and from food-rich substrates in the lower intertidal zone, preventing female defence of an area suitable for both breeding and feeding. U. vocans, however, burrows in areas sufficiently rich to support feeding, leading to relatively low female mobility and defence of burrows that are also suitable breeding sites. 5. Adaptive radiation of the genus Uca in the Americas is manifest by trends toward smaller adult size, higher population densities, more frequent microgeographic sympatry and increased terrestriality, compared to species in the western and Indo-Pacific regions. We outline the general features of the selection mechanisms tying each of these trends to the evolution of resource—defence mating systems. Intraspecific variation in the courtship behaviour and site of mating in U. lactea and U. vocans supports our contention that resourse—defence behaviour tends to occur at high population densities. Additional data are needed to evaluate the other hypotheses critically.     

Comparative Studies of Reproductive Behavior in Mantis Shrimps and Fiddler Crabs

Comparative ethological studies of closely related species canplace into clear perspective the functions of behavioral traitsas species diverge and adapt to different environments. In thispaper we contrast the reproductive behavior of species withintwo crustacean groups: the mantis shrimps (stomatopods) andthe fiddler crabs (genus Uca). For the stomatopods, we identifyprobable selective relationships between features of species'ecology and their mating systems. Population density, the kindand availability of shelters in which these shrimps live, theintensity of predation and spawning cycles all play importantroles in molding reproductive behavior in this group. Associationsbetween the ecology and mating systems of fiddler crabs havebeen discussed recently elsewhere. Here we focus on a comparativeanalysis of sexual communication in these crabs. A study ofcompetitive courtship signaling and mate choice in the fiddlercrab Uca beebei has shown that certain male courtship signalsare highly attractive probably because they exploit female sensory-responsesystems that have been molded by selection for escape from predators.Interspecific comparison of male courtship displays and theresponses of females to these displays suggest that sensoryexploitation may play an important role in the evolution ofsexual signals in the genus. Comparative studies have advancedour understanding of how natural and sexual selection affectthe reproductive behavior of both stomatopods and fiddler crabs.     

On the Mode of Activation of Sequestered Messengers in Artemia salina

Activation of the dormant embryos of Artemia salina was marked by a rapid increase in ³²P uptake which reached a stationary phase after 6 h of activation. The increase in ³²P uptake by whole cysts was paralleled by its incorporation into nucleotides. Fractionation of acid-soluble nucleotides and alkaline hydrolysate of nucleic acids on Dowex-1-formate column revealed the ³²P radioactivity to be exclusively localised in AMP. Analysis of the labelled RNA species extracted at different stages of development indicated a preferential labelling of small molecular weight species till the emergence of the embryos, followed by the de novo synthesis of messenger and stable RNA species in later stages of development. During early development, polyadenylated RNA species were localised in the par-ticulate fraction sedimenting at 16,000 rpm and their location shifted to the soluble fraction as development proceeded. Activation of preformed messengers by phosphorylation of the adenylate residue of their poly A stretches and translocation of the capacitated messengers to the cytosol via a RNP-membrane complex is proposed as a trigger of embryonic differentiation.     

Size Bias in Visual Searches and Trapping of Brown Treesnakes on Guam

ABSTRACT The accidental introduction of the brown treesnake (BTS; Boiga irregularis) to the island of Guam after World War II set off a chain of bird, bat, and lizard extirpations. Fortunately, many of the eliminated species have the potential to be restored following population reduction or eradication of the snake. The primary operational tool for population reduction is an effective snake trap, but areas subjected to long-term trapping continue to support BTS, suggesting that some adult snakes are refractory to trapping. We closed a 5-ha area to BTS emigration and immigration and surveyed the population using trapping and visual surveys to determine whether a refractory stratum of adult snakes existed and if trapping was effective for snakes of all sizes. Our surveys included 101 trapping occasions and 109 visual surveys over 309 days, resulting in 2,522 detections of 122 individuals. We detected 44 of 45 supplemented snakes by this intensive sampling effort, which also revealed that trapping was fully effective for snakes >900 mm in snout—vent length (SVL), partially effective for snakes 700–900 mm SVL, and totally ineffective for smaller juveniles (350–700 mm SVL). Visual searching was effective for snakes of all sizes. As BTS mature at approximately 950-1, 050 mm SVL, continuous trapping should suffice to eliminate recruitment in the absence of immigration. Immigration or inadequate effort is most likely responsible for the persistence of BTS in areas subject to long-term trapping. Thus, current efforts to capture trap-refractory adult snakes with alternate control tools are less likely to be successful than immigration barriers alone or in combination with elevated capture effort.     

Low Genetic Diversity In Willaertia Magna From Wide Geographical Sources, and Characterisation of Willaertia Minor N. Sp. (Heterolobosea, Vahlkampfidae)

ABSTRACT. Twelve isolates assigned to Willaertia magna by morphological and physiological criteria, including the type material, were characterised at 23 enzyme-encoding loci. Isolates from Australia, Bangladesh, Belgium, England, France, Indonesia, Japan, Korea, Madagascar and Papua New Guinea showed fixed alleles at 18 loci and polymorphism at Acp, Lap, Me, 6Pgd and Pgm 1. the maximum difference in pairwise comparisons was 8.7%, consistent with intra-specific variation in the related genus Naegleria. an additional isolate, assigned to Willaertia by features of its flagellate and cyst stages, represents a new species. Willaertia minor n. sp. shares alleles with W. magna at only three loci, has a maximum growth temperature of 38°C and is significantly smaller in all stages than W. magna. Flagellates appear to divide only once and fail to replace the lost flagella, producing biflagellate daughter cells. Cysts and trophozoites are more readily mistaken for Naegleria , but lack perinuclear granules, as do the mature cysts of W. magna.     

Effects of dissolved organic matter and ultraviolet radiation on the accrual, stoichiometry and algal taxonomy of stream periphyton

1. We investigated the effects of dissolved organic matter (DOM) and ultraviolet‐B (UVB) radiation on periphyton during a 30‐day experiment in grazer‐free, outdoor artificial streams. We established high [10–12 mg carbon (C) L⁻¹] and low (3–5 mg C L⁻¹) concentrations of DOM in artificial streams exposed to or shielded from ambient UVB radiation. Periphyton was sampled weekly for ash‐free dry mass (AFDM), chlorophyll (chl) a , algal biovolume, elemental composition [C, nitrogen (N) and phosphorus (P)], and algal taxonomic composition. 2. Regardless of the UVB environment, increased DOM concentration caused greater periphyton AFDM, chl a and total C content during the experiment. Increased DOM also significantly increased periphyton C : P and N : P (but not C : N) ratios throughout the experiment. Algal taxonomic composition was strongly affected by elevated stream DOM concentrations; some algal taxa increased and some decreased in biomass and prevalence in artificial streams receiving DOM additions. UVB removal, on the other hand, did not strongly affect periphyton biomass, elemental composition or algal taxonomic composition for most of the experiment. 3. Our results show strong effects of DOM concentration but few, if any, effects of UVB radiation on periphyton biomass, elemental composition and algal taxonomic composition. The effects of DOM may have resulted from its absorption of UVA radiation, or more likely, its provision of organic C and nutrients to microbial communities. The strong effects of DOM on periphyton biomass and elemental composition indicate that they potentially play a key role in food web dynamics and ecosystem processes in forested streams.