Choosing hunting sites with little information: patch-choice responses of crab spiders to distant cues

Adult female crab spiders (Misumena valid) released in a meadowmoved to milkweed stems more often than to other sites and alsomoved to flowering milkweed stems more often than to equallyavailable nonflowering ones. Preferences of the spiders as apopulation were similar during initial and second trials, butgiven individuals did not repeat their initial choices moreoften than predicted by chance. Thus, spiders probably respondeddirectly to cues present, rather than to information retainedfrom earlier experience. Spiders placed at the base of floweringmilkweed stems did not choose these sites more often than predictedby chance or more often than did spiders released in the grass,suggesting that this increase in localization provided no additionalinformation for choosing a hunting site. However, spiders placedat the bases of nonflowering stems selected them less oftenthan predicted. Second trials with these spiders also resembledinitial trials, but, as with the spiders released in the grass,given individuals did not repeat their initial performance moreoften than predicted. Thus, spiders released in the grass probablyresponded directly to cues present, as did spiders releasedat the base of stems. The choices of these spiders, combinedwith flexible giving-up times, caused the spiders to be concentratedon flowering stems. Although this strategy results in a better-than-randompattern of selecting hunting sites, the spiders lose considerableforaging time in the process.     

Pick-up lines: cues used by male crab spiders to find reproductive females

The low population density of the crab spider Misumena vatiaand the high percentage of gravid adult females begs the question,how do adult males find reproductive females? We explored oneof the potential mate-locating tactics of adult males, theirline-following behavior. M. vatia do not build webs; however,they do lay down silken lines during their movements. In boththe field and the laboratory, adult males (but not juvenilemales) regularly followed lines of other individuals, as wellas their own, sometimes multiple times. However, they did notdistinguish the direction in which lines were laid, and theyeven followed lines of other species. Males responded mechanicallyto these lines, but not chemically. They followed lines ofpenultimate and adult virgin females, as well as new and old lines, with similar high frequency. Neither washing the lineswith water nor washing them with acetone (to remove possiblewater-soluble or organically soluble pheromones) affected theirchoice of lines. Due to the low activity of females, theirlines may be several days old, over which time any possible chemical information would likely dissipate. Therefore, pheromonesseem unlikely to aid males searching for unmated adult females.Nevertheless, even searching males that follow lines largelyindiscriminately should derive a benefit because concentrationsof lines could denote the presence of females and thereby maximizenumbers of females encountered.     

Exploitation of floral signals by crab spiders (Thomisus spectabilis, Thomisidae)

Flowers exhibit characteristics through which they exploit thesensory biases of pollinating insects, and both signaler andreceiver benefit from this interaction, either through reproductiveservice or food reward. However, the preferences of pollinatorsfor certain flower traits such as color or odor might be exploitedby predators that target pollinating insects. Crab spiders,Thomisus spectabilis, position themselves on flowers to preyon pollinators such as honeybees, Apis mellifera. We gave bothhoneybees and crab spiders the choice between two randomly chosenwhite Chrysanthemum frutescens, including olfactory signalsin one experiment and excluding odor in a second experiment.When olfactory signals were included, crab spiders and honeybeesclearly preferred the same flower out of a pair. However, agreementlevel was at chance in the absence of olfactory signals. Wealso analyzed the visual flower characteristics that might influencethe decision of the animals. Neither the size of flowers (diameterof flower and diameter of reproductive flower center) nor thereflectance properties (receptor excitation values in ultraviolet,blue, and green; overall brightness) influenced the choicesof crab spiders and honeybees. Therefore, odor seems to be thefloral signal that bees use to identify high-quality flowersand that crab spiders exploit to encounter honeybees.     

Fine-scale substrate use by a small sit-and-wait predator

Substrate choice is one of the most important decisions thatsit-and-wait predators must make. Not only may it dictate theprey available but also the cover for the predator which mayconceal it from prey or its own predators. However, while ona particular substrate the behavior and use of that substratemay vary widely. When naïve, newly emerged crab spiderlingsMisumena vatia (Thomisidae) occupied flowering goldenrod Solidagocanadensis, their behavior differed markedly on inflorescenceswith relatively sparse and densely packed flower heads as wellas on experimentally thinned and unthinned inflorescences. Initially,the spiderlings most often hunted at the thinned sites and hidamong the dense flower heads at the unthinned sites, a differencethat disappeared in all broods tested after 2–3 h, possiblybecause of the growing hunger of the initially concealed individuals.Prey capture (dance flies) in the thinned sites initially significantlyexceeded that in unthinned sites but subsequently did not differ.However, spiderlings encountered their principal predator, thejumping spider Pelegrina insignis, significantly more oftenon unthinned than thinned inflorescences. Even though usagepatterns initially differed strikingly, spiderlings did notdiffer in their rates of quitting the two types of sites. Theseresults suggest a trade-off between foraging and predator avoidancethat changes in response to increasing hunger over time.     

A test of sexual cannibalism models, using a sit-and-wait predator

Sexual cannibalism, the capture and consumption of one sex by the other resulting from sexual interactions, provides an intriguing set of conditional fitness payoffs. Usually these payoffs will only benefit the cannibal, but under certain circumstances advantages might accrue to the cannibalized individual as well. Here I test three models of sexual cannibalism with data from a sit-and-wait predator, the crab spider Misumena vatia , whose lifestyle differs from the species for which these models were generated. The models include both precopulatory mechanisms (no gametes exchanged) and postcopulatory ones (gametes exchanged) perceived to generate adult fitness payoffs and situations in which cannibalism is not adaptive in the adult stage (aggressive spillover). In M. vatia , females sometimes cannibalize males before mating can occur. Precopulatory cannibalism is unlikely to reward female M. vatia significantly because of the diminutive size of the males, but it could be part of a broader syndrome that provides the females with large food payoffs earlier in the life cycle. Although the frequency of cannibalism is low (3.8–7.6%), this level suffices to be an important selective factor, a point accentuated by the extremely cautious behaviour of the males toward the females. I then compare the traits associated with these models to the species for which they were designed and certain others for which adequate data also exist. None of the sets of responses to these traits closely resembles each other in the different species. These results suggest that sexual cannibalism has developed in response to a heterogeneous collection of lifestyles and phylogenetic constraints.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 427–437.     

Sexual size dimorphism and male contest in wild Siamese fighting fish

Larger males of the wild Siamese fighting fish Betta splendens were more successful in male contests. There were no differences in fighting duration among treatments. Comparing agonistic behaviour between large and small males in ±1 and ±2 S.D treatments, larger males attacked, chased and performed total agonistic behaviour more than smaller males. There were no differences between larger and smaller males concerning other agonistic behaviour during fighting. Females presented with two potential mates of different sizes did not prefer larger males.     

Female hunger can explain variation in cannibalistic behavior despite male sacrifice in redback spiders

Male Australian redback spiders (Latrodtctus hasselti Thorell:Theriidae) place their abdomens directly over their mate's mouthpartsduring copulation, increasing the likelihood of sexual cannibalism.Male sacrifice may be adaptive because cannibalized males increasetheir paternity relative to those that are not eaten. Despitemale sacrifice behavior, however, up to 50% of laboratory matingsmay end without sexual cannibalism. Here, I report a similarpattern in the field, where males were not cannibalized in 35%of observed matings (6/17). I examined variation in female cannibalisticbehavior by evaluating the following three hypotheses for theoccurrence of cannibalism from the female perspective: (1) themistaken identity hypothesis proposes that females sometimescannibalize males because they mistake them for prey, (2) themate rejection hypothesis predicts that females cannibalizemales who are unacceptable as mates, and (3) the feeding opportunismhypothesis predicts that hungry females are more likely to becannibalistic Field observations refuted die first two hypotheses:females recognized males as potential mates (i.e., nonprey),and cannibalized and noncannibalized males were not phenotypicallydifferent. The feeding opportunism hypothesis was supported.In staged field matings, cannibalistic females were hungrierthan their noncannibaUstic counter-parts. Moreover, a logisticregression analysis indicated that hunger was a significantpredictor of cannibalism. Because redback males are below thetypical prey size that females accept, well-fed females areless likely to consume their mates, despite the vulnerable matingposture. These results indicate that, although males facilitatesexual cannibalism, their fate may depend on the female's physicalcondition.     

Important crab resources inhabiting Hokkaido waters

In this paper the fishing grounds, fluctuations in catches, history of exploitation, classification, distribution, migration, life history and resource trends of the edible crabs of the waters around Hokkaido are discussed. The Hokkaido crab fisheries developed along with the canning industry and there is now an increasing demand for boiled crab. At first, the most popular species was the king crab, Paralithodes camtschatica, but as its numbers declined other species such as the horsehair crab, Erimacrus isenbeckii, the banasaki crab, Paralithodes brevipes, the snow crab, Chionoecetes opilio, and the red snow crab, Chionoecetes japonicus, became popular. This report is based on the results of the latest research into the ecology and resources of the Erimacrus isenbeckii and Paralithodes brevipes species, which are currently the focus of resource conservation in Hokkaido.     

Risky mate search and male self-sacrifice in redback spiders

Male redback spiders twist their abdomens onto the fangs oftheir mates during copulation and, if cannibalized (65% of matings),increase their paternity relative to males that are not cannibalized.The adaptive male sacrifice hypothesis proposes that this increasedreproductive payoff from a single mating outweighs the residualreproductive value of a cannibalized male, because high mortalityduring mate searching restricts alternative mating opportunities.It has been reported that redback male residual reproductivevalue is low because males are functionally sterile after onemating—a putative intrinsic constraint that could arguablyfavor self-sacrifice in the absence of ecological restrictionson multiple mating. However, sterility and self-sacrifice mayboth arise as aspects of a terminal investment strategy if theprobability of multiple mating is sufficiently low. Here I reportfield data that support the adaptive male sacrifice hypothesis.More than 80% of redback males die without finding a potentialmate in nature. Data from two observational field studies andone release experiment suggest that in the absence of cannibalism,male redbacks would expect fewer than one mating opportunityin a lifetime. This expectation was not significantly higherfor a large male or one in good condition. A simple quantitativeanalysis confirms that even if males are assumed to be fertilethroughout life, the measured mortality rate during mate searchin combination with previously documented paternity benefitsof cannibalism is sufficient to ensure that self-sacrifice isadaptive for male redback spiders.     

The effect of colour variation in predators on the behaviour of pollinators: Australian crab spiders and native bees

1. Australian crab spiders exploit the plant–pollinator mutualism by reflecting UV light that attracts pollinators to the flowers where they sit. However, spider UV reflection seems to vary broadly within and between individuals and species, and we are still lacking any comparative studies of prey and/or predator behaviour towards spider colour variation. 2. Here we looked at the natural variation in the coloration of two species of Australian crab spiders, Thomisus spectabilis and Diaea evanida, collected from the field. Furthermore, we examined how two species of native bees responded to variation in colour contrast generated by spiders sitting in flowers compared with vacant flowers. We used data from a bee choice experiment with D. evanida spiders and Trigona carbonaria bees and also published data on T. spectabilis spiders and Austroplebeia australis bees. 3. In the field both spider species were always achromatically (from a distance) undetectable but chromatically (at closer range) detectable for bees. Experimentally, we showed species‐specific differences in bee behaviour towards particular spider colour variation: T. carbonaria bees did not show any preference for any colour contrasts generated by D. evanida spiders but A. australis bees were more likely to reject flowers with more contrasting T. spectabilis spiders. 4. Our study suggests that some of the spider colour variation that we encounter in the field may be partly explained by the spider's ability to adjust the reflectance properties of its colour relative to the behaviour of the species of prey available.     

Exploring the effect of the Cardinium endosymbiont on spiders

Spiders have recently emerged as important diversity hot spots for endosymbiotic bacteria, but the consequences of these symbiotic interactions are largely unknown. In this article, we examined the evolutionary history and effect of the intracellular bacterium Cardinium hertigii in the marbled cellar spider Holocnemus pluchei. We showed that Cardinium infection is primarily transmitted in spider populations maternally via egg cytoplasm, with 100% of the progeny from infected mothers being also infected. Examination of a co‐inherited marker, mitochondrial DNA (mtDNA), revealed that Cardinium infection is associated with a wide diversity of mtDNA haplotypes, showing that the interaction between Cardinium and H. pluchei has a long‐term evolutionary dimension and that horizontal transmission among individuals could also occur. Although Cardinium is well known to exert sex ratio distortion or cytoplasmic incompatibility in various arthropod hosts, we show, however, that Cardinium does not interact with the reproductive biology of H. pluchei. A field survey shows a clear geographical structuring of Cardinium infection, with a marked gradual variation of infection frequencies from ca. 0.80 to 0. We discuss different mechanistic and evolutionary explanations for these results as well as their consequences for spider phenotypes. Notably, we suggest that Cardinium can either behave as a neutral cytoplasmic element within H. pluchei or exhibit a context‐dependent effect, depending on the environmental conditions.     

Distribution of social crab spiders in eucalypt forests

Abstract Social crab spiders (Thomisidae, Diaea) are found in Eucalyptus forests of varying latitude, altitude and species composition in southern Australia. Neither temperature nor rainfall differ between areas where social Diaea are found, suggesting that spiders have a preferred climatic range, and that they change altitudes at different latitudes to maintain their preferred range. In these areas, those Eucalyptus species that hosted Diaea had smaller leaves and fruit than eucalypts that did not, which suggests that the spiders may choose trees based on leaf morphology.