Seasonal growth of Laminaria solidungula in the Canadian High Arctic in relation to irradiance and dissolved nutrient concentrations

Pontomyia is a genus of tiny flightless marine midges, with 4 described species, found on tropical or subtropical sea shores of the western Pacific Ocean. They are rarely noticed because of their small size (about 1.5 mm) and ephemeral adult life (1 to 3 h). They usually occur in association with seagrasses and seaweeds on which the larvae feed. The oar-like wings of adult males propel them over the water surface, but do not enable them to fly. Adult females are not only wingless but virtually legless. Emergence of the adults in the field may occur either during the day or at night. In a laboratory aquarium population of P. cottoni Womersley, maintained for about 18 months in a 12 h light-12 h dark cycle, males began to emerge at the end of the light period, whereas females emerged only after the onset of darkness. Field observations suggest that emergence times are probably influenced also by tides.     

Life history of the golden ring cowry Cypraea annulus (Mollusca: gastropoda) on Okinawa Island,Japan

Two mark-recapture studies, regular population censuses, field observations, and laboratory culture were used to study the life history of the tropical marine gastropod Cypraea annulus Linnaeus, 1758 from Cape Maeda, Okinawa Island, from April 1984 to March 1986. In the field, the average rate of increase in shell length of marked juvenile snails was 1.0±0.3 mm wk^-1 (N=13), with a maximum of 1.5 mm wk^-1 (initial shell length 10.6 to 17.2 mm). In the laboratory, the maximum growth rate of juveniles was 3.0 mm wk^-1 with food ad libitum. Snails with primordial teeth on the shells grew at a rate of 0.1 to 0.5 mm wk^-1 for about 2 wk. The adults continued to grow at a similar rate for an additional 3 to 6 wk, and ceased detectable growth when some females started spawning egg masses. In the littoral zone on Okinawa Island, snails reproduced throughout the year. Egg masses brooded by three females (shell length 17.4 to 21.3 mm) in the field contained 90 000 to 133 000 ova. In the laboratory, brooding periods of three females lasted 6,8, and 9 d. The estimated average frequency of spawning was 5 egg masses female^-1 yr^-1. The mean shell length of adult females (20.3 mm) was significantly larger than that of adult males (19.6 mm). The life-history strategy of C. annulus is characterized by rapid growth, high fecundity, and repeated spawning throughout the year.     

Vertical distribution and migration of fishes of the lower mesopelagic zone off Oregon

An exceptionally large midwater trawl (50 m² mouth area) with 5 opening and closing codends was towed horizontally in the lower mesopelagic zone at depths of 500, 650, 800 and 1000 m off Oregon (USA) from 1–6 September, 1978. In comparison to more conventional trawls, ours collected more fish, including rare species and large individuals of common species. Comparison of collections made by day and by night revealed that 12 of the 15 most common species probably migrated vertically. Bathylagus milleri evidently migrates from 650 m during the day to 500 m at night. Cyclothone acclinidens and C. atraria were more abundant by night than by day at 800 m, possibly due to an upward migration from deeper depths at night. C. pseudopallida, C. signata, Chauliodus macouni, Tactostoma macropus and Stenobrachius leucopsarus were more abundant by day than by night at 500 m, suggesting that they migrated out of this depth horizon at night. Lampanyctus regalis, and large individuals of B. pacificus were more abundant by night than by day at 500 m, possibly because they migrated upward from near 650 m. Many species exhibited trends of increasing or decreasing size with depth, and several species showed changes in migratory behavior with size. For example, only small (<240 mm) T. macropus migrated vertically, whereas only large (>110 mm) B. pacificus appeared to migrate. Depths of maximum abundance of congeneric species were usually separated. B. milleri and B. pacificus had similar distributions by day, but the former was shallower at night. S. leucopsarus tended to live shallower than S. nannochir both day and night. Congeners always occurring at the same depth were Cyclothone pseudopallida and C. signata (both most abundant at 500 m) and C. acclinidens and C. atraria (both most abundant at 800 m).     

Demography of the intertidal brown alga Pelvetia fastigiata in Southern California,USA

The demography of Pelvetia fastigiata was studied from 1973 through 1977 in the intertidal zone near La Jolla, California, USA. Populations were usually small and relatively stable as new recruits replaced older individuals that were lost; however, in 1977 high recruitment substantially increased total abundances. The survivorship of recruits was similar regardless of the season or year of recruitment. Mortality rates decreased with age, and the mean life expectancy continued to increase for at least 1 yr. Losses of juveniles were high; 50% lived 80 d and only 9% survived to the age of first reproduction (about 1.5 yr). Plants present at the start of the study had higher survivorship rates than new recruits, 19 and 7%, respectively, living more than 3 yr. Individuals within aggregations of large plants lived longer than most dispersed individuals, and aggregations remained intact throughout the study in spite of low recruitment. Established individuals had low mortality rates and populations probably relied on these persistent individuals for reproduction.     

Native legume establishment on acidic coal mining overburden at Collie,Western Australia

Nitrogen is often provided to impoverished overburden dumps through the establishment of legumes. Low indigenous soil nutrient levels, summer drought conditions and an acidic mining overburden represent major obstacles to successful rehabilitation of open-cut coal mining at Collie in southwest Western Australia. In this study,Acacia pulchella, a native Western Australian species often used in rehabilitation of mined lands, was shown to nodulate and grow in coal mining overburden with pH values less than 4.0 under glasshouse conditions. Plant growth (both top and root dry weight), nodule fresh weight, and nodulation success was best at a pH near 5.0, a value only slightly lower than the typical soil pH of the native jarrah (Eucalyptus marginata) forest. Acetylene reduction rates were reduced by acidity and ranged from 8.2 m C₂H₄g^–1hr^–1 at pH 6.77 to 3.0 m C₂H₄g^–1hr^–1 at a pH of 3.98. Four additional plant species were found to occur and to nodulate on acid overburden material at Collie.     

Energy budgest,growth and filtration rates in Mytilus edulis at different algal concentrations

Growth of Mytilus edulis L. was measured in aquaria with through-flowing sea water at different levels of constant algal concentrations. The amount of food and oxygen consumed by the mussels were measured over given periods as well as the changes in dry organic weight during the same periods. From these parameters it was possible to make simple energy budgets and to compare the estimated growth with actual growth, and, further, to determine growth efficiences at different food levels. Energy budgets were made for mussels grown at algal concentrations of 0, 1.6×10³, 3.0×10³ and 26.0×10³ Phaeodactylum tricornutum cells x ml^-1. The estimated growth was found to be close to actual growth at algal concentrations above maintenance level and the net growth efficiency was found to be between 18% (3.0×10³ cells x ml^-1) and 61% (26×10³ cells x ml^-1). It has been shown that the filtration rate is independent of algal concentrations between about 1.5×10³ to 30×10³ P. tricornutum cells x ml^-1. Outside this range a decrease in filtration rate was noticed.     

Territoriality among male red-winged blackbirds

Summary We examined male site fidelity and territorial movements in a population of individuallyidentifiable red-winged blackbirds (Agelaius phoeniceus) over an 8-year period. Based on general theory and knowledge of the ecology of this population, we predicted that males would be site conservative, and that they would voluntarily change territories only when they could expect to improve significantly their reproductive success. Because males are absent from their territories for only short periods, and probably have accurate comparative information only about nearby territories on which to base their decisions, we predicted that most moves would be over short distances. About 70% of males that bred for more than 1 year retained their original territories between breeding years. Most of the 30% of males that changed territories moved less than 200 m, often to adjacent territories. As predicted, males moving less than 200 m tended to have better reproductive success after moving than before, whereas long distance movers did not improve their success after moving. Territorial male redwings appear to monitor breeding activity on nearby territories and move when significant potential benefits are perceived and opportunities exist. Long-distance moves may be involuntary ones. The amount of information possessed about present and alternative sites, and the time and opportunity required to collect the information, are probably major constraints influencing site conservatism in this and other territorial species.     

Abundance and production rates of floating diatom mats (Rhizosolenia castracanei and R. imbricata var. shrubsolei) in the Eastern Pacific Ocean

Abundance and production rates were measured on freefloating mats composed of the diatoms Rhizosolenia castracanei and R. imbricata var. shrubsolei in the California Current and boundary waters of the central North Pacific during October, 1980. Mats ranged from 3.0 to 10.6 cm in maximum length and had a mean volume of 6.4 ml. Production rates of the diatom mats averaged 4.0 g C colony^-1 h^-1, more than 10³ times higher than that of an equal volume of surrounding water. However, because of their low density at these sites, about 1 mat m^-3, diatom mats contributed only about 1% of the total primary production. The two large Rhizosolenia species comprised almost all of the phytoplankton biomass within the mats. Rhizosolenia species rarely occurred in seawater between mats, where the phytoplankton community was dominated by a diverse array of nannoplankton. The Rhizosolenia species in the mats appeared to be in healthy condition and contained intracellular bacteria. The very high production rates of the colonies indicate high nutrient demand and, since these particular diatoms lack the symbiotic cyanobacterium Richelia intracellularis, which is implicated in N-fixation in other species of Rhizosolenia, our results indicate that other nutrient sources must be present. We discuss the potential role of the intracellular bacteria in nitrogen fixation.     

Innate antipredator responses of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) depend on predator species and their diet

The ability to discriminate between more dangerous and less dangerous predators can have serious fitness advantages for fish juveniles. This is especially true for hatchery-reared fish young used for stocking, because their post-release mortality is often much higher than that of wild-born conspecifics. We tested whether two coexisting fish predators and their different diets induce innate behavioral responses in predator-naive Arctic charr (Salvelinus alpinus) young originating from an endangered hatchery-bred population used for re-introductions. We predicted the antipredator responses of charr to be stronger towards chemical cues of brown trout (Salmo trutta) and pikeperch (Stizostedion lucioperca) than towards odorless control water. More pronounced antipredator behavior was predicted in treatments with predators fed on charr than when their diet consisted of another sympatric salmonid, European grayling (Thymallus thymallus), or when they were food-deprived. The Arctic charr young showed strong antipredator responses in all brown trout treatments, whereas odors of the less likely predator pikeperch were avoided with conspecific diet only. Freezing was the most sensitive antipredator behavior, as it was completely absent in control treatments. We found considerable individual variation in the amount and strength of antipredator responses. Although almost half of the charr failed to show antipredator behavior towards the piscivores, those with the innate ability showed highly sensitive recognition of predator odors. Our results indicate that the innate antipredator behavior of the juvenile fish is already finely tuned to respond specifically to chemical cues from different fish predators and even their diets.Communicated by J. Krause     

Comparison of feeding habits of myctophid fishes and juvenile small epipelagic fishes in the western North Pacific

To examine the potential trophic competition between myctophids and small epipelagic fishes in the nursery grounds in spring, we compared the stomach contents of dominant myctophids (Symbolophorus californiensis, Ceratoscopelus warmingii and Myctophum asperum; n = 179) and juvenile epipelagic fishes (Japanese sardine, Sardinops melanostictus, Japanese anchovy, Engraulis japonicus, chub mackerel, Scomber japonicus, and spotted mackerel, S. australasicus; n = 78) that were simultaneously collected at nighttime with a midwater trawl net around the Kuroshio-Oyashio transition zone in the western North Pacific. It was clear that the neritic copepod Paracalanus parvus s.l. was the most abundant species in NORPAC samples (0.335 mm mesh size) taken at the same stations. Diets of dominant myctophid fishes differed from those of the juvenile epipelagic fishes; Japanese sardine and anchovy mostly preyed upon P. parvus s.l. (23.6% of stomach contents in volume) and Corycaeus affinis (16.1%), respectively. Both chub and spotted mackerels mainly preyed upon the seasonal vertical migrant copepod, Neocalanus cristatus (15.9 and 14.7%, respectively). On the contrary, myctophid fishes probably do not specifically select the abundant neritic copepods. Namely, S. californiensis mostly preyed upon a diel vertical migrating copepod, Pleuromamma piseki (22.7 and 30.6% in stomach of juvenile and adult, respectively), while C. warmingii and M. asperum preyed on Doliolida (43.0% in stomach of juvenile C. warmingii), appendicularians (11.0% in stomach of juvenile M. asperum), and Ostracoda (6.3% in stomach of adult C. warmingii). Feeding habits of myctophid fishes seem adapted to their prey animals; low rate of digested material (less than 30% in volume) in stomachs of S. californiensis may be linked to the movement of P. piseki, hence S. californiensis can easily consume this copepod at night since they are more concentrated at night than daytime. High rate of digested material (over 40%) of M. asperum and adult C. warmingii suggest that they feed not only at night but also during the daytime in the midwater layer. Thus, myctophid fishes actually fed in the surface layer but less actively than the small epipelagic fishes. These results suggest that the potential for direct food competition between myctophids and small epipelagic fishes is low in the nursery ground, but there remains a possibility of indirect effects through their prey items, since the above gelatinous animals feed on common prey items as juveniles of Japanese sardine and anchovy.     

Dietary selectivity in the field and food preferences in the laboratory for two herbivorous fishes (Cebidichthys violaceus and Xiphister mucosus) from a temperate intertidal zone

Diets and food selectivity of two stichaeid fishes (Cebidichthys violaceus and Xiphister mucosus) from the rocky intertidal zone of the central California coast (USA) were studied at each season of the year by gut content analysis and abundance measurements of potential macrophyte food items. Both fishes, after reaching a standard length of about 44 mm, were almost exclusively herbivorous. The bulk of the diet consisted of 8 to 10 species of chlorophytes and rhodophytes. These main dietary components were chiefly annual seaweeds with high surfaceto-weight ratios (sheetlike forms or small, highly branched forms). Perennial seaweeds were eaten in relatively large amounts only during the winter. Macrophytes eaten in only trace amounts included about 20 species of chlorophytes, chrysophytes, phaeophytes, rhodophytes and a spermatophyte. The small amount of animal material in the diet (never more than 2% by weight) could well have been ingested incidentally while the fishes were feeding on seaweeds. Food preference tests with up to 19 macrophyte species in the laboratory revealed that both fishes chose to eat three annual rhodophytes (Smithora naiadum, Porphyra perforata and Microcladia coulteri) in preference to Ulva lobata, an annual chlorophyte that was more abundant in the diets of field-caught specimens.     

Operational sex ratios and behavioural sex differences in a pipefish population

In the pipefish Syngnathus typhle, only males brood embryos in specially developed brood pouches, supplying oxygen and nutrients. Laboratory studies have shown that this elaborate paternal care has led to sex-role reversal in this species: males limit female reproductive rate, females are the primary competitors for mates and males exercise greater selectivity in accepting mates. In the first field study of this pipefish, we describe mating behaviour in the wild and test the hypothesis that temporal variations in the operational sex ratio (OSR) determine sex differences in mating behaviour. Our study comprised two reproductive seasons of two sequential mating periods each, the latter separated by a lengthy interval of male brooding. During mating periods, females displayed to all males without wandering and males moved about searching for females, without reacting to all females. The OSR was least female-biased (or even male-biased) at the onset of the breeding season, when most pipefish were simultaneously available to mate, but became strikingly female-biased as males' pouches were filled. The OSR remained substantially female-biased during the second mating period, because few males became available to remate at any one time. As hypothesised, female-biased OSRs resulted in more female-female meetings. As well, females were above the eelgrass more often than brooding males, thus exposing themselves to conspecifics and/ or predators. In the second year, males arrived earlier than females on the breeding site and male pregnancies were shorter, because of higher water temperatures, so rematings occurred earlier. Males met more often during that year than the previous one, but male competitive interactions were still not observed. The field results support laboratory studies and demonstrate that behaviours associated with female-female competition are more prominent when the OSR is more female-biased. Correspondence to: A. Vincent     

Interactions between fangblennies (Plagiotremus rhinorhynchus) and their potential victims: fooling the model rather than the client?

Lepidophagous (scale-eating) blue-striped fangblennies (Plagiotremus rhinorhynchus Bleeker 1852) are often found sympatrically with the bluestreak cleaner wrasse (Labroides dimidiatus Valenciennes 1839). They have some resemblance to the juvenile L. dimidiatus and have previously been reported as aggressive cleaner wrasse mimics. We observed 14 P. rhinorhynchus on a small area in the barrier reef near Hoga Island, Indonesia to assess the effects of client size on the behaviour and attack success of fangblennies: our results suggest that fangblennies are selective with regard to victim size; fish avoided by the fangblennies are significantly larger than those not avoided and attack success is significantly higher at intermediate victim size classes. The behaviour of the victims also has a significant direct effect on the foraging success of the fangblennies; where the potential victim posed, 63.6% were ignored by the fangblenny and only 7.4% of attacks were successful on posing fish as opposed to a surprise attack success rate of 71.6%. Overall, victims which exhibited the pose behaviour were significantly smaller in size. It appears likely that the predatory strategy of these fangblennies varies with victim size and that mimicry plays a minor role in attracting potential victims. We suggest that in common with other mimetic fish the resemblance of fangblennies to juvenile bluestreak cleaner wrasse allows them to actively hunt in areas where adult cleaners are common thus, indirectly improving their feeding opportunities.     

Differential parasitism of least flycatchers and yellow warblers by the brown-headed cowbird

Summary Brown-headed cowbirds (Molothrus ater) parasitized yellow warblers (Dendroica petechia) six times more frequently than least flycatchers (Empidonax minimus) nesting in the same riparian habitat in southern Manitoba. This difference was surprising because least flycatchers were higher quality hosts; flycatchers always accepted cowbird eggs while warblers did so on only about half the occasions. Both hosts were equally good foster parents, at least until fledging; thus, once an egg was accepted there was probably no further advantage in parasitizing one species over the other. The probability that a nest was parasitized decreased with increasing nest height, perhaps as a consequence of the cowbird's habit of searching for nests close to the ground. Since least flycatchers nested higher in the canopy than yellow warblers, a greater proportion of flycatcher nests probably were not detected by cowbirds. Nevertheless, nest height alone could not account fully for the lower incidence of parasitism on flycatchers. Clutch initiation in flycatchers peaked 6 days after warbler clutch initiations and 5 days after that of cowbirds. Despite the later peak in initiations, flycatcher nests were always available as potential hosts over the entire cowbird laying season and it was not until new clutch initiations by warblers declined in late summer that flycatchers were actually used as hosts. Because least flycatchers responded more aggressively than yellow warblers to a model female cowbird at the nest, we concluded that greater nest defense by flycatchers may have also reduced the rate of brood parasitism in this host. Together, our results suggest the large difference in the frequency of parasitism between these two hosts was primarily a product of nest location but that differences in host nest-defense and breeding season asynchrony may have contributed to preferential host selection.Offprint requests to: J.V. Briskie     

Plasticity in echolocation signals of European pipistrelle bats in search flight: implications for habitat use and prey detection

We studied the echolocation and hunting behavior of three aerial insectivorous species of bats (Vespertilionidae: Pipistrellus) in the field in order to characterize the signals used by the bats and to determine how call structure varies in relation to habitat structure (uncluttered versus cluttered space). We documented free-flying, naturally foraging wild pipistrelles in various habitats using multiflash stereophotography combined with simultaneous sound recordings. Then we reconstructed the bat's flight position in three-dimensional space and correlated it with the corresponding echolocation sequences. In all three species of pipistrelles, signal structure varied substantially. In echolocation sequences of the search phase we found a consistent association of signal types with habitat types. In uncluttered habitats (obstacles more than 5 m from the bat) pipistrelles emitted almost exclusively narrowband signals with bandwidths less than 15 kHz. In cluttered habitats (obstacles less than 5 m from the bat) they switched to signals with bandwidths of more than 15 kHz. Wideband signals were also used when the bats were turning in cluttered and uncluttered spaces and for an instant after turning away from obstacles. Prey detection occured only when the outgoing signal did not overlap with the returning echo from potential prey. The bats also avoided overlap of echoes from potential prey and obstacles. Based on the results of this study, we propose an overlap-free window within which pipistrelles may detect potential prey and which allows predictions of minimum distances to prey and clutter-producing objects. Correspondence to: E.K.V. Kalko     

Growth and population structure in the infaunal bivalve Yoldia eightsi in relation to iceberg activity at Signy Island,Antarctica

Growth rates in the infaunal nuculanid bivalve mollusc Yoldia eightsi (Couthouy) were assessed for field populations in Factory Cove, Signy Island, Antarctica, between. February and April 1992. Daily increments in length (maximum shell dimension) ranged from 2.3 m d^-1 for a 30 mm individual to 5.1 m d^-1 for a 10 mm specimen. These growth rates were converted to annual increments, based on a growing season for the Signy population of around 5 mo, and ages for the largest individuals in the population (35 mm in length), were calculated to be 65 yr. Specimens of 43 mm in length have been found near this site and, if their growth rates are similar to this population, their ages would be in the order of 120 yr. Size distributions from two sites in the same Y. eightsi bed 300 m apart showed significant differences. At the more exposed site the distribution was dominated by small juveniles, with 86% of the population 10 mm in length, while 13% were >20 mm in length. At the less exposed site the values were 27% <10 mm and 56% >20 mm. Icebergs have often been seen grounded on the Y. eightsi bed in this study, especially on the exposed outer portion. This factor, in association with inhibition of larval settlement by high densities of large individuals, is proposed as an explanation of the observed population distributions and the absence of very large specimens in the Factory Cove population.     

Sperm storage and mating in the deep-sea squid Taningia danae Joubin, 1931 (Oegopsida: Octopoteuthidae)

Spermatangium implantation is reported in the large oceanic squid Taningia danae, based on ten mated females from the stomachs of sperm whales. Implanted spermatangia were located in the mantle, head and neck (on both sides) or above the nuchal cartilage, under the neck collar and were often associated with incisions. These cuts ranged from 30 to 65 mm in length and were probably made by males, using the beak or arm hooks. This is the first time wounds facilitating spermatangium storage have been observed in the internal muscle layers (rather than external, as observed in some other species of squid). The implications of these observations for the mating behavior of the rarely encountered squid T. danae are discussed.     

Use of Stress‐Hormone Levels and Habitat Selection to Assess Functional Connectivity of a Landscape for an Amphibian

Abstract: The influence of landscape matrix on functional connectivity has been clearly established. Now methods to assess the effects of different land uses on species’ movements are needed because current methods are often biased. The use of physiological parameters as indicators of the level of resistance to animal movement associated with different land uses (i.e., matrix resistance) could provide estimates of energetic costs and risks to animals migrating through the matrix. To assess whether corticosterone levels indicate matrix resistance, we conducted experiments on substrate choice and measured levels of corticosterone before and after exposure of toads (Bufo bufo) to 3 common substrates (ploughed soil, meadow, and forest litter). We expected matrix resistance and hormone levels to increase from forest litter (habitat of the toad) to meadows to ploughed soil. Adult toads had higher corticosterone levels on ploughed soil than on forest litter or meadow substrates. Hormone levels did not differ between forest litter and meadow. Toads avoided moving onto ploughed soil. Corticosterone levels in juvenile toads were not related to substrate type; however, hormone levels decreased as humidity increased. Juveniles, unlike adults, did not avoid moving over ploughed soil. The difference in responses between adult and juvenile toads may have been due to differences in experimental design (for juveniles, entire body used to measure corticosterone concentration; for adults, saliva alone); differences in the scale of sensory perception of the substrate (juveniles are much smaller than adults); or differences in cognitive processes between adult and juvenile toads. Adults probably had experience with different substrate types, whereas juveniles first emerging from the water probably did not. As a consequence, arable lands could act as ecological traps for juvenile toads.     

Diet and food preferences of the adult horseshoe crab Limulus polyphemus in Delaware Bay,New Jersey,USA

Adult horseshoe crabs Limulus polyphemus (L.) feed on a wide variety of infaunal and epifaunal invertebrates during their spring spawning migration in Delaware Bay, New Jersey, USA. Comparison of the gut contents with estimates of available prey showed that the most abundant potential prey item, the bivalve Gemma gemma, was avoided. The thinner shelled but comparatively scarce clam Mulinia lateralis was a preferred prey item. In the laboratory, crabs fed on G. gemma when it was the only available item but not when M. lateralis or soft-shell clams, Mya arenaria, were offered in conjunction. Large M. lateralis (>10mm) were preferred to small M. lateralis; there was no discrimination between M. lateralis and M. arenaria of the same size. Male and female horseshoe crabs had similar gut contents and laboratory feeding preferences, despite the fact that females are larger than males. Crabs spawning later in the summer contained more food than did crabs collected at the peak of spawning activity.     

Social spider defense against kleptoparasitism

Summary Because of the large amount of webbing they provide, social spider colonies often host other satellite spider species referred to as kleptoparasites or food stealers. Such kleptoparasites may take advantage of increased prey capture rates associated with large spider aggregations. This study investigated the relationship between a cooperatively social spider species, Anelosimus eximius (Araneae: Theridiidae), which lives in the undergrowth of tropical rainforests in Peru, and its kleptoparasite, Argyrodes ululans (Araneae: Theridiidae), which specializes in foraging in An. eximius webs. Although large aggregates of spiders may be more attractive to kleptoparasites, the benefits of group defense may offset this cost. Natural colonies were observed, and enclosed field colonies containing fixed numbers of host spiders were manipulated in order to determine whether kleptoparasite success is affected by the number of social spiders that are available for defense. Prey was less likely to be stolen by Ar. ululans when a greater number of host An. eximius spiders were involved in prey capture. When hosts detected a kleptoparasite earlier and chased it more often, prey was more likely to be successfully defended. Ar. ululans was more successful in stealing small prey items in all colonies and gave up more readily on very large prey (> 11 mm). I conclude that communal living and group defense in An. eximius confer protection from the kleptoparasite Ar. ululans.