Seothyra

About: Seothyra is a(n) research topic. Over the lifetime, 7 publication(s) have been published within this topic receiving 246 citation(s). The topic is also known as: Buck spoor spiders.

The influence of food supply on foraging behaviour in a desert spider

Abstract: We tested the alternative hypotheses that foraging effort will increase (energy maximizer model) or decrease (due to increased costs or risks) when food supply increased, using a Namib desert burrowing spider, Seothyra henscheli (Eresidae), which feeds mainly on ants. The web of S. henscheli has a simple geometrical configuration, comprising a horizontal mat on the sand surface, with a variable number of lobes lined with sticky silk. The sticky silk is renewed daily after being covered by wind-blown sand. In a field experiment, we supplemented the spiders' natural prey with one ant on each day that spiders had active webs and determined the response to an increase in prey. We compared the foraging activity and web geometry of prey-supplemented spiders to non-supplemented controls. We compared the same parameters in fooddeprived and supplemented spiders in captivity. The results support the "costs of foraging" hypothesis. Supplemented spiders reduced their foraging activity and web dimensions. They moulted at least once and grew rapidly, more than doubling their mass in 6 weeks. By contrast, food-deprived spiders increased foraging effort by enlarging the diameter of the capture web. We suggest that digestive constraints prevented supplemented spiders from fully utilizing the available prey. By reducing foraging activities on the surface, spiders in a prey-rich habitat can reduce the risk of predation. However, early maturation resulting from a higher growth rate provides no advantage to S. henscheli owing to the fact that the timing of mating and dispersal are fixed by climatic factors (wind and temperature). Instead, large female body size will increase fitness by increasing the investiment in young during the period of extended maternal care.

Environmental factors affecting the web and activity of a psammophilous spider in the Namib Desert

Abstract: For web-building spiders, shifting sand dunes would seem an inhospitable habitat. Nonetheless, one species of web-building spider Seothyra henscheli (Eresidae) is locally abundant in the Namib Desert dunes. Here we describe the structure of the web and examine the influence of certain environmental factors on web design and spider activity. Wind-blown sand covers the web's capture elements and disrupts foraging activity. Webs in wind-exposed areas are subject to greater sand-loading and/or scouring than in sheltered areas. Variation among habitats in web size may be explained by differences in local wind regimes: webs in wind-exposed sites have smaller capture elements than those in sheltered sites. Construction of a new web and web repair following disturbance are energetically expensive and expose the spider to risks of predation, overheating and desiccation. Environmental constraints on foraging and the cost of web-building help to explain the seasonal pattern of activity of S. henscheli.

Foraging at the thermal limit: burrowing spiders ( Seothyra , Eresidae) in the Namib desert dunes

Abstract: In the Namib Desert dunes, the web of Seothyra sp. (Eresidae) comprises sticky silk lining the edges of a horizontal mat on the sand surface. The spider sits in a silk-lined burrow attached to the mat. Arthropods become entangled in the sticky silk of the mat and are attacked and pulled into the burrow by the spider. We investigated the influence of sand surface temperature on the activity of spiders during the summer. We determined the range of thermal conditions encountered by spiders, their temperature tolerance and the influence of temperature on foraging activity and prey handling behavior. The environmental temperatures available to Seothyra vary from 17–33° C at the coolest time of day to 33–73° C at the hottest. When prevented from retreating into burrows, spiders showed signs of thermal stress at about 49° C, whereas unrestrained spiders continued to forage at web temperatures above 65° C by moving between the hot surface mat and the cooler burrow. Spiders responded quicker to prey stimuli during the hot hours of the day and completed prey capture sequences in significantly less time at surface temperatures above 49° C than below. Furthermore, captured arthropods succumbed more quickly at high surface temperatures. Our study supports the hypothesis that web design and thermoregulatory behaviors enable Seothyra to hunt under extreme thermal conditions.

A test of habitat selection at two spatial scales in a sit-and-wait predator : a web spider in the Namib Desert dunes

Abstract: 1. Distribution and web characteristics were examined for the dune-burrowing spider Seothyra henscheli in the Namib Desert to determine how these spiders relate to spatially heterogeneous habitat features that influence foraging costs and benefits, growth rate and survival. In particular, we set out to determine the relative importance of (i) habitat selection, (ii) site-dependent growth and survival, and (iii) restricted dispersal, to the occurrence of S. henscheli in different macro- and microhabitats. 2. Spider webs were mapped along transects that crossed dune regions with dense vegetation hummocks, steep dune plinths and bare, gravelly interdune plains. Spiders were more abundant in the hummocks than in the interdunes and plinths. Translocation of spiders from the hummocks to the interdunes and plinths confirmed that spider density was correlated with habitat quality. 3. In a comparison of web sites with dummy sites in terms of factors related to sand stability and food availability, it was found that, except for the avoidance of extreme conditions, web-site characteristics did not differ from dummy sites. Web geometry indicated that good sites were characterized by stable sand and abundant ants. However, many spiders occurred at poor sites when good sites were vacant nearby. 4. It is concluded that S. henscheli do not actively choose web sites, but have restricted dispersal. Site-dependent growth and survival may explain the pattern of local abundance. Site tenacity may be a result of the spiders' inability to predict site quality coupled with high costs of relocation. It is inferred that spiderlings tend to remain near their mother's site, which has a previous record of success. 5. Tolerance of variable conditions of dunes and the ability to sustain populations even at poor sites enable these spiders to occur in extreme desert conditions.

Costs of aggregation: shadow competition in a sit-and-wait predator

Abstract: Shadow competition, when sedentary foragers closer to a source of food reduce its availability to those further away, is predicted to increase with the size and density of a group. We tested the occurrence of shadow competition and examined its consequences for a burrowing spider Seothyra henscheli (Eresidae) in the Namib Desert. Differences between individual spiders occurring inside or on the periphery of clusters compared to solitary spiders were examined in a natural population, by experimental manipulation of densities and by computer simulation of the experimental manipulation. Spiders in the population grew more slowly in clusters than did solitary spiders and this was confirmed by the experiment. The experiment showed that spiders grew more rapidly on the periphery of a cluster than inside it, but that survival showed the opposite trend. The largest effect was in the highest density, where all spiders maintained active webs throughout the experiment, indicating a state of hunger. Modeling indicated that such effects may be explained by the way ants, the principal prey of the spiders, reach spider webs at different locations within the patches of different densities. Modeling confirmed that shadow competition adequately explains the patterns of foraging, growth and survival of sedentary foragers such as these spiders, and is likely to have wider implications for other sit-and-wait predators.