Menardi Llc

[Yrko Philogene]

Metamenardi and M. bourneti are two species of spiders inhabiting caves and other subterranean habitats. The occurrence of both species within the same cave has never been proved convincingly and several authors hypothesized a complete niche differentiation mainly based on microclimatic conditions.In order to study the apparent niche differentiation of the two species, we studied several populations of M. menardi and M. bourneti occurring in six caves in the Western Italian Alps (NW Italy). A series of squared plots were monitored monthly from March 2012 to February 2013. At each survey, we counted individuals and we collected the main environmental variables at each plot, namely distance from cave entrance, structural typology (wall, floor or ceiling), light intensity, wind speed and counts of potential prey. Moreover, temperature and relative humidity were continuously logged in each cave. We run several statistical models (GLMMs) in order to relate the counts of individuals to the environmental parameters. The distance from the cave entrance, structural typology and prey availability resulted most important factors driving the abundance of both species within the cave. On the other hand, despite life cycles appeared very similar, the two species seems to exhibit different tolerance to the microclimatic variations within the cave, which emerged as the main factors determining the differentiation of their niche. At least in our study area, M. bourneti tolerates broad microclimatic fluctuations and is potentially able to colonize a wide variety of caves. On the other hand, when the climatic conditions in a cave are suitable for M. menardi (narrow ranges of relatively low temperature and high humidity), M. bourneti is excluded.

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In the last decade there has been a renewed interest in the study of behavioural adaptations to environmental constraints with a focus on adaptations to challenging habitats due to their reduced ecological complexity. However, behavioural studies on organisms adapted to nutrient poor subterranean habitats are few and far between. Here, we compared both morphological traits, in terms of relative leg lengths, and behavioural traits, captured in the geometry of the spider web, between the cave-dwelling spider, Meta menardi, and two aboveground species from the same family (Tetragnathidae); Metellina mengei and Tetragnatha montana. We found that the webs of the cave spider differed significantly from the two surface-dwelling species. The most dramatic difference was the lack of frame threads with the radii in the webs instead attaching directly to the surrounding rock, but other differences in relative web size, web asymmetry and number of capture spiral threads were also found. We argue that these modifications are likely to be adaptations to allow for a novel foraging behaviour to additionally capture walking prey within the vicinity of the web. We found only limited evidence for morphological adaptations and suggest that the cave orb spider could act as a model organism for studies of behaviour in energy-poor environments.

The study of behavioural adaptations in response to environmental drivers is impeded by a number of factors including our limited knowledge of the phylogenetic relationships of most organisms and hence possible evolutionary constraints, the restrictions arising from the ghost of selection past in many dynamic environments that limits the potential for further selection and the very complex biotic and abiotic interactions present in most ecosystems1. One fruitful approach for overcoming some, or most, of these impediments is to investigate novel behaviours in resource-limited environments, which are usually more stable, less species rich and less complex than more benign ecosystems.

Spiders are some of the most dominant predators in cave ecosystems with more than a 1000 species exclusively found underground with an equal, if not larger, number of species spending part of their lifecycle in subterranean habitats14. A large proportion of these species are web-building sit-and-wait hunters; more than two thirds of recorded cave spiders found in European caves fit into this category, and surprisingly, given the scarcity of flying prey, at least 9 species in the orb weaving family Tetragnathidae inhabit caves15.

Cave orb spiders in the genus Meta have been anecdotally reported to show behavioural adaptations to nutrient poor subterranean habitats by modifying their webs, including eliminating the frame and reducing overall silk investment, and by potentially engaging in off-web prey capture15,30,31,32. In this study, we compare the web geometry of the European cave orb spider Meta menardi with two surface-dwelling tetragnathids; the closely related Metellina mengei and the more distantly related Tetragnatha montana33. As orb spiders generally have poor vision, construct their webs at night and can build normal webs in complete darkness in the laboratory18,34, we do not expect the darkness in the caves to affect web geometry per se. However, given the sensitivity of orb webs to fluctuations in temperature, humidity, wind speed and prey type25,35, we hypothesise that the stable conditions inside caves, and the rarity of suitable flying prey, will result in significant differences between the webs of cave and aboveground spiders.

During web-building, orb spiders measure distances between spiral loops with their legs36 and leg length affects the distances between spiral loops37. We therefore also compared leg length and general morphology between our three species to control for any effect of this on web geometry. Since cave adapted animals usually have elongated limbs3 our expectation was that Meta menardi has longer relative leg lengths than the two aboveground species.

We observed, in accordance with our hypothesis, that webs of the cave-dwelling orb spider Meta menardi showed statistically discernible and visually apparent differences to the webs of related aboveground species. Most notable was the lack of the frame that usually enclose the capture spiral and radii of orb webs. This resulted in the radii acting as mooring threads by attaching directly to the surrounding rock substrates. Thus, these cave webs had a much larger number of mooring threads than standard orb webs, which could provide information to the spider about prey walking on the rock surface in vicinity of the web. To our knowledge, this study, utilizing the unique opportunities for quantifying foraging behaviour offered by spider webs, is one of very few studies on cave-dwelling animals that demonstrate a significant difference in behaviour compared to closely related surface-dwelling counterparts6,38.

Morphological adaptations to the subterranean environment have been studied in much greater details than behavioural adaptations with examples of convergent evolution in a number of traits including loss of eyes, loss of pigments and limb elongation3,5,39. To complement our behavioural studies, we therefore also compared limb elongation in the cave orb spiders given that they show no evidence of eye or pigment loss15,40. However, in our comparative study, M. menardi, contrary to our hypothesis, did not have the longest relative leg length for either leg I or III, although it did have statistically significantly longer relative leg lengths than its closest surface relative, Metellina mengei. The facultative cave spider Metellina merianae has relative leg lengths in between these two species41, so more detailed studies are needed to determine if some degree of limb elongation might be present in cave orb spiders. The large overall size of M. menardi could potentially also be viewed as an adaptation to the subterranean environment as an increased body size is a frequent morphological adaptation seen in cave arachnids3. Meta spiders are generally among the largest members of the family Tetragnathidae15, so their size could indicate some degree of morphological adaptation, although it is worth noting that large surface-dwelling araneid spiders are relatively common in the tropics.

A relatively smaller web combined with the lower number of capture spirals and the slightly increased relative mesh height, suggests that the webs of M. menardi are not as important for prey capture as in other species. However, there was no difference in the number of radii, resulting in M. menardi having a higher density of radii and potentially more stopping pontential, which, combined with their relatively smaller capture area, may suggest that they rely on capturing fewer, but larger prey47,48. This is though not supported by the prey captured in their web, which seems to consist mainly of small gnats, mosquitoes and caddisflies15,32, whereas woodland Tetragnatha and Metellina are known to feed on relatively larger prey including large mosquitoes and tipulids49,50, although Metellina mengei from Wytham Woods seems to predominantly feed on small aphids51. Finally, aerial prey (mainly mosquitoes) has been observed to escape quickly from M. menardi webs52, suggesting they might not successfully retain larger and stronger insects should such prey be intercepted.

The comparative approach adopted in this study was somewhat skewed as all cave spiders measured were late instar juveniles, while the aboveground spiders measured were all adults. However, while this may have impacted some of our quantitative results, it is unlikely to have had any influence on the qualitative differences and conclusions presented in this paper as orb webs generally do not undergo strong ontogenetic changes57. The changes they do show either relate to web asymmetry, as older and larger spiders build webs with larger lower parts due to faster gravity-assisted downwards running or to derivative web features such as free sectors not present in juvenile spiders58,59,60. The tetragnathids studied here, all built standard orb webs with the exception of the lack of frame threads in subadult cave spiders, and as they built inclined webs, the link between size and asymmetry is expected to be weak43,44. Thus, the major differences such as number of frames, number of radii and relative web size between the cave spider and the two aboveground spiders found in this study are therefore likely to be due to habitat rather than ontogeny, especially as observations on adult Meta menardi suggest they construct similar webs to those in our study30,61. However, we cannot rule out that the differences found in asymmetry are caused by ontogeny.

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