Communities of terrestrial animals often contain closely related species that overlap in how they use the environment. Consequently, this poses the challenge of understanding how and why closely related species coexist and thrive together. This phenomenon could be explained by variations in the use of specific resources, such as the microhabitats. We studied this possibility in a community of multiple congeneric species within Sclerosomatidae (Arachnida, Opiliones). The factors that promote coexistence among Opiliones species remain largely unexplored. Specifically, we aimed to explore (1) whether the most common Leiobunum species in four riparian forests in Illinois, USA, differ inter- and intra-specifically in morphological traits (body width and leg length) and in the substrates they use for daytime roosting. Additionally, we explored (2) whether morphological traits and substrate use are correlated. We found that 6 categories of species and sexes varied in body size and leg length, as well as the proportion of microhabitats in which they roosted. These variations seemed to correlate and allowed us to sort species and sexes into three phenotypes: (a) tree-roosters with small bodies and longer legs, (b) foliage-roosters with larger bodies and medium to short legs, and (c) mixed-roosters with smaller bodies and intermediate to short legs. Hence, this morphological and microhabitat-use association might render species/sexes better able to navigate and use their habitats, enabling them to forage more successfully for food and escape predators. Our study highlights the importance of considering morphological and ecological mechanisms that enable multiple related species to coexist in sympatry.

Study sites

We surveyed above-ground Opiliones of Leiobunum from July 17th to August 14th, 2024. We measured their morphological traits and noted the substrates on which they were found at four field sites in Cook County, Northeastern Illinois, USA (41°57'N; 87°50’W, 179 meters in elevation; Fig. 1A). All sites are within a 30 km radius and are located within the watersheds of two major rivers, with protected riparian forests. Two sites (Bunker Hill and LaBagh Woods) are along the North Branch of the Chicago River, and the other two sites (Camp Pine Woods and Schiller Woods) are along the Des Plaines River. The general vegetation structure, canopy height, substrate availability, and distance to the river were roughly similar across the specific locations where we sampled in the forest trails. We sampled between 10:00 and 17:30 hrs and under sunny, partly cloudy, and overcast (but not rainy) conditions.

Sampling

To locate Opiliones, we searched all visible substrates for their characteristic movement and body shape from 0 to 3 m in height. Once we located an individual, we grasped them by as many legs as possible to prevent the voluntary release of limbs (“autotomy”, Escalante & O’Brien, 2024).

Regarding morphological traits, we measured body width and the length of one leg of the fourth pair using digital calipers to the nearest 0.01 mm, as both are standard measures for body size (Escalante et al., 2020; Fowler-Finn et al., 2014, 2018). For this, we held each individual dorsal-side up and measured the maximum body width. We then held the individual by multiple legs, dorsal side up, extended the fourth leg, gently pressed on the femur-patella joint to extend the leg, and measured the length of the fourth leg from the base of the coxa to the end of the tarsus.

Regarding ecological traits, we recorded the substrate type on which each individual was found roosting. We categorized the roosting substrates as tree (the trunk of any tree), log (fallen tree on the ground), foliage (any living leaf, stems, or inflorescences), and leaf litter (any dead leaf litter, twigs, and/or trees or branches found on the ground).

Identification

In the field, we photographed individuals and assigned them morphospecies IDs based on morphological characteristics, including the coloration of the legs, pedipalps, and body, and the overall limb length and body size. We then assigned morphospecies to the currently described species and sexes based on available literature (Edgar, 1971; Fowler-Finn et al., 2018, 2019; Kahn et al., 2018; Shultz, 2018a). Supplemental identification based on photographs was provided by additional Opiliones researchers.