Population genomics in two cave-obligate invertebrates confirms extremely limited dispersal between caves

A Case for Below-Ground Dispersal? Insights into the Biology, Ecology and Conservation of Blind Cave Spiders in the Genus Troglodiplura (Mygalomorphae: Anamidae)

Previously described from only fragments of exoskeleton and juvenile specimens, the cave spider genus Troglodiplura (Araneae: Anamidae), endemic to the Nullarbor Plain, is the only troglomorphic member of the infraorder Mygalomorphae recorded from Australia. We investigated the distribution of Troglodiplura in South Australia, collecting and observing the first (intact) mature specimens, widening the number of caves it has been recorded in, and documenting threats to conservation. Phylogenetic analyses support the placement of Troglodiplura as an independent lineage within the subfamily Anaminae (the 'Troglodiplura group') and provide unequivocal evidence that populations from apparently isolated cave systems are conspecifics of T. beirutpakbarai Harvey & Rix, 2020, with extremely low or negligible inter-population mitochondrial divergences. This is intriguing evidence for recent or contemporary subterranean dispersal of these large, troglomorphic spiders. Observations of adults and juvenile spiders taken in the natural cave environment, and supported by observations in captivity, revealed the use of crevices within caves as shelters, but no evidence of silk use for burrow construction, contrasting with the typical burrowing behaviours seen in other Anamidae. We identify a range of threats posed to the species and to the fragile cave ecosystem, and provide recommendations for further research to better define the distribution of vulnerable taxa within caves and identify actions needed to protect them.

Exploratory behaviour divergence between surface populations, cave colonists and a cave population in the water louse, Asellus aquaticus

Abstract

Behaviour is considered among the most important factors in colonising new habitats. While population divergence in behaviour is well-documented, intraspecific variation in exploratory behaviour in species with populations successfully colonising and adapting to extreme (compared to the ‘typical’) habitats is less understood. Here, by studying surface- vs. cave-adapted populations of water louse (Asellus aquaticus), we tested whether (i) adaptation to the special, ecologically isolated cave habitat includes a decrease in explorativeness and (ii) recent, surface-type cave colonists are more explorative than their surface conspecifics from the source population. We repeatedly tested dispersal related novel area exploration and dispersal speed in both the presence and absence of light. We found that surface populations showed higher behavioural activity in dark than in light, and they were more explorative and dispersed faster than their cave conspecifics. Recent colonists showed a trend of higher dispersal speed compared to their source surface population. We suggest that extreme and isolated habitats like caves might work as ‘dispersal traps’ following successful colonisation, because adaptation to these habitats includes the reduction of explorativeness. Furthermore, we suggest that individuals with higher explorativeness are likely to colonise markedly new environments. Finally, we provide experimental evidence about surface A. aquaticus moving more in dark than in light.

Significance statement

Environmental conditions in caves are differing drastically from those of the surface. Consequently, animals colonising subterranean habitats are subject to different selective forces than those experienced by the ancestral surface-living population. Behaviour is believed to be a key factor in successful colonisation to novel habitats; however, intraspecific behavioural variation in species with both surface- and cave-adapted populations is less known. Here, we compared dispersal related novel area exploration and dispersal speed across surface and cave-adapted populations of the freshwater crustacean Asellus aquaticus. Our results show that cave-adapted A. aquaticus are significantly less explorative and disperse slower than surface-type populations, indicating that caves may act as ‘dispersal traps’, where adaptation includes the loss of explorativeness. Also, recent cave colonists show a trend to be faster dispersers than peers from the surface source population, suggesting that individuals with higher explorativeness are likely to colonise markedly different new environments.

Life-history traits drive spatial genetic structuring in Dinaric cave spiders

The subterranean ecosystem exerts strong selection pressures on the organisms that thrive in it. In response, obligate cave-dwellers have developed a series of morphological, physiological, and behavioral adaptations, such as eye reduction, appendage elongation, low metabolic rates or intermittent activity patterns, collectively referred to as troglomorphism. Traditionally, studies on cave organisms have been hampered by the difficulty of sampling (i.e., small population sizes, temporal heterogeneity in specimen occurrence, challenges imposed by the difficult-to-access nature of caves). Here, we circumvent this limitation by implementing a museomics approach. Specifically, we aim at comparing the genetic population structures of five cave spider species demonstrating contrasting life histories and levels of troglomorphism across different caves in the northern Dinarides (Balkans, Europe). We applied a genome-wide hybridization-capture approach (i.e., HyRAD) to capture DNA from 117 historical samples. By comparing the population genetic structures among five species and by studying isolation by distance, we identified deeper population structuring and more pronounced patterns of isolation by distance in the highly troglomorphic Parastalita stygia and Stalita pretneri ground dwellers, while the three web-building Troglohyphantes species, two of which can occasionally be found in surface habitats, showed less structured populations compatible with higher dispersal ability. The spatial distribution of genetic groups revealed common phylogeographic breaks among lineages across the studied species, which hint at the importance of environmental features in driving dispersal potential and shaping underground diversity.