Towards understanding partial adaptation to the subterranean habitat in the European cave spider, Meta menardi: An ecocytological approach

Abstract

The European cave spider, Meta menardi , is a representative of the troglophiles, i.e. non-strictly subterranean organisms. Our aim was to interpret the cytological results from an ecological perspective, and provide a synthesis of the hitherto knowledge about M. menardi into a theory of key features marking it a troglophile. We studied ultrastructural changes of the midgut epithelial cells in individuals spending winter under natural conditions in caves, using light microscopy and TEM. The midgut diverticula epithelium consisted of secretory cells, digestive cells and adipocytes. During winter, gradual vacuolization of some digestive cells appeared, and some necrotic digestive cells and necrotic adipocytes appeared. This cytological information completes previous studies on M. menardi starved under controlled conditions in the laboratory. In experimental starvation and natural winter conditions, M. menardi gradually exploit reserve compounds from spherites, protein granules and through autophagy, and energy-supplying lipids and glycogen, as do many overwintering arthropods. We found no special cellular response to living in the habitat. Features that make it partly adapted to the subterranean habitat include starvation hardiness as a possible preadaptation, an extremely opportunistic diet, a partly reduced orb, tracking and capturing prey on bare walls and partly reduced tolerance to below-zero temperatures.

Unique behavioural modifications in the web structure of the cave orb spider Meta menardi (Araneae, Tetragnathidae)

“…However, while orb spiders are known to alter life history and silk properties due to starvation 70 , 71 , only minor quantitative changes in orb geometry, including increased area, but reduced number of capture spirals, were found 72 , 73 as opposed to the large-scale quantitative and qualitative differences observed in this study. In addition, Meta menardi does not show any specific physiological adaptations to starvation 74 . Overall, the changes we observed in the webs of M. menardi are unlikely to be caused by behavioural flexibility alone, although more detailed laboratory studies of web-building behaviour of the cave spider are needed to fully eliminate this possibility.…”

Exploring the homogeneity of terrestrial subterranean communities at a local spatial scale

“…Winter abundance of non-troglobionts was generally low, following the general reduction of activity in surface habitats in winter and the typical dormancy pattern in most arthropods. In winter, we collected few external species in pitfall traps installed far from the entrance; these were mostly species overwintering in caves -a seasonal behaviour documented in several taxa (Chelini et al, 2011;Lipovšek et al, 2016Lipovšek et al, , 2019Balogová et al, 2017;Mammola & Isaia, 2018).…”

Starvation hardiness as preadaptation for life in subterranean habitats

“…Unfortunately, TEM analysis of gastropods was not possible because during sectioning procedure on microtome, abundant, hard spherites were torn in large numbers out of slices, causing extensive damage in other sample portions; such slices were useless for the TEM analysis. Ultra-thin sections (75 nm) were analysed by a Zeiss EM 902 transmission electron microscope (details in 45 ).…”

“…According to hitherto studies, beside quiescence and diapause, some non-troglobiont terrestrial invertebrate species conduct a nonfeeding, prevalently resting overwintering accompanied by growth, moulting and gonad ripening in caves 45 , 53 – 56 . Moreover, some other non-troglobionts that otherwise feed in caves throughout the year when food is available, undergo quiescence- and dormancy-resembling starvation and resting because of the rare opportunities to feed 45 . Thus, responses of organisms to low food supply are heterogeneous, as reported also for other traits 14 .…”