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.
Highlights
The European cave spider, Meta menardi (Latreille, 1804) (Araneae, Tetragnathidae), is a ubiquitous species inhabiting the twilight zone of many hypogean habitats across Europe[1,2,3,4,5,6,7]
According to the general ecological classification of subterranean animals[35,36,37], trogloxenes are species regularly found in the subterranean habitat, but unable to complete their entire life cycles therein, and troglobionts are those regularly found in the subterranean habitat and completing their entire life cycles therein[37]
(1) What changes appear in the midgut diverticula epithelial cells? And which energy-supplying compounds do M. menardi spend while wintering in natural conditions? (2) Are there any differences in this respect between experimental starvation under controlled and starvation under natural conditions in caves? (3) How can this knowledge contribute to understanding adaptatiogenesis in spiders to the subterranean habitat? we merged relevant hitherto knowledge and established the theory on the nature of the adaptation of M. menardi being intermediate between the epigean and the deep subterranean spider species
Summary
The European cave spider, Meta menardi (Latreille, 1804) (Araneae, Tetragnathidae), is a ubiquitous species inhabiting the twilight zone of many hypogean habitats across Europe[1,2,3,4,5,6,7]. We chose the midgut epithelium cells, since these show rapid response to the feeding conditions of an individual[44] We carried out these experiments during the growing period−in spring and in autumn−with relatively abundant prey in the entrance cave sections[33], and in winter, when prey is usually scarce[43]. We found that during the growth period, M. menardi accumulate reserve compounds in spherites and protein granules, and energy-supplying lipids and glycogen, all of which form an adaptive response to potential starvation[43] This response is, in general, the same as in invertebrates with winter dormancy in their life cycle, e.g. Scoliopteryx libatrix[45]. In starved M. menardi, macroautophagy−referred to as autophagy48,49−is an indicative pro-survival process[33,50]
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