Despite its inhospitable appearance and lack of any ... - Udine Cultura

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110 microparasellid and cirolanid isopods, ameirid and ectinosomatid harpacticoids and mysids - are related to taxa that are still living in marine environments and are called thalassoid stygobionts. The current distribution of stygobiont species is the result of an ever-changing mosaic of communities which have evolved on a geological time-scale and whose composition is still evolving. A particularly interesting aspect of the development of stygodiversity, which has long been debated by scientists, is the large number of relict species in underground communities. Some underground systems conserve species and even entire taxonomic groups related to now extinct ancient surface fauna (in one of his books, the French biologist René Jeannel spoke of cave-dwelling “living fossils”). Events like the Pleistocene glaciations, sea regressions and transgressions, and the salinity crisis of the Mediterranean have long been the focus of scientific debates on the origin of underground fauna. Some scientists believe stygobionts to be the “survivors” of surface populations which had taken refuge underground to escape harsh surface conditions (this “refugium” hypothesis goes back to the ideas of Charles Darwin). Others view colonisation as a continuous, ongoing process (the “active colonisation” hypothesis). Karstic spring (Friuli Venezia Giulia) According to the “refugium” theory, tropical regions, which were not affected by catastrophic events, would have scarce underground fauna; most importantly, surface populations, the ancestors of stygobionts, would generally have become extinct in geographical areas which underwent drastic climatic changes. Further knowledge has proved that both these theories were partly wrong, and suggested a new evolutionary scenario, according to which the present structure of stygodiversity has several causes. First, the extinction of surface populations is not a mandatory prerequisite for speciation in The endemic gastropod Bythiospeum calepii underground habitats. We already mentioned this when describing the coexistence of pigmented, eyed species of crustaceans with depigmented, anophthalmic or totally blind ones - many cases are known in Italy, especially for crustaceans of the genera Proasellus, Synurella and Gammarus. These are examples of ongoing active colonisation of groundwater not associated with dramatic changes in the surface environment. Stygobionts have also been found in all kinds of habitats in tropical areas - lava tubes on volcanic islands, cenotes in Mexico, caves in Somalia, anchialine systems in Australian deserts and in the caves of small islands in the Caribbean, and in Brazilian caves, inhabited by multitudes of blind fish - thus definitively contradicting the “refugium” hypothesis as the only explanation for the origin of underground communities. Although groundwater colonisation may occur independently of unfavourable conditions in the environments lying above, dramatic climatic and geological changes in surface habitats may in fact suddenly interrupt the genetic flow between hypogean and epigean populations. This explains the origin of “relicts”, both eco-geographical (separated from their closest surface relatives - still living and sometimes well diversified) and phyletic (the last survivors of a now extinct surface evolutionary line). Phyletic relicts are very interesting from the scientific viewpoint. Entire taxonomic groups (like bathynellaceans and thermosbaenaceans) belong to this category, and their closest relatives can no longer be found among the 111
112 animal groups still living on the surface. However, since the chronology of colonisation and speciation is hard to establish, a solution has been found in the so-called “molecular clocks”, which synchronise DNA mutations in living organisms with palaeogeographical and palaeoclimatic events. An example is afforded by the examination of Sardinian, Provençal and Tuscan species of stenasellid isopods, living evidence of the ancient tectonic fragmentation of the Tyrrhenian plate and drifting of the Sardinian-Corsican continental plate and its fragments from what is now Provence towards present-day Italy. Although necessary for speciation, vicariance (i.e., the splitting of the area of the old species into two parts separated by boundaries) is not only associated with these ancient, wide-ranging events, and may occur on any scale, from small, isolated fractures in karstic systems, to continents. Vicariant events may be geoclimatic and ecological: obviously, the efficiency of these “boundaries” is closely related to the ecology of the species, especially their aptitude for dispersal. Many stygobiont species are not prone to dispersal, and show Affinities between Provençal, Sardinian-Corsican and coastal Tyrrhenian fauna are explained by their common palaeogeographic history Niphargus similis, found in relict sites in glacialised areas of the Alpine chain limited geographic distribution (they are strict endemics). Furthermore, the low fecundity, benthic larval development and low dispersal potential of many interstitial crustaceans suggest that continuous and jump dispersal are quite rare in these groups. This is why stygobionts can be used as excellent historical (palaeogeographical) indicators, their descriptive capacity being similar to that of true fossils. Among the events that modelled present-day Italian stygofauna, the best known are certainly glaciations and sea regressions. The Quaternary glaciations depleted underground fauna in large areas of Italy, thus leading to the total absence of entire stygobiont genera of gastropods, copepods, isopods and amphipods north of the boundary of the Würmian glaciation. Sea regression, associated with glacial eustatism, trapped several taxa in coastal sediments, giving rise to stygobization. Very ancient regressive events led fauna of marine origin to become relicts, enigmatically confined and unevenly distributed in continental groundwater far from coastlines (amphi-Atlantic, Caribbean-Mediterranean, Caribbean-Mediterranean-Australian). In this case, the grandiose movements of plate tectonics were the main cause of relictualisation and, in the Mediterranean area, gave rise to the so-called Tethyan relicts, which date back to the disappearance of the ancient Tethys Sea in the Oligocene. 113
Page 1 and 2: Fauna FABIO STOCH Despite its inhos
Page 3 and 4: 44 45 Methods for sampling and anal
Page 5 and 6: 48 Shells of gastropods Iglica voba
Page 7 and 8: 52 collected in phreatic waters in
Page 9 and 10: 56 ● Cyclopoid copepods. So far,
Page 11 and 12: 60 those associated with percolatin
Page 13 and 14: 64 Species of genus Niphargus; top:
Page 15 and 16: 68 ■ Decapods Italian fauna has o
Page 17 and 18: 72 Stygofaunal provinces in Italy T
Page 19 and 20: 76 aquifers in the Sabini mountains
Page 21 and 22: 80 ■ The ecology of aquifers Acco
Page 23 and 24: 84 85 The discovery of a new submer
Page 25 and 26: 88 anastomoses, often adjacent to t
Page 27 and 28: 92 93 The saline springs of Poiano:
Page 29 and 30: 96 Modified sensory setae on the an
Page 31 and 32: 100 Theoretical evolutionary sequen
Page 33 and 34: 104 Sketch showing degrading phases
Page 35: 108 109 Stygodiversity in the Presc

Despite its inhospitable appearance and lack of any ... - Udine Cultura

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