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

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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
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