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The discovery of a new submerged world: anchialine environments
Diana Maria Paola Galassi
Anchialine environments were first
discovered in 1966, when the Austrian
scientist Rupert Riedl described them
as “marginal caves”. Since then,
experts have debated the correct
definition of anchialine systems. Today,
they agree on defining them as caves
or other underground aquatic habitats
near the coastline of islands and
continents, supplied by continental
freshwater and with underground
connections to the sea. Consequently,
the water of anchialine pools is
brackish, and light-weight freshwater
generally floats on top of heavier
seawater.
The most typical feature of anchialine
environments is the absence of a
surface connection with the sea, which
manages to reach far inland through
deep infiltration passages in limestone
and volcanic rocks.
The most fascinating, extraordinary
examples are the well-known Mexican
cenotes, small bodies of crystalline
brackish water, like blue eyes glittering
in the tropical forests of Mexico and
Belize, not far from the coast. In Italy,
typical anchialine pools are found in
the Grotta Zinzulusa, Abisso and Buco
dei Diavoli (Salento), together with
other water-tables in Apulia,
groundwater in Porto Palo (Sicily), the
Grotta Verde, Grotta di Nettuno and
Grotta del Bue Marino (Sardinia) and
the Grotta di Punta degli Stretti
(Argentario, Tuscany).
Anchialine pools are marked by few
food resources, total darkness, and
vertical gradients of salinity and
oxygen concentrations. Although in the
past anchialine ecosystems were
believed to support themselves on
allochthonous organic matter (deriving
from the rock above and from
seawater), today we know that part of
the organic matter is locally
synthesised by chemo-autotrophs.
The most fascinating aspect that
makes these environments true treasure
troves of biodiversity is their exclusive
fauna. Examples are remipedes, the
most primitive class of living
crustaceans, which have been found in
anchialine caves on the Bahamas, in
lava tubes on the island of Lanzarote
and, more recently, in Australia.
Remipedes, like many other animal
groups found in these habitats, are true
living fossils, whose distribution,
enigmatically uneven in several areas
of the world, dates back to the breakup
of the ancient Tethys Sea.
In addition to these unique organisms,
there are also other extraordinary
animals, some of which are Italian
endemics with restricted distribution,
like the sponge Higginsia ciccaresei,
the thermosbaenacean Monodella
stygicola, the mysid Stygiomysis
hydruntina, and the decapod
Typhlocaris salentina.
It is worth noting that the copepod
Muceddina multispinosa was recently
discovered in the Grotta Verde (Capo
Caccia, Alghero, Sardinia), a species
with disjunct distribution also found in
anchialine environments on the islands
of Mallorca and Lanzarote (Spain).
Generally speaking, anchialine habitats
host heterogeneous assemblages
which includes strictly marine animals
and, to a lesser degree, freshwater
organisms.
Typically anchialine species are closely
associated with the particular
environment in which they live, and
have never been found in other types of
underground habitats. They are also
stygobionts, and have marked
specialised features. Their origin dates
back to various geological times, from
the Tertiary to the more recent
Sea entrance to the Grotta Zinzulusa (Salento, Apulia)
Pleistocene. Some researchers believe
that their ancestors lived in the abysses
of the sea; others trace their origin back
to organisms living in shallow seawater
on the continental shelf.