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

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The lack of eyes or their functional atrophy is generally accompanied by
hypertrophic (excessively developed) alternative sensory organs for life in a
dark world, where smelling or touching the surrounding space is the only way
of sensing the approach of predators or potential prey, finding a partner, or
making one’s way in three-dimensional space, large or small. The body
surfaces of stygobionts are therefore covered with sensory organs, which have
different shapes according to species. For instance, crustaceans have
aesthetascs, setae with many chemoreceptors sensitive to chemical stimuli,
and thigmoreceptors, end-organs which respond to touch and enable
organisms to find their way around by feeling the lake bed or single sand
particles in interstitial environments. Stygobiont crustaceans living in free
water sometimes have longer antennules than their surface relatives. These
modified cephalic appendages are used to sense at distance: it is better to
know in advance if a predator is coming, before it is too late to escape! In
stygobionts, the compensatory length of sensory appendages is contrasted
by clearly rudimentary locomotory appendages, which are much smaller than
those of their close relatives living on the surface, and generally with fewer
segments, setae and spines. This adaptation is actually a type of
specialisation, as it is typical, or even exclusive, to interstitial species.
The interstitial environment of hyporheic zones of rivers, springs and karstic
springs covered by alluvial sediments has one great disadvantage: living
Body elongation favours movement in the interstitial habitat
spaces are restricted. In these narrow habitats, movement is confined, and
walking species, let alone swimmers, are rare. Most of their time is spent
moving around single sand particles, feeding on the bacterial biofilm covering
the surface. Long legs would only hinder movement. This morphological
adaptation is accompanied by a reduction in body size, as stygobiont species
are generally smaller than their surface relatives. It is no coincidence that
shorter legs and smaller bodies are often associated. The most probable
reason is that these adaptations are the result of developmental heterochrony.
Heterochrony. This is a deviation from
the typical developmental sequence of
formation of organs and parts as a
factor in evolution, both in animals
with discontinuous development (with
larval stages separated by moults)
and those with continuous, gradual
transformations into adults.
Although there are various types of
heterochrony, the most commonly
found in underground environments
are progenetic paedomorphosis (also
known as progenesis) and neoteny.
Why has heterochrony been so
successful in the colonisation of
groundwater?
Progenesis is the process by which
Evolutionary dynamics of stygobiont progenesis
features of the sexually mature animal
develop precociously. The result of this deviation is a reproductively mature
adult which preserves the morphological and/or physiological characteristics
typical of juvenile or larval stages. These small adults often look like larvae
and, if they are metameric (segmented) animals or have segmented body
appendages, they may have fewer metameres and segments than they would
if they had developed normally. But what is the advantage of being small?
Obviously, it enables these animals to creep and wriggle into tiny interstitial
spaces. Although heterochrony is the result of modifications - genome
alterations which are generally disadaptive - it actually provides free admission
to interstitial environments. It may therefore be a pre-adaptation, i.e., nonadaptive,
and at times a neutral character or set of characters acquired in their
original habitats (i.e., surface water) which may become useful when
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