guest post by Maureen Emily Niño
Caves are unique, yet harsh habitats characterized by perpetual darkness, high humidity, low oxygen levels, limited nutrients, and low temperature fluctuations – factors otherwise considered unfavorable in normal environments. Yet, these systems can be home to cavefishes. And among these cavefishes, a particular group belonging to the genus Sinocyclocheilus stands out due to the exceptionally high diversity and extensive morphological variations of its members.
Eyeless golden-line fish (Sinocyclocheilus
anophthalmus), a blind cavefish. © 2011 sclereid0309. some rights reserved
(CC BY-NC-ND 4.0).
Sinocyclocheilus is the most diverse group of cavefishes with 75 currently
recognized living species. They can only be found in cave and surface waters of
the Yunnan-Guizhou Plateau in China. The Yunnan-Guizhou Plateau is an extensive
karst landscape composed of many caves. The fishes can be categorized into 3
types based on their habitat preference: a) surface-dwellers which can
only be seen swimming outside the caves, in surface rivers and lakes adjacent
to the Plateau, b) cave-associated species or troglophiles which
are either found in cave entrances, within the caves, or swimming back and
forth these two areas, and c) cave-restricted species or troglobites
which can only be found inside the caves. Troglobites cannot survive
outside caves and are completely dependent on caves for their entire life cycle.
Now, you might be wondering: if the cave environment is so harsh, then
why did the Sinocyclocheilus cavefishes choose to live in caves?
When scientists analyzed the mitochondrial DNA of the Sinocyclocheilus
cavefishes, it was revealed that approximately 7.4 million years ago during
the Mid-Miocene, their surface-dwelling ancestor moved into caves for refuge,
at the time of the aridification of China when surface waters all dried
up. Then, this ancestor has diversified into many different Sinocyclocheilus
species until the Pleistocene. At present, there are 75 recognized living
species, 56 of which are cave-dwellers which exhibit troglomorphic traits or
odd adaptations associated with cave-living.
In most cave-dwelling animals (i.e. troglophiles and troglobites), including
cavefishes, it is generally observed that they commonly respond to the unusual
conditions of their new cave environment through the development of
troglomorphic traits such as reduced or completely lost eyes, less pigmentation,
smaller body size, more elongated limbs, heightened non-visual sensory organs,
presence of horns, etc. In contrast, their surface-dwelling relatives are typically
pigmented and have normal eyes. While the presence of troglomorphic traits
easily distinguishes most cave-dwelling animals from their surface-dwelling relatives,
such is not only the case for the Sinocyclocheilus cavefishes. The cave-dwelling
members of this genus display an extensive array of morphological variations. In
addition to cave-dwelling members which exhibit troglomorphic traits (e.g, have
reduced to blind eyes), there are also cave-dwelling Sinocyclocheilus species
which resemble surface-dwelling members (e.g. have normal eyes).
How did this group of distinct cavefishes end up with an extensive
variation in their morphology? What are the patterns of morphological
variations? How do these patterns relate to habitat utilization and
geographical distribution of Sinocyclocheilus? Will studying this group
of endemic Chinese cavefishes help us better understand evolution in caves?
A new multi-species
evolutionary model
In the recently published
study in BMC Ecology and Evolution, the authors’ combined analysis of
morphology (presence of eyes & horns), phylogeny, dating, rate shifts in
the evolution of body size and eye size, and distribution revealed the
following patterns:
1. All Sinocyclocheilus preferred cave-dwelling – they would move into
caves whenever there is an opportunity. This is because the surface-dwelling
ancestor most likely developed a cave-loving habit before moving into the
caves. Karstic landscapes are characterized by surface waters (lakes, rivers,
streams) which are typically connected to lakes and rivers found inside the
caves. The drying up of surface waters is a common occurrence in these
landscapes and whenever that happens, the ancestor fish may have temporarily retreated
into the caves to avoid desiccation. This may have led Sinocyclocheilus
to prefer dwelling in caves.
2. A general pattern observed is that as species become troglobitic, their
eyes degenerate and their body size decreases. This is probably because a
smaller body size helps them move in between narrow spaces in caves. In
addition, smaller eyes and body size also means that they will need less energy
to function inside the caves where food is limited. Yet, there are also
troglobitic species which do not fit in this pattern. For instance, there are
some troglobites that are large and normal-eyed, similar to their
surface-dwelling relatives.
3. Troglobitic species can have eyes which are normal, micro-sized, or blind.
Therefore, eye morphology is not always correlated with the type of habitat
utilized by these species.
4. Horned species originated from non-horned species and all horned species
are troglobites, but not all troglobites have horns. This suggests that the
presence of horns is a new trait that takes a long time to develop and for this
reason, it is only restricted in some troglobites.
5. Lastly, of these troglomorphic traits, the most striking is the complete
loss of eyes. There are at least 8 different species of blind Sinocyclocheilus
cavefishes. The evolution of blindness occurred independently at least 3
times. This means that 3 different species of surface-dwelling ancestors each
evolved separately into one or more different blind species.
All these complex morphological patterns together with Sinocyclocheilus’
unusually high diversity led scientists to assert that Sinocyclocheilus can
serve as an invaluable multi-species model for studying the evolution of
troglomorphic traits. In other words, learning more about this group of endemic
Chinese cavefishes will certainly help us better understand how evolution
occurs in caves.
The Sinocyclocheilus cavefishes
are teaching us a lot about evolution in caves, but sadly, they are not exempted
from threats to their existence. Within
this group, several species are highly restricted to their specific caves, and with
small population sizes. This makes them highly susceptible to threats such as
invasive alien species introductions, habitat degradation, and water pollution.
Currently, nine Sinocyclocheilus species are listed in the IUCN Red List
(IUCN, 2021). Knowing this, it is important that further studies are conducted
not only to obtain more knowledge on how troglomorphic traits have evolved in
these unique species, but also to draw more attention on the establishment of
conservation measures for their protection and preservation.
References
IUCN. (2021). The IUCN Red List of Threatened Species. Version 2021-1. https://www.iucnredlist.org. Downloaded on 20 May 2021.
Mao, T.R., Liu, Y.W., Meegaskumbura, M., Yang, J., Ellepola, G. Senevirathne, G., Fu, C.H., Gross, J.B., & Pie, M.R. (2021). Evolution in Sinocyclocheilus cavefish is marked by rate shifts, reversals, and origin of novel traits. BMC Ecology and Evolution, 21(45). https://doi.org/10.1186/s12862-021-01776-y.
Sclereid0309. (2011). Synocyclocheilus anophthalmus. Flickr. https://www.flickr.com/photos/sclereid/8157222890/in/photostream/
Blog owner's note: As
a culminating activity to my MS Biology class in Biogeography, I asked
my students to write a blog post on a topic in biogeography. We welcome
constructive comments on this student piece.
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