A Consideration of Fishless Karsts
If you examine a map of the karst areas of the world it is quickly obvious that there are major areas of limestones in which no subterranean fishes have ever been collected. And in all cases this is not because the areas are remote and unexplored. On the contrary, most are very well known and have been explored for many years. So it is possible that there are in fact no subterraean fishes within these areas. If this is so we need to consider explanations for this absence. The following discusion is by no means comprehensive and the explanations by no means conclusive but this is an initial exploration of such explanations. Comments very welcome.
The following karst areas have no known troglobitic fishes
Florida has an extensive network of, mostly, phreatic caves (Upchurch et al. (2018) and over 200 species freshwater fishes (Robins et al. 2018). It also has a long history of cave diving exploration of its flooded caves including some world class penetrations and major diving expeditions carried out by world class cave divers. Despite this abundace of potential habitat space, species as potential colonists and extensive observation, no troglobitic species have ever been collected or even seen.
Fishes have been observed in caves in Florida. Marshall (1947) reported on the cyprinid Notropis harperi in streams and rivers emerging from the many vauclusian risings and from caves in Florida. This species was also studied by Hubbs and Crowe (1956), Relyea and Sutton (1972, 1973) and Brockman and Bartone (1977). Howell (1960) made a thorough study of the variability and habitats of this species. Relyea and Sutton (1973) record Yellow Bullhead, Ictalurus natalis, in two caves in Alachua County Florida, Bat Cave and Martin’s Cave, both of which are 12 km from the nearest surface drainage. Adults and juveniles are present in both locations and appear to feed successfully as they are healthy and not emaciated. The presence of juveniles so far from a surface water supply suggests that the animals breed in the cave. There are a number of anatomical abnormalities in the fishes, primarily the absence of any of pelvic, caudal and adipose fins. This may be because of inbreeding in a small, closed population. Relyea and Sutton suggest that this species is preadapted to cave life because it has an elaborate chemo-sensory system. Taking all of these points together there is a very strong suggestion that these populations are stygophilic. Streever (1992) records the death of a large number of yellow bullheads at the Peacock Springs Cave System in Suwanee County, Florida. Whether these animals are also troglophilic is not known. This is the limit of what we know about subterranean fishes in Florida.
The absence of troglobitic fishes cannot be so simple as too little time or the fact that Florida was covered by sea water at xxxx. It has a documented subterranean fauna including 24 aquatic troglobites in 627 caves (Hobbs 2012). This includes 10 troblobitic crayfishes in two genera (Hobbs 1974, Hobbs, Hobbs and Daniel 1977, Walsh 2001, Crandall and De Grave 2017).
If, for whatever reason, freshwater fishes have found it impossible, or undesirable, to colonise the Florida subsurface what about colonisation from the sea? There seems no reason why this should not be a possibility. The Bahamas and Cuba, both no more that 400km from Florida, have 2 and 4 species respectively, all in the marine Family Bythitidae. And marine incursion onto the peninsula should not, in principle, have impeeded this. However, again, this seems not to have happened and we are left with the enigma of a fishless Florida.
Has 1 aquatic troglobite in 1300 caves (area 43000km2) (Hobbs 2012). Hobbs (2012) reports that this region is arid and this probably explains the very small number of aquatic troglobites and the absence of fishes.
The Dinaric karst, Slovenia and Croatia
The Dinaric karst is one of the most important areas on Earth for subterranean biodiversity (Culver and Sket 2000, Sket 1997, Sket 2004, Sket 2005, Sket 2012, Sket, Paragamian and Trontelj 2004) and is home to one of the most iconic of all subterranean animals, the Olm, Proteus anguinus (Amphibia: Proteidae). But there are no troglobitic fishes. A first, and probably simplistic, explanation for the absence of fishes is comptetion from Proteus for scarce resources. However this is probably not the explanation as there are a number of troglophilic and trogloxenic fish species known to occur in groundwater in this area.
Voros et al. (2017) and (2018) show that the genus Proteus may contain several isolated and well differentiated taxa and that there may be grounds for the recognition of several species within the genus.
Mulu, Sarawak, Malaysia
The Nullarbor Plane, Australia
The Nullarbor Plain in southern Australia is one of the largest karst outcrops on the planet at c. 2,000,000 km2. It has many world class caves that have been well explored over the past 50 years, including major diving penetrations of extensive phreatic passages (Gillieson 2004, Ford and Williams 2007:403-408) . The water is of noted clarity, but depite this no fishes have ever been seen. In fact the situation with regard to aquatic fauna generally is very stark - there appears to be no aquautic fauna, with the possible exception of one recent record of Copepoda swimming among the unique microbial curtains that are present in considerable numbers (S. Eberhard pers. comm., reported in Tetu et al. 2015). Richards (1971) made a very thorough survey of the subterranean fauna and found no aquatic animals at all. She explained this as follows:
"Liquid Medium and aquatic cavernicoles
No aquatic fauna has been discovered in any of the Nullarbor 1akes.This may be due to the phreatic development of the deep caves and their lakes, and the general topography of the Nullarbor Plain. Despite evidence of higher sea levels at least 30.3 m above the present, no evidence has yet been found of marine invasion of the caves, and there seems little possibility that marine fauna could have entered the limestone interstitially and become isolated in the lakes by later withdrawal of the sea. When the caves opened to the surface, Jennings (1961, 1963) believes the climate was not much moister than to-day. There was no normal surface drainage in the form of streams in the vicinity of the caves, and so no freshwater fauna to colonize the lakes. Consequently there were no avenues for colonization of the lakes."
"The almost total absence of aquatic fauna in the cave waters of the aquifer is surprising. Salinity is not the cause of this absence as there is one known aquatic troglobitic species, an amphipod crustacean, in the hypersaline waters of Nurina. Neither is the lack of nutrients responsible, for biofilms are abundant in the cave waters and [these] are remakable. So far, microbiological studies have been restricted to the calcite precipitating microbial mantles. They are communities of microorganisms consisting primarily of Pseudomonas spp. and Pseudalteromonas spp. as well as nitrite oxidisers (Holmes et al. 2001, Tetu et al. 2013, Tetu et al. 2015) "
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