Recently, thanks to a friend, I got a wonderful monograph on livebearers from a British antique shop [1]. Along with a number of otherwise hard to find facts that will warm the systematic zoologist, I found here also an unobtrusive information about the blind livebearer Amblyopsis spelaeus (from Greek: amblys = weak, opis = sight), called “Cave Livebearer”. Although I have been studying viviparous animals for a long time, this information was absolute news to me. After an immediate phone call to Mr. Dokoupil’s apartment in Bratislava, it was clear that I had stumbled upon a really well tucked away piece of information about a livebearer that had escaped the attention of aquarists, despite the fact that De Kay had already described it in 1842! However, its viviparity was only reported by G. W. Nikolski in 1957 when he successfully propagated it in an aquarium. It is strange that even after 50 years it is not listed among the viviparous fishes (not even in FishBase), although it was systematically included among them. The Amblyopsoidea, as well as the Poecilioidea, were until recently suborders of the order Cyprinodontiformes. Now, admittedly, in the new system according to Nelson (2006) they belong to the order Percopsiformes.
Despite the gap of ½ century during which the fish has been forgotten by aquarists, information is very difficult to find. I can therefore present only a few data, mostly coming from the above book.
The fish was first found in the huge Mammoth Cave in Kentucky. Later they were recorded in many other underground waters flowing through limestone caves in the central United States. They have even been found in the reservoirs of deep wells, demonstrating connections even in very remote areas. Specifically, they are relatively abundant in groundwater in Kentucky, Ohio, and Indiana.
Although there is only fresh water in the caves where they now live, it is likely that their ancestors were marine fish. This assumption is based on the fact that, of all the species of fish and amphibians living at these localities, A. spelaeus is the exclusive host of parasitic crustaceans, which belong to the group of marine crustaceans.
Males grow up to 80 mm, females up to 120 mm. Their body shape resembles that of mudfishes (Umbra), but they are more slender. The heavily degenerated eyes are visible only as small grey-blue dots under the skin. The optic nerves are reduced, but the optic lobes of the brain are developed in the same way as in fish with functional eyes. The loss of vision is compensated by excellent hearing and a large number of taste buds arranged in transverse rows on the head.
This fish is completely devoid of any colouration. The body is almost transparent, with only a faint grey-greenish tinge on the back. It has not yet been shown whether Amblyopsis can change colour under the influence of light, as, for example, the cave tetra Anoptichthys jordani.
In keeping with their natural habitats, i.e. caves and groundwater, they require a temperature of 12 to 16 °C. Good filtration is essential, and stronger water flow is suitable. The tank must be shaded, as although blind, they show considerable restlessness when exposed to bright light. A perfect covering of the aquarium is very important, as the fish jump frequently and well. Especially when disturbed. Most of the time they swim vigorously just below the surface, but the slightest tremor around the tank will alarm them and they sink to the bottom.
They also take almost 100% of their food from the bottom. Threadworms, tadpole larvae, meat, etc. They don’t take food that floats in the water column at all. Only rarely do they take food from the surface. This is of course due to the fact that there are no insects deep in the caves.
Due to the low water temperatures and therefore slow metabolism, they live to a long age (14 years has been proven), and because the groundwater is very poor in food, they are able to starve for up to 2 years!
From the genus Amblyopsis, another species is described, rosae Eigenmann, 1899, of which I have only traced that it reaches half the size (i.e. about 6 cm) and no mention of viviparity, among other things. But the assumption would be there.
[1] Jacobs, K. (1971): Livebearing Aguarium Fishes, Studio VISTA Ltd., London, 459 pp.
[2] http://lib.washington.edu
[3] http://www.fws.org