- Before 1896, when I had the opportunity of studying the fauna of Lake Tanganyika on the spot, it was known that there existed, in the so-called Sea of Ujiji, one animals, the affinities of which are undoubtedly marine. This was the medusa Limnocnida, which Dr. Boehm saw as he crossed the lake in 1883.
It was known further that the jelly-fish was associated in Tanganyika with a number of strange molluscan forms, for the empty shells of what appeared to be some six entirely new genera of gasteropods had been brought home by Captain Speke, Joseph Thompson, and Mr. Hore. As the animals contained in these shells have not hitherto been known, their classification by the conchologists with existing fresh-water types have always appeared extremely doubtful, and from the first Mr. Edgar Smith, who described the greater number of these forms, has held the opinions that they might eventually turn out to have the same oceanic characters as the jelly-fish.
J. E. S. Moore, 1898
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The resemblance of the Lake Tanganyika Paludomidae to unrelated saltwater species is thought to the product of similar environmental pressures. Both marine and freshwater snails face predation from fish and crustaceans, which has resulted in an escalating arms race of defensive and offensive adaptations. Piercing teeth and crushing claws are met with spines, narrow apertures and thickened ridges on the shells. (And it's not confined to aquatic habitats. Some terrestrial species, such as Opisthostoma, resist attack in exactly the same way.)
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It is not surprising that Moore thought Lake Tanganyikan snails were land-locked relicts of an ancient sea. But the resemblance to marine species is a product of convergence, of similar environmental pressures resulting in similar solutions.
Ain't evolution grand?
References
Moore, J.E.S. (1898). On the Zoological Evidence for the Connection of Lake Tanganyika with the Sea. Proceedings of the Royal Society of London 62: 451 – 458.
West, K., Cohen, A. & Baron, M. (1991). Morphology and behavior of crabs and gastropods from Lake Tanganyika, Africa: Implications for lacustrine predator-prey coevolution. Evolution 45: 589 – 607
Wilson, A.B., Glaubrecht, M. & Meyer, A. (2004). Ancient lakes as evolutionary reservoirs: evidence from the thalassoid gastropods of Lake Tanganyika. Proceedings of the Royal Society of London B 271: 529 – 536.