In a post that appeared on this site some seven years ago, I briefly introduced you to the sculpins of Lake Baikal. Sculpins, to quickly recap, are a group of bottom-dwelling fish found in Eurasia and North America, both in marine and freshwater habitats. At some point, a representative of the freshwater sculpins entered the massive Siberian lake known as Baikal, where it gave rise to one of the world's classic adaptive radiations.
To date, about thirty species of sculpin have been described from Lake Baikal. The level of morphological divergence between these species is such that they have been classified in the past into three separate families: while some were placed in the widespread family Cottidae, others were placed in two families endemic to Baikal, the Abyssocottidae and Comephoridae. However, phylogenetic analyses indicate that all the Baikalian sculpins originated from a single ancestor, and the entire clade is nested not only within the Cottidae but also within the genus Cottus (Kontula et al. 2003). Some of the Baikalian sculpins, such as the relatively basal Leocottus kessleri, retain a habitus and lifestyle similar to those of other sculpins elsewhere. Others, such as the golomyankas of the genus Comephorus, have become remarkably modified.
The greatest diversity of Baikalian sculpins has resulted from their radiation into the lake's deep waters, which reach over 1600 metres (Sideleva 1996). This is a habitat unparalleled in any other freshwater lake. The only other great lakes reaching even comparable depths are the rift lakes Malawi and Tanganyika in Africa (the great lakes of North America, in contrast, are reasonably shallow). In the African lakes, the water quickly becomes anoxic below a fairly shallow top layer, and so the depths are devoid of multicellular life. Baikal, in contrast, is oxygenated all the way down (in this post, I speculated that this was due to Baikal's hydrothermal vents; it seems I was wrong. Baikal is oxygenated because the change in surface water temperature between summer and winter results in water circulating between layers and drawing oxygen down; in the tropical great lakes, where surface temperature remains fairly constant all year round, this circulation doesn't happen). The bulk of Baikal's deep-water sculpins make up the prior family Abyssocottidae, and exhibit adaptations similar to those seen in many marine deep-water fish. Their retinal structure has become simplified as a result of low light conditions. Their scales are reduced, and the lateral line system is composed of neuromasts exposed directly on the surface of the skin rather than contained in sub-surface canals and exposed to the outside environment via pores. The convergences between 'abyssocottids' and marine deep-sea fishes are so marked that some authors previously used them to argue for a direct marine ancestry of the Baikal fish (perhaps through a direct connection between Baikal and the sea that was once thought to have existed in the past), but this has been firmly quashed by the more recent molecular analyses. Instead, the majority of Baikal's deep-waters sculpins form a single clade that originated from shallower-water ancestors; the only exception is the genus Procottus, which includes both shallow-water and deep-water species (Kontula et al. 2003).
Possibly sister to this deep-water clade are the aforementioned two species of golomyanka in the genus Comephorus. The golomyankas are without question the most bizarre members of the Baikalian sculpin radiation. They have become adapted to a pelagic mode of life, swimming in the open water column and feeding on Baikal's similarly remarkable pelagic amphipod Macrohectopus branickii (and as remarkable as Lake Baikal's sculpins are, they are nothing compared to its amphipods). It is not a simple matter for a sculpin to swim freely: they lost their swim bladders at an earlier stage in their evolution, so their native position is quite closely associated to the water's bed. To correct for this ancestral lack of buoyancy, golomyankas have lost their covering of scales and developed a low-density body structure that contains a high proportion of oil, about one-third of their total mass. Their pectoral fins have become greatly enlarged, covering about twice the area of the remainder of the body. The end result is that golomyankas are close to neutral buoyancy, and able to simply float in water column, waiting to ambush passing prey.
Golomyankas are also distinctive in their reproductive biology. Other sculpins lay their eggs in nests among stones, where they are tended by the male until they hatch. This includes the Baikalian genus Cottocomephorus, which has adopted a partially pelagic life comparable to that of Comephorus, but not to the same extent (Cottocomephorus species resemble Comephorus in having enlarged pectoral fins, but are otherwise more typically sculpin-like). Golomyankas, in contrast, are viviparous, releasing active larvae directly into the water column. Golomyankas are by far the most abundant fish in Lake Baikal, and a major component in the diet of other fish species (including, when young, other golomyankas). They are one of the key components in making Lake Baikal what it is, the world's only freshwater sea.
Kontula, T., S. V. Kirilchik & R. Väinölä. 2003. Endemic diversification of the monophyletic cottoid fish species flock in Lake Baikal explored with mtDNA sequencing. Molecular Phylogenetics and Evolution 27 (1): 143–155.
Sideleva, V. G. 1996. Comparative character of the deep-water and inshore cottoid fishes endemic to Lake Baikal. Journal of Fish Biology 49 (Suppl. A): 192–206.