MOLECULAR PHYLOGENETIC PERSPECTIVE ON THE EVOLUTION OF THE DEEP-SEA FISH GENUS CYCLOTHONE (STOMIIFORMES, GONOSTOMATIDAE)

A portion of mitochondrially encoded 12S and 16S ribosomal RNA genes w ere sequenced from 13 currently recognized species of the midwater dee p-sea fish genus Cyclothone (Stomiiformes: Gonostomatidae) and three g onostomatid outgroup taxa. Phylogenetic analyses using maximum parsimo ny and maximum likelihood methods were performed on unambiguously alig ned, combined sequences (803 bp) of the two genes. The resultant tree topologies from the two methods were congruent, being robust and suppo rted by various tree statistics, enabling the evolutionary history of Cyclothone to be described in detail. The molecular phylogeny demonstr ated striking inconsistencies with previously proposed ''natural group s,'' although the latter could be confidently refuted by the molecular data. The most significant characteristic of the evolutionary history of Cyclothone was the independent acquisition of an apomorphic depth habitat from the relatively ancestral, lower mesopelagic habitat, by e ach of three major distinct lineages that had diverged earlier in thei r evolution. Moreover, such macroevolutionary habitat shifts had been necessarily accompanied by morphological and ecological novelties, pre sumably originating from paedomorphosis. Repeated evolution of such ch anges strongly suggests ontogenetic plasticity in Cyclothone which cou ld enable these fishes to acquire larval-like, simple organization of body structure. Such a body plan could help them subsist in food-poor surroundings and regulate reproductive variables that take advantage o f increasing larval survival toward shallower depths. Recent speciatio n events, on the contrary, have produced contemporary sister species o f allopatric (or microallopatric) distributions, but few morphological and ecological differences. Even if remarkable miniaturization has oc curred, such as in the Mediterranean endemic C. pygmaea, it had to hav e been a simple truncation of ancestral species' ontogeny without atte ndance of any discernible paedomorphic features. On the basis of the f ossil record, geological history of the Mediterranean region, and ecto therm molecular divergence rate, it was estimated that Cyclothone radi ation had already started in the early-middle Miocene (17-20 million y ears ago).