Parasites of the lake sturgeon, Acipenser fulvescens, systematics and biogeography
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Sturgeons (Osteichthyes: Acipenseridae) are evolutionary relics with a wide distribution in the northern hemisphere. As basal actinopterygian fishes with a basic diadromous life history and a markedly diverse assemblage of host specific parasites, sturgeons provides ideal opportunities for exploring the historical biogeography of host parasite relationships. However, the considerable database for sturgeon parasites in Eurasia has, in the past, contrasted with the lack of comparable data from North American sturgeons, a discrepancy which has also prevented any meaningful analyses of their biogeography and host associations. Consequently, a systematic and biogeographical study of the parasites of a major endemic North American species of sturgeon, Acipenser fulvescens Rafinesque, was undertaken. This study resulted in the description of Diclybothrium atriatum (Monogenea) and Spinitectus acipenseri (Nematoda), and synonymy of Truttaedacnitis clitellarius and T. lebedevi (Nematoda). The North American Skrjabinopsolus manteri (Digenea) is considered a subspecies of the European S. semiarmatus, based on a re-examination of both species. Morphological and systematic studies on the Diclybothriidae, Deropristiidae and the genera Truttaedocnitis and Spinirectus have established the monophyly of certain groups specific to sturgeons, identified phylogenetic relationships, and provided a basis for interpreting their coevolutionary history. The history of sturgeon parasites is one of colonization by host switching and subsequent variable periods of co-evolution, leading to the formation, through dispersal and vicariance, of small highly distinct monophyletic assemblages whose species show strong associations with major discrete continental and oceanic formations. While the present association of lake sturgeon with two major drainage systems (Arctic and Atlantic) is reflected in the biogeography and systematic relationships of its parasites, the palaeontology, phylogeny and parasite relationships of sturgeons support the historical connection between North America and Northeast Asia and between A. fulvescens and A. baeri. It is hypothesized that a former contiguous distribution of Nearctic and Palaearctic (Siberian) sturgeons, was subsequently fragmented by orogeny, marine transgressions, and ultimately Pleistocene glaciations which caused major displacement of the lake sturgeon lineage and its association with the Gulf of Mexico drainages. This history follows the over all restriction of sturgeons in the freshwater Arctic drainages resulting in modification of a basic diadromous life history which has structured the parasite assemblage into one comprising freshwater species.