Galápagos and Californian sea lions are separate species: Genetic analysis of the genus Zalophus and its implications for conservation management.

Jochen Bw Wolf,Fritz Trillmich,Diethard Tautz

doi:10.1186/1742-9994-4-20

Abstract

BackgroundAccurate formal taxonomic designations are thought to be of critical importance for the conservation of endangered taxa. The Galápagos sea lion (GSL), being appreciated as a key element of the Galápagos marine ecosystem, has lately been listed as 'vulnerable' by the IUCN. To date there is, however, hardly any scientific evidence, whether it constitutes a separate entity from its abundant Californian neighbour (CSL). In this paper, we delineate the taxonomic relationships within the genus Zalophus being comprised of the Galápagos sea lion, the Californian sea lion and the already extinct Japanese sea lion (JSL).ResultsUsing a set of different phylogenetic reconstruction approaches, we find support for monophyly of all three taxa without evidence of reticulation events. Molecular clock estimates place time to common ancestry of the Galápagos sea lion and the Californian sea lion at about 2.3 ± 0.5 mya. Genetic separation is further suggested by diagnostic SNPs in the mitochondrial and nuclear genome. Microsatellite markers confirm this trend, showing numerous private alleles at most of the 25 investigated loci. Microsatellite-based estimates of genetic differentiation between the Galápagos sea lion and the Californian sea lion indicate significant genetic differentiation. Gene diversity is 14% lower in the Galápagos sea lion than in the Californian sea lion, but there is no evidence for recent bottleneck events in the Galápagos sea lion.ConclusionBased on molecular evidence we build a case for classifying the Galápagos sea lion (Zalophus wollebaeki), the Californian sea lion (Zalophus californianus) and the Japanese sea lion (Zalophus japonicus) as true species. As morphological characters do not necessarily fully reflect the rapid divergence on the molecular level, the study can be considered as a test case for deriving species status from molecular evidence. We further use the results to discuss the role of genetics in conservation policy for an organism that already is under the general protection of the habitat it lives in.

Highlights

Accurate formal taxonomic designations are thought to be of critical importance for the conservation of endangered taxa
Gene diversity is 14% lower in the Galápagos sea lion than in the Californian sea lion, but there is no evidence for recent bottleneck events in the Galápagos sea lion
Based on molecular evidence we build a case for classifying the Galápagos sea lion (Zalophus wollebaeki), the Californian sea lion (Zalophus californianus) and the Japanese sea lion (Zalophus japonicus) as true species

Summary

Introduction

Accurate formal taxonomic designations are thought to be of critical importance for the conservation of endangered taxa. Even in long-managed populations ecological, geographical and genetic borders, core to the ESU concept, are not always clearly delineated This is well illustrated in the case of the endemic Galápagos sea lion, whose geographical range is well defined (Fig. 1), while it is unclear if it constitutes a unique ecological and evolutionary entity. As other marine organisms in the Galápagos it deserves special attention from a conservation viewpoint, being subject to extreme changes in food availability triggered by climatic fluctuations of the El Niño Southern Oscillation events. During such an event, mortality drastically increases and can lead to the loss of entire cohorts [2].

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