Short Communication
Mitochondrial DNA evidence for deep genetic divergences in allopatric populations of the rocky intertidal isopod Ligia occidentalis from the eastern Pacific

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Abstract

Nucleotide sequences from the cytochrome c oxidase subunit I (COI) gene were used to test for genetic differentiation in the rocky intertidal isopod crustacean, Ligia occidentalis (Ligiidae), from the eastern Pacific. Phylogenetic analyses showed that individuals of L. occidentalis from southern California, USA to Manzanillo, Colima, Mexico partitioned into 15 highly-divergent clades. Mean Kimura 2-parameter (K2P) genetic distances among clades ranged from 13.2% to 26.7%. These values are similar to interspecific genetic distances found in a wide variety of crustaceans, including Ligia spp., suggesting that the taxon L. occidentalis represents a complex of cryptic species.

Introduction

The rock louse, Ligia occidentalis Dana, 1853, is a common rocky intertidal isopod distributed from Oregon, USA south to Chamela Bay, Jalisco, Mexico, including the entire Gulf of California (Espinosa-Pérez and Hendrickx, 2006). Several aspects of the life history of L. occidentalis suggest that dispersal capability is probably extremely limited, which leads to the prediction that substantial genetic differentiation should occur throughout its range. Development of the young is direct, occurring in brood sacs (Warburg, 1993), thus dispersal is limited to the juvenile and adult stages. Juveniles and adults, however, inhabit a narrow ecological niche, limited mainly to the upper intertidal splash zone of rocky beaches where they feed on algae and scavenge on dead animals and plants (Morris et al., 1980). Rock pools are used by these isopods to replenish water lost by dehydration, but they are unable to survive for prolonged periods in seawater. Throughout the range of L. occidentalis, rocky beach habitat is discontinuous, sometimes being interrupted by long stretches of sandy beaches, especially along the mainland coast of the Gulf of California (Thomson et al., 2000). In the present study we test the prediction that these physical dispersal barriers, together with life history traits, will result in restricted gene flow and substantial population structure in L. occidentalis by analyzing DNA sequences from a segment of the mitochondrial cytochrome c oxidase subunit I (COI) gene from individuals sampled over a broad portion of its range.

Section snippets

Sampling

We sampled 163 L. occidentalis from 44 coastal localities in the eastern Pacific and Gulf of California, from Long Beach, California, USA to Manzanillo, Colima, Mexico (Fig. 1). We also included an available GenBank sequence for L. occidentalis from San Diego, California, USA (Accession No. AF255780). Sample size per locality varied from 1 to 17 individuals. Specimens were preserved in 95% ethanol.

Molecular analyses

Total genomic DNA was extracted from legs or thoracic muscle using the DNeasy™ (QIAGEN Inc.,

Results

Haplotype diversity (h) in the 610 bp COI segment varied from 0.389 to 1.000 among the 15 assigned clades (see below) of L. occidentalis; nucleotide diversity (π) ranged from 0.00070 to 0.04536 (Table 1). None of the values for Tajima’s D were significant. Mean G + C (guanine + cytosine) content ranged from 39.1% to 44.4% which falls within the range of values for isopods (Costa et al., 2007).

Bayesian and MP phylogenetic trees yielded similar topology (Fig. 2) and confirmed the extensive genetic

Discussion

Based on the limited dispersal capability, lack of a pelagic larval stage and restricted ecological niche of L. occidentalis we expected to find a high degree of genetic differentiation among populations of this widely-distributed rocky intertidal isopod. The significant population structure found within four of the identified clades of L. occidentalis was consistent with our prediction, but the overall degree of genetic diversification revealed in the phylogenetic analyses was unexpected.

Acknowledgments

We thank R. Cudney, L.A. Hurtado, M. Mateos, O. Morales, M. Pires, D. Reznick, and J. Torre for collecting specimens, R. Wetzer for kindly providing the specimen of L. exotica, and R.C. Brusca, L.A. Hurtado, M. Mateos and T. Watts for technical assistance and/or suggestions on the manuscript. Samples in Mexico were collected under permit FLOR0030 from the Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT) issued to F. Molina-Freaner. This work was supported by funds from the

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