Mar Biol (2008) 155:243–247 DOI 10.1007/s00227-008-1005-0

S H O R T CO M MU N I C A T I O N

Molecular dating and biogeography of the neritic krill Nyctiphanes M. Eugenia D’Amato · Gordon W. Harkins · Tulio de Oliveira · Peter R. Teske · Mark J. Gibbons

Received: 6 November 2007 / Accepted: 16 May 2008 / Published online: 10 June 2008 © Springer-Verlag 2008

Abstract The genus Nyctiphanes (Malacostraca, Euphausiacea) comprises four neritic species that display antitropical geographic distribution in the PaciWc (N. simplex and N. australis) and Atlantic (N. couchii and N. capensis) Oceans. We studied the origin of this distribution applying methods for phylogenetic reconstruction and molecular dating of nodes using a Bayesian MCMC analysis and the DNA sequence information contained in mtDNA 16S rDNA and cytochrome oxidase (COI). We tested hypotheses of vicariance by contrasting the time estimates of cladogenesis with the onset of the major barriers to ocean

circulation. It was estimated that Nyctiphanes originated in the PaciWc Ocean during the Miocene, with a lower limit of 18 miilion years ago (Mya). An Atlantic–PaciWc cladogenic event (95% HPD 3.2–9.6) took place after the closure of the Tethyan Sea, suggesting that dispersal occurred from the Indo-PaciWc, most likely via southern Africa. Similarly, the antitropical distribution pattern observed in the eastern Atlantic Ocean likely resulted from recent Pliocene–Pleistocene (95% HPD 1.0–4.97) northward dispersal from the southern hemisphere. Our results imply that dispersal appears to have had a signiWcant role to play in the evolution of this group.

Communicated by T. Reusch.

Introduction

Electronic supplementary material The online version of this article (doi:10.1007/s00227-008-1005-0) contains supplementary material, which is available to authorized users.

Nyctiphanes is one of only two strictly neritic euphausiid genera in the Order Euphausiacea (Casanova 1984; van der Spoel et al. 1990; Maas and Waloszek 2001) and comprises four species that inhabit the productive temperate waters on either side of the equator in both the PaciWc and Atlantic Oceans. They often dominate zooplankton biomass, thus forming the basis of economically important food chains (Pillar et al. 1992). Antitropical species distribution patterns such as that observed in Nyctiphanes can be explained by either founder dispersal or vicariance hypotheses of cladogenesis (Lindberg 1991 and references therein). We attempted to infer the most likely causes for the current disjunct geographic distribution patterns of Nyctiphanes spp. by dating the cladogenetic events of the reconstructed phylogeny of the group under diVerent molecular clock models. If the fragmented distribution patterns in Nyctiphanes are the result of vicariance, the divergence time estimates should postdate the origin of the major barriers to dispersal; the closures of the Tethyan Sea »14 million years ago

M. E. D’Amato · M. J. Gibbons Biodiversity and Conservation Biology Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa G. W. Harkins · T. de Oliveira South African National Bioinformatics Institute, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa P. R. Teske Molecular Ecology Laboratory, Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia M. E. D’Amato (&) Biotechnology Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa e-mail: [email protected]; [email protected]

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(Mya) (Rögl and Steininger 1983), the Indonesian Seaway »13 Mya (White 1994), and the Central American Seaway »3.1 Mya (Keigwin 1978). Founder dispersal was considered the most likely explanation for cladogenesis when species divergence time estimates were signiWcantly diVerent from the estimated dates of vicariant events documented in the literature. Vicariance versus dispersal hypotheses have been tested in other benthic (Teske et al. 2007 and references therein) and pelagic marine species (Bowen and Grant 1997; Grant and Bowen 1998). To our knowledge, this is the Wrst study in which biogeographic hypotheses have been tested in euphausiids in a Bayesian framework under diVerent evolutionary models.

Mar Biol (2008) 155:243–247

data given the absence of a fossil record and the substantially diVerent body sizes observed among species in this group. The Yule process (Yule 1924) speciation model was used as a prior for the MCMC runs to infer the time to most common recent ancestor (TMCRA) in all the specieslevel analyses. Nyctiphanes capensis Hansen was collected oV South Africa and Namibia in February 2001, N. couchii Bell was collected oV Scotland in 2001, N. australis G.O. Sars was collected oV New Zealand in 2001, and N. simplex Hansen was collected oV California in 2000 (Fig. 1).

Results Methods The molecular phylogenetic relationships of Nyctiphanes spp. were reconstructed using several diVerent methods of tree reconstruction and information from the mitochondrial cytochrome oxidase I gene (CO1) and mitochondrial large subunit of the ribosomal RNA coding gene (16S rDNA). Molecular clock hypotheses were tested (Felsenstein 1981; Drummond et al. 2006) and molecular dating techniques using strict and relaxed molecular clock methods (Drummond et al. 2006) employed to estimate the divergence dates of species in the genus Nyctiphanes. We applied molecular rates from calibrated molecular clocks for crustacean COI and 16S (Schubart et al. 1998). A coestimate of nucleotide substitution model parameters, phylogeny and divergence was obtained using the Bayesian Markov chain Monte Carlo method implemented in BEAST v1.4.6 (Drummond and Rambaut 2006) employing probabilistic calibration prior to appropriately incorporate calibration uncertainties (Drummond and Rambaut 2006). This method is well suited to analyse the euphausiid

Fig. 1 Geographic distribution of Nyctiphanes species. All sites except the southeast PaciWc population of N. simplex were sampled. Semitransparent grey arrows indicate former open seaways; the onset of the current barriers is indicated in Mya (million years ago). Solid grey arrows indicate sea currents with their date of origin in Mya

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Congruent topologies were recovered in all the phylogenetic analyses irrespective of the gene fragment used, the method of tree construction employed, or the molecular clock model applied (Fig. 1, Supplementary Material). For the sake of brevity, only the Bayesian phylogeny of the 16 euphausiid species based on 16S is shown in Fig. 2. In all trees, the monophyly of Nyctiphanes was highly supported with both high bootstrap and clade credibility values recovered. This genus forms a subclade within a clade comprising the species Meganyctiphanes norvegica, Nematoscelis megalops, Thysanoessa inermis, T. longipes and T. macrura, and distinct from the clade containing the Euphausia species. This result is in agreement with a morphological assessment of the systematic position of this group (Casanova 1984; Maas and Waloszek 2001), who hypothesised that Nyctiphanes were closely related to Meganyctiphanes and Pseueuphausia, and Nematoscelina (Thysanoessa, Stylocheiron, Nematoscelis and Tessarabrachion) but not with the result of van der Spoel et al. (1990) who thought Nyctiphanes was more closely related to Euphausia.

Mar Biol (2008) 155:243–247

245

L. vannamei E. similis E. krohni E. frigida E. tricantha E. superba E. crystallorophias E. pacifica N. megalops T. inermis T. longipes T. macrura M. norwegica

26.46 Mya [20.27 - 34.15]

***

N. simplex N. australis

7.92 Mya [4.43 - 13.05] 5.71 Mya [3.25 - 9.35] 2.7 Mya [1.0 - 4.97] Central American Seaway closure

***

45

40

35

30

25

20

15

***

Benguela current onset

Indonesian Seaway closure

Tethys Sea closure

Antartic Convergence onset

***

10

N. australis N. capensis N. capensis N. couchii

5

Time (Mya)

Fig. 2 The 16S consensus tree of the relaxed molecular clock analysis. The tips of the tree correspond to the sampled species in the present, branch lengths reXect the mean of the posterior probability density, i.e. internal nodes are scaled to time of origin. The posterior probability density for the time of most recent common ancestor (TMRCA) for the speciation of N. couchii and N. capensis is presented in dark blue, the TMRCA of the speciation event between N. australis, N. capensis and N. couchii is presented in green, the TMRCA of the speciation of N. simplex from the other three Nyctiphanes species presented is shown in

yellow and the TMRCA of the speciation event from the sampled Nyctiphanes and other euphausiids in shown in red. The TMRCA means and the 95% highest probability density (HPD) intervals is shown by the horizontal bars in the internal nodes of the Nyctiphanes clade and are indicated in text in the Wgures. Posterior probabilities >0.9 are represented by triple asterisks in the internal nodes of the tree. This tree was calculated using the Yule speciation process and an evolutionary rate of 0.60% per million years were used as priors in the relaxed molecular clock analysis

Within the Nyctiphanes clade, N. capensis and N. couchii are inferred to be sister species to which N. australis is the sister taxon, and all three species share a common ancestor with N. simplex (Fig. 2). The intra- and interspeciWc nucleotide p-distances for both the data sets are available as supplementary material (Table 1, Supplementary Material). Similar estimates of the TMRCA were recovered for both data sets under both the strict and relaxed molecular clock models with the widest 95% highest posterior density intervals (HPDs) obtained under the latter model (Table 2, Supplementary Material). Replicate MCMC runs in BEAST under the relaxed clock model estimate that the Nyctiphanes species shared a common ancestor around

7.924 Mya (95% HPD 4.431–13.059 Mya; 16S) and 9.815 Mya (95% HPD 5.748–18.149 Mya, COI) (Table 1). Under the same model, it was further estimated that N. australis, N. capensis and N. couchii shared an MRCA around 5.714 Mya (95% HPD 3.247–9.352 Mya, 16S) and 5.35 Mya (95% HPD 3.199–9.569 Mya, COI; Table 2 Supplementary Material). For the N. australis and N. capensis species pair, the TMRCA was estimated at 2.70 (95% HPD 1.002–4.973 Mya, 16S) and 2.575 Mya (95% HPD 1.377– 4.977 Mya; COI). The values of the eVective sample size (ESS) exceeded 9,000 for all runs, suggesting acceptable mixing and suYcient sampling of the MCMC chains (Rambaut and Drummond 2004).

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Discussion The similarity of the TMRCA estimates obtained for the diVerent mitochondrial genes under both the strict and relaxed molecular clock models suggest that the signal for the TMRCA can be recovered even when the model may be misspeciWed. Almost identical results were obtained among replicate runs of the Bayesian MCMC analyses suggesting that tree space has been eVectively sampled (Drummond et al. 2006). In all the phylogenetic analyses the PaciWc species N. simplex and N. australis occupy the most basal branches in the Nyctiphanes clade, indicating that these are the oldest extant Nyctiphanes lineages. If we assume that contemporary and ancestral geographical distributions are similar, this may imply an origin in the PaciWc rather than in the Prototethys Sea during the Late and Post Cretaceous epochs (Van der Spoel et al. 1990). The onset of the Antarctic convergence »23 Mya that led to the establishment of the temperate environment in the southern hemisphere (White 1994), precedes the date indicated for the Nyctiphanes simplex node in the phylogenetic trees (Fig. 2). Therefore, it is possible that Nyctiphanes originated in the south PaciWc as a neritic krill group subsequent to the establishment of colder conditions along the highly productive coastlines that were thought to have existed in this region at this time (White 1994). The establishment of a temperate environment in the North PaciWc as a consequence of the closure of the Indonesian Seaway (White 1994), is thought to have occurred subsequent to that in the southern hemisphere. Thus the presence of N. simplex in the northern PaciWc is possibly a result of recent dispersal from the south. Evidence for trans-tropical dispersal in the East PaciWc at the time of the Pliocene–Pleistocene boundary has been well-documented for several taxa (Bowen and Grant 1997; Grant and Bowen 1998; Lindberg 1991; Dawson 1946; Valentine 1955; Emerson 1952). The origin of the N. australis species lineage postdates the two major geotectonic events in the Miocene; the closures of the Tethyan and Indonesian seaways around 14 and 13 Mya, respectively (Rögl and Steininger 1983; White 1994), which led to the establishment of novel ocean circulation and climatic patterns. The opening of new environments may have inXuenced the origin and diversiWcation of this group in the Indo-West PaciWc and Indian Ocean, giving rise to the N. australis and proto-Atlantic lineages. The sister species relationship between N. australis and the Atlantic species group suggests that the colonization of the Atlantic perhaps by a proto-Atlantic species, likely resulted from dispersal from the Indian Ocean and transport along the African coastline rather than through the Drake passage, where the water temperatures are far colder than those experienced across the current geographical distribution range of contemporary Nyctiphanes species (Rabassa

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Mar Biol (2008) 155:243–247

et al. 2005). Planktic dispersal via the former route has been well-documented in the literature (Haq 1982; Connolly et al. 2003; Gibbons 1995, 1997; Gibbons et al. 1995). Consistent with this hypothesis, N. capensis is currently endemic to the Benguela current, which was established approximately 10 Mya (Siesser 1980). In turn, the sister species relationship between N. capensis and N. couchii suggests colonization of the North Atlantic must have occurred by means of a dispersal event from the southern hemisphere, possibly during a period of glacial cooling and consequent contraction of the tropical regions. Dispersal via this route during the Pleistocene has been well-documented for several marine taxa (Bowen and Grant 1997; Grant and Bowen 1998; Koufopanou et al. 1999; Stepien and Rossenblatt 1996; Stillman and Reeb 2001; Burridge and White 2000; Hilbish et al. 2000; Grant and Leslie 2001). At this time the South Atlantic underwent a period of intensive cooling due to both the reorganization of ocean circulations after the closure of the Central American Seaway and the Pliocene glaciations (Dupont et al. 2005 and references therein). With the exception of the association between the N. capensis–N. couchi divergence time and the Wnal closure of the Central American Seaway, the estimated dates of cladogenesis within this genus do not correspond with the other vicariant events documented in the literature (Fig. 2). We therefore conclude founder dispersal rather than vicariance may have played a major role in the generation of the current disjunct geographical distribution among Nyctiphanes species. Acknowledgments We would like to thank G. Tarling, S.N. Jarman and J. Gomez for supplying us with specimens of Nyctiphanes couchii, N. australis and N. simplex. Financial support for MED was provided by the National Research Foundation—Royal Society SET Program. Gordon W. Harkins is funded by the Atlantic Philanthropies and NRF Grant 62302. The Bioinformatics Capacity Development Research Unit from the South African Medical Research Council funds Tulio de Oliveira. Peter Teske was supported by a postdoctoral research fellowship from the NRF and an overseas study grant from the Ernest Oppenheimer Memorial Trust. Sampling and laboratory procedures comply with the current laws of the country.

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Molecular dating and biogeography of the neritic krill ... - Springer Link

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