Mar Biol (2006) 150:111–119

DOI 10.1007/s00227-006-0335-z

RESEARCH ARTICLE

Phylogenetics of American scallops (Bivalvia: Pectinidae) based on partial 16S and 12S ribosomal RNA gene sequences

Carlos Saavedra · Juan B. Peña

Received: 20 October 2005 / Accepted: 13 April 2006 / Published online: 12 May 2006© Springer-Verlag 2006

Abstract Pectinids constitute one of the most con-spicuous groups of marine bivalves, and include someof the most important species from the point of view ofWsheries and aquaculture. In spite of this, their system-atics and evolution are not well understood. Only twomolecular phylogenetic analyses based on relativelywide taxonomic samplings have been published. Thesestudies largely neglected American species, some ofwhich are central for testing current models of pectinidevolution and diversiWcation, or are commercially valu-able. We have sequenced 820 nucleotide base pairs ofthe 12S and 16S ribosomal RNA genes in nine speciesof pectinids belonging to six genera living along Amer-ican coasts. Sequences from homologous regions of 19other species were gathered from public databases. Weconstructed phylogenetic maximum-parsimony andmaximum-likelihood trees of this set of 28 taxa. Ourphylogenetic analysis indicates that Crassadoma ispolyphyletic, and cementation to the substrate as a lifehabit could have appeared independently in two geo-graphic chlamydinid lineages. Nodipecten is placed inthe subfamily Pectininae, and the suspected close rela-tionship of Amusium, Euvola and Pecten within thissubfamily is also supported. Zygochlamys patagonicaappears in the Chlamydinae subfamily, as expected.The existence of a separate subfamily Palliolinae is

suggested but not supported statistically. The positionof Argopecten, Aequipecten and Flexopecten within thesubfamily Pectinidae, suggested by a recent study,could not be conWrmed, and we argue that it could bedue to a combination of long branch attraction andincomplete sequencing.

Introduction

One of the primary aims of marine biology is thedescription of the diversity of marine life and the classi-Wcation of marine organisms according to modern taxo-nomic methods in a consistent systematic framework.This task is fundamental for understanding the ecologyand evolution of marine organisms as well as forinformed actions towards the exploitation of marineresources (Levinton 2001). Taxonomy and systematicshave been carried out for decades by comparative stud-ies of morphology, anatomy and ecology. Bivalves pos-sess shells which are easily collected, and thereforewere among the Wrst marine organisms to be studied indetail. However, the high variability of shell morphol-ogy in response to diVerent environments has oftenmade it impossible to produce evolutionarily coherentsystems of classiWcation in many groups of bivalves.Phylogenetic analysis of DNA sequences providesmeans to overcome these diYculties due to their powerto recognize diVerent taxa and to produce detailed spe-cies phylogenies (Baco et al. 1999; Distel 2000; Ó Foig-hil and Taylor 2000).

Pectinids comprise some of the most conspicuousspecies of bivalves, including many species of the high-est commercial value as food (Shumway 1991; Peña2003). Pectinid shells exhibit an extraordinary diversity

Communicated by S.A. Poulet, RoscoV

C. Saavedra (&) · J. B. PeñaInstituto de Acuicultura de Torre de la SalConsejo Superior de Investigaciones CientíWcas, 12595 Ribera de Cabanes, Castellón, Spaine-mail: saavedra@iats.csic.es

123

112 Mar Biol (2006) 150:111–119

of shape and sculpture, which has resulted in a varietyof classiWcation systems based on morphology (Thiele1935; Korobkov 1960; Hertlein 1969; Habe 1977;Waller 1986, 1991, 1993, 2006). The main characteris-tics of these systems have been reviewed by Matsum-oto and Hayami (2000) and Barucca et al. (2004).Pectinids also exhibit a rich fossil record, which hasresulted in a large body of paleontological literatureand in detailed hypotheses about the evolution anddiversiWcation of the family (Smith 1991; Beu andDarragh 2001; Waller 1969, 1991, 1993, 2006; and refer-ences therein).

In spite of all this, molecular systematics and phylog-enetics of pectinids have been lagging behind otherbivalve groups. Early molecular studies were more orless congruent with phylogenies based on morphologi-cal traits (Rice et al. 1993; Kenchington and Roddick1994; Steiner and Müller 1996; Frischer et al. 1998;Giribet and Carranza 1999; Canapa et al. 2000a, b;Steiner and Hammer 2000), but their limited amountof sequencing and/or taxonomic sampling renderedmany of their conclusions uncertain. Only two studiesperformed a larger taxonomic sampling. Matsumotoand Hayami (2000) studied the mitochondrial cyto-chrome oxidase I (COI) gene sequences of 19 speciesof pectinids from Japan and their surrounding areas.Barucca et al. (2004) studied 24 species for two mito-chondrial genes (12S and 16S ribosomal RNA). Thesetwo studies gave support to the classiWcation systemdevised by Waller (1991, 1993, 2006). They groupedspecies in basically two wide subgroups, correspondingto the subfamilies Pectininae and Chlamydinae. Mat-sumoto and Hayami (2000) also found that Parvamus-sium intuscostatum appeared as a separate branchbasal to the main two subgroups, giving support to theexistence of a separate family Propeamusiidae. Thestudy of Barucca et al. (2004) also found that Delecto-pecten vitreus made a distinct, basal clade to theremaining Pectinids, supporting Waller’s (1991, 2006)view of a separate subfamily Camptonectinae.

Many of the molecular phylogenetic studies of mol-luscs or bivalves have included American scallop spe-cies (Rice et al. 1993; Kenchington and Roddick 1994;Steiner and Müller 1996; Giribet and Carranza 1999;Campell 2000; Steiner and Hammer 2000). Frischeret al. (1998) performed a phylogenetic analysis ofsequences of the 18S rRNA gene focused speciWcallyon the phylogenetics of scallop taxa. Unfortunately,most of these studies included only a limited number(3–7) of pectinid taxa, which lessens the strength oftheir conclusions. American species were almost com-pletely absent from the more extensive studies ofMatsumoto and Hayami (2000) and Barucca et al.

(2004). Matsumoto and Hayami (2000) included onlyone species present in America (Chlamys islandica),and Barucca et al. (2004) included two (Argopectenirradians and again C. islandica).

A detailed study of the molecular phylogenetics ofAmerican scallop species is highly desirable for tworeasons. First, accounts of pectinid fauna living on theAmerican coasts indicate that there is an importantfraction of species that belong to endemic genera(Keen 1971; Coan et al. 2000; Forcelli 2000; Lodeiroset al. 1999; Mikkelsen 2004). In spite of their impor-tance from the point of view of marine biodiversity, thesystematic status and phylogenetic relationships ofthese species are poorly known. Second, the Americanpectinid fauna is of paramount relevance to understandthe evolution and diversiWcation of the pectinid familyas a whole, because some of the Wnest studies dealingwith the subject have been based on analyses of Amer-ican pectinid taxa (Smith 1991; Waller 1991, 1993,2006). This calls for detailed comparisons of fossil-based and molecular-based phylogenies (Waller 2006).

Here we present the results of a phylogenetic studyof 10 American pectinid species. They span sevendiVerent genera, including three American endemics(Argopecten, Euvola, Placopecten), one of sub-Antarc-tic distribution (Zygochlamys), and three more of glo-bal distribution (Crassadoma, Nodipecten, Chlamys).We have sequenced the same two mitochondrial RNAregions (12S, 16S) scored by Barucca et al. (2004),which has allowed us to perform a joint analysis of thetwo data sets. We have placed the American specieswithin the phylogenetic framework of the family Pec-tinidae outlined by previous studies, and addressedsome of the pending problems of pectinid phylogeny.

Materials and methods

Species sequenced for this study, together with theirsampling localities and accession numbers, are given inTable 1. We sequenced 2–4 individuals from each spe-cies, to take into account intraspeciWc polymorphisms.When possible, more than one species per genus wasstudied, to allow for phylogenetic replication. Thesequences of Pecten maximus and P. novaezelandiaecame from our previous studies (Saavedra and Peña2004, 2005) (accessions AJ972435, AJ972436,AJ972445, AJ972446, AM039771, AM039772,AM039781 and AM039782). The sequences of theremaining species came from the studies of Baruccaet al. (2004), and we refer to that publication for acces-sion numbers. Barucca et al. (2004) included in theiranalyses Wve species for which they could not obtain

123

Mar Biol (2006) 150:111–119 113

the 12S sequence. We excluded those species from ouranalyses. Some of the species studied by Barucca et al.are referred to in this article by updated latin names:Flexopecten glaber, instead of Chlamys glabra; Mim-achlamys varia, instead of Chlamys varia; and Crassad-oma multistriata, instead of Chlamys multistriata. Theshells of the sequenced specimens were available forthree species: Nodipecten nodosus, Euvola ziczac andZygochlamys patagonica. They have been deposited inthe Museo Nacional de Ciencias Naturales in Madrid(Spain), with accession numbers MNC 15.07/5409,15.07/5410 and 15.07/5411, respectively.

We ampliWed fragments of mitochondrial 12S and16S ribosomal RNA genes by PCR. Primers for 16SrRNA were 16S-ai and 16-Sbi (Palumbi 1996). Foramplifying the 12S rRNA fragment, we used primer12Sbi of Palumbi (1996) in combination with primer12S-F (5�-AGACATGGATTAGATACCCATTTAT-3�).This primer is a modiWcation of Palumbi’s (1996) 12Saiprimer. PCR conditions were as in Saavedra and Peña(2004). Sequencing was carried out with the PCR prim-ers, in an Applied Biosystems capillary sequencer atthe DNA Sequencing Service of the University ofValencia (Spain).

Sequences were edited in BioEdit (Hall 1999). Forsome species, the regions near the primers showedlow quality sequences, and therefore were excludedfrom the dataset. A draft alignment was carried outwith CLUSTAL X (Thompson et al. 1994), as imple-mented in BioEdit, using the default options. Then,the alignment of the species studied by Barucca et al.

(2004) (EMBL Align database accession no. 000577)was used to reWne the draft. Two highly variableregions that were excluded by Barucca et al. (2004)were also excluded by us. Base composition heteroge-neity was tested with PAUP* v. 4.10b (SwoVord1998).

Phylogenetic analysis was based on the maximumparsimony (MP) and maximum likelihood (ML) meth-ods, and performed with PAUP*. For MP analyses,gaps were considered a Wfth character. A heuristicsearch was performed with 100 random stepwise addi-tions of taxa, tree bisection reconnection (TBR) andbranch swapping. ModelTest v.3.06 (Posada and Cran-dall 1998) was used to Wnd the model of evolution thatbest Wt our dataset. We used the Akaike informationcriterion to select the model, following the proceduresdescribed in Posada and Buckley (2004). The selectionprocedure rendered the TVR + I + G model as the onewith the best Wt. Model parameters were estimatedfrom the data. ML trees were constructed by a heuris-tic search with 10 random stepwise additions of taxa,TBR and branch swapping. As in Barucca et al. (2004),Spondylus gaederopus (Fam. Spondilydae) was used asthe outgroup taxon in all analyses.

Statistical support for the clades was assessed bybootstrapping (Felsenstein 1985) based on 1,000 (MP)or 200 (ML) replicates. We used the list of bipartitionsfound during the bootstrapping processes in both MPand ML procedures to evaluate in detail alternativearrangements of taxa, especially for nodes with lowbootstrap support.

Table 1 American pectinid taxa sequenced for this study, distribution range, sampling locations and sequence accession numbers

Name Range Sampling locality Number of individuals sequenced

Accession 12S Accession 16S

Argopecten purpuratus SE PaciWc Bahía Mejillones, Chile 3 AM039762, AM039763 AJ972426, AJ972427Argopecten ventricosus Central E

PaciWcBahía La Paz, Baja

California Sur, Mexico3 AM039764–AM039766 AJ972428–AJ972430

Crassadoma gigantea NE PaciWc McMullin Group Is., BC, Canada

4 AM039773, AM039774 AJ972437, AJ972438

Euvola vogdesi Central E PaciWc

Bahía La Paz, Baja California Sur, Mexico

2 AM039767, AM039768 AJ972431, AJ972432

Euvola ziczac Caribbean, Central W Atlantic

Araya Peninsula, Venezuela

2 AM039769, AM039770 AJ972433, AJ972434

Nodipecten nodosus Caribbean, Central W Atlantic

Araya Peninsula, Venezuela

2 AM039775, AM039776 AJ972439, AJ972440

Nodipecten subnodosus Central E PaciWc

Laguna Manuela, Baja California Sur, Mexico

2 AM039777, AM039778 AJ972441, AJ972442

Placopecten magellanicus NW Atlantic Commercial, USA 2 AM039779, AM039780 AJ972443, AJ972444Zygochlamys patagonica SE PaciWc,

SW AtlanticOV Mar del Plata,

Argentina2 AM039783, AM039784 AJ972447, AJ972448

123

114 Mar Biol (2006) 150:111–119

Results and discussion

We obtained the sequence of the two target regions forall 25 individuals of the 9 species scored. After combin-ing the two gene fragments and subsequent alignment,the total length of the sequence studied was 827 nucle-otides. Some individuals of the same species showedthe same sequence. In these cases, a single sequencewas included in the analysis. The alignment has beensubmitted to the EMBL Align database (accessionALIGN_000833).

There was no signiWcant heterogeneity in base com-position across species (P = 0.99). Average base com-position was 28.5% A, 17.6% C, 26.5% G and 27.4% T.Nucleotide variation was observed at 499 positions.Insertion/deletions were present at 183 sites, account-ing for 37% of variable sites. Substitutions were mainlytransitions. Three hundred and forty-three characterswere parsimony-informative (including gaps).

Models of evolution selected by hLRT and AICdiVered (Table 2). Only two models were containedwithin the 99% AIC conWdence set of models, asshown by the cumulative weights. The two modelsincluded a Wxed proportion of invariable sites andgamma-distributed rates among sites, and diVered onlyin the number of substitution rate categories (5 inTVM + I + G and 6 in GTR + I + G). The diVerencesin AIC values between these two models were verysmall, as shown by a �i value of only 1.14 for the secondbest model. However, hLRT selected a Tamura–Neimodel (Tamura and Nei 1993) with a Wxed proportionof invariable sites and gamma-distributed rates amongsites as the model that best Wt the data. Interestingly,the likelihood of this model was much lower than thatof the two models chosen with the AIC criterion: itranked sixth at likelihood scores, and Wfth at AIC val-ues, with a �i of 48 (Table 2). This illustrates nicely thatthe Akaike information criterion performs better thanhierarchical likelihood ratio tests (hLRT) for modelselection, as indicated by Posada and Buckley (2004).

We obtained a single most parsimonious tree, whichis shown in Fig. 1. ML trees obtained with the twomodels selected by AIC had the same topology, andonly the TVM + I + G was used for bootstrapping. Thistree is shown in Fig. 2. The MP and the ML trees coin-cided in their major topological features, which pointsto a good consistency of the phylogeny. The two treesdisagreed only in the placement of a few clades withlow bootstrap support. On the other hand, there weresome disagreements between our trees and thoseobtained by Barucca et al. (2004) using the same generegions, which is not unexpected given that we used ashorter DNA fragment, excluded the species whosesequence was incomplete in Baruca et al.’s study, andadded 19 new sequences. However, all the groups withhigh bootstrap support in Barucca et al. (2004, theirFig. 2) were recovered again by our analyses. We referto that publication for discussion of those clades, andwill concentrate on the results derived from the inclu-sion of American taxa, from the highest to the loweststatistically supported cases.

Crassadoma and the cementing life habit

The genus Crassadoma was created by Bernard (1986)to accommodate Hinnites giganteus (the former nameof Cr. gigantea), a species that shows a cementing lifehabit, on the grounds that cementing was obligatory inthis species of Hinnites, while it was facultative in otherHinnites. However, Harper (1991, cited in Waller 1993)pointed out that the cementing style was the same ingiganteus as in the remaining Hinnites. Waller (1993)extended the genus Crassadoma to include othercemented and non-cemented Hinnites species, and cre-ated the tribe Crassadomini, which included the genusCaribachlamys as well.

Our analysis indicates that the American Cr. gigan-tea is closer to Chlamys islandica than to the othermember of the Crassadoma genus included in thisstudy (Cr. multistriata, from the eastern Atlantic and

Table 2 Evolutionary models for pooled pectinid 12S and 16S sequences and their support statistics, obtained with ModelTest (Posadaand Crandall 1998)

Model ¡ln L K AIC �i Weight cumWeight

TVM + I + G 7,394.7285 9 14,807.4570 0.0000 0.6390 0.6390GTR + I + G 7,394.3013 10 14,808.6025 1.1455 0.3604 0.9994TVM + G 7,403.1294 8 14,822.2588 14.8018 0.0004 0.9998GTR + G 7,402.6855 9 14,823.3711 15.9141 0.0002 1.0000TrN + I + G 7,420.7769 7 14,855.5537 48.0967 2.30e¡11 1.0000TIM + I + G 7,419.8501 8 14,855.7002 48.2432 2.14e¡11 1.0000HKY + I + G 7,422.6152 6 14,857.2305 49.7734 9.94e¡12 1.0000

123

Mar Biol (2006) 150:111–119 115

western Indian Ocean). Cr. multistriata, in turn, hasMimachlamys varia as sister species. These results haveimplications regarding the origin of C. gigantea. Grantand Gale (1931) and Waller (1991) assumed that thisspecies originated in the eastern PaciWc from a memberof the Chlamys group, presumably C. hastata. Waller(1993) later suggested that an origin from an Atlanticspecies would be more likely, given the highly derivedtraits of C. gigantea in comparison with otherChlamydinae and the absence of Crassadoma in theIndo-west PaciWc. Our results clearly support thatC. gigantea originated from the C. islandica group ofspecies, to which C. hastata belongs (Waller 1991). Ourresults also support the view that the cementing habitappeared independently in several lineages (Waller1991, 1993). However, these lineages were not diVerent

waves of Crassadoma moving from the eastern Atlan-tic to the western Atlantic and then to eastern PaciWc,as suggested by Waller (1993), but diVerent Chlamydi-nid lineages. Finally, our results agree with the view ofHinnites as a polyphyletic genus (Waller 1991, and ref-erences therein), and indicate that Crassadoma is alsopolyphyletic.

Nodipecten, Euvola and the subfamily Pectininae

Euvola are usually viewed as the American counter-parts of Pecten scallops from the eastern Atlantic andIndo-PaciWc (Waller 1991). The two genera are sup-posed to share a common ancestor with Amusium, agenus living in the Indo-PaciWc and western Atlantic,which is represented in this study by the Australian

Fig. 1 Maximum parsimony tree of 27 pectinid species based oncombined 12S and 16S sequences. Numbers after species’ namesrefer to diVerent individuals. Numbers at nodes indicate % boot-strap support based on 1,000 replicates. For clarity, bootstrap val-ues for clades including only conspeciWc or congeneric sequences

have been excluded. These values are > 90%, with the exceptionof the clade containing Pecten maximus and P. novaezelandiae(81%). The outgroup branch has been shortened to Wt the page.Taxa have been grouped in three subfamilies, according to Waller(2006). CI = 0.50, RI = 0.70, RC = 0.35

Spondylus gaederopus

Mimachlamys nobilis

Amusium pleuronectes

Mirapecten mirificus

Adamussium colbecki

Chlamys islandica

Laevichlamys cuneata

Argopecten ventricosus 3

Crassadoma gigantea 3Crassadoma gigantea 1

Crassadoma multistriataMimachlamys varia

Semipallium amicumSemipallium dringi

Coralichlamys madreporarumLaevichlamys wilhelminae

Zygochlamys patagonica 4Zygochlamys patagonica 3

Placopecten magellanicus 2Placopecten magellanicus 1

Gloripallium palliumMirapecten rastellum

Nodipecten nodosus 2Nodipecten nodosus 1Nodipecten subnodosus 2Nodipecten subnodosus 1

Pecten maximus 8Pecten maximus 6Pecten novaezelandiae 2Pecten novaezelandiae 1

Euvola ziczac 6Euvola ziczac 3Euvola vogdesi 3Euvola vogdesi 2

Argopecten purpuratus 1Argopecten purpuratus 2

Argopecten ventricosus 2Argopecten ventricosus 1

Flexopecten glaberAequipecten opercularis

Subfam. Palliolinae

Subfam. Pectininae

Subfam. Chlamydinae

100

67

73

31

100100

17

43

18

32 94

72

36

48

99100

99

62

100

19

97

123

116 Mar Biol (2006) 150:111–119

species A. pleuronectes. Amusium has been oftenincluded in a separate subfamily (Amusiinae), andeven in the family Propeamusiidae. Waller (1972) pro-posed the placement of Amusium in the Pectinidaerather than in the Propeamusiidae. Later, the molecu-lar studies of Matsumoto and Hayami (2000) and Bar-ucca et al. (2004) showed Amusium as clearlybelonging in the Pectininae. Our analysis supports theclose relationship of Euvola, Amusium and Pecten.Waller (2006) has proposed, on the base of detailedmorphological analyses of living and fossil species, thatthe lineage that led to Pecten diverged Wrst in the phy-logeny of this group, and therefore Amusium andEuvola are more closely related. Our two trees pro-duced diVerent branching orders of the three genera,with low bootstrap support, so the issue cannot be

solved with the present data. Matsumoto and Hayami(2000), studying COI amino acid sequences, foundAmusium at the basis of the Pectininae clade (includ-ing the tribe Decatopectinini), with a good bootstrapsupport, but did not include any Euvola species in theiranalysis.

The most important result of this study regardingthe subfamily Pectininae is the placement of Nodipec-ten. Some authors have placed this genus within theChlamydinae (Hertlein 1969), but our results give sup-port to those that placed it within the Pectininae (Thi-ele 1935; Waller 1986). Waller (2006) proposed that theclade formed by the tribes Amusiini and Pectininiwould have originated from a common ancestor thatbelonged to the lineage that would give rise to theAequipecten and Flexopecten lineage. However, Waller

Fig. 2 Maximum likelihood tree of 27 pectinid species. Numbers after species’ names refer to diVerent individuals. Numbers at nodesindicate bootstrap support based on 200 replicates. Bootstrap values for nodes including only conspeciWc or congeneric sequences havebeen excluded

Spondylus gaederopus

Adamussium colbecki

Amusium pleuronectes

Mimachlamys nobilis

Argopecten purpuratus 1Argopecten purpuratus 2

Mirapecten mirificus

Chlamys islandica

Laevichlamys cuneata

Argopecten ventricosus 3

Coralichlamys madreporarumLaevichlamys wilhelminae

Semipallium amicumSemipallium dringi

Zygochlamys patagonica 4Zygochlamys patagonica 3

Crassadoma multistriataMimachlamys varia

Crassadoma gigantea 3Crassadoma gigantea 1

Placopecten magellanicus 2Placopecten magellanicus 1

Gloripallium palliumMirapecten rastellum

Nodipecten nodosus 1Nodipecten nodosus 2Nodipecten subnodosus 2Nodipecten subnodosus 1

Pecten maximus 8Pecten maximus 6

Pecten novaezelandiae 2Pecten novaezelandiae 1

Euvola ziczac 6Euvola ziczac 3Euvola vogdesi 3Euvola vogdesi 2

Argopecten ventricosus 2Argopecten ventricosus 1

Flexopecten glaberAequipecten opercularis

100

56

37

98

90

6

31

99

23

6

95

65

23

36

2895

100

26

100

34

96

123

Mar Biol (2006) 150:111–119 117

(2006) did not include Nodipecten in this scenario.Previously, he had considered that Nodipecten couldbe phylogenetically close to the tribe Camptonectinae(Waller 1986). Our results suggest that the origin of theAmusini and Pectinini tribes was in a common ancestorshared with present-day Nodipecten.

Placopecten and the subfamily Palliolinae

Waller (2006) has proposed the subdivision of the fam-ily Pectinidae in four subfamilies: Camptonectinae,Pectininae, Chlamydinae and Palliolinae. Molecularphylogenetic support for the Camptonectinae, Pectini-nae and the Chlamydinae was obtained by diVerentstudies, notably those of Matsumoto and Hayami(2000) and Barucca et al. (2004). The Palliolinae wouldinclude the genera Placopecten and Adamussium, andalso other northeastern Atlantic genera such as Pallio-lum and Pseudamussium (Waller 2006). Evidence forthe subfamily Palliolinae in our data is given by Placo-pecten magellanicus and Adamussium colbeckii form-ing a separate clade in the MP tree (Fig. 1), theirappearing as neighbour branches in the ML tree(Fig. 2), and the inspection of the bipartitions obtainedduring the bootstrapping process, which indicates thatP. magellanicus formed a separate clade with A. col-beckii in 63.4% of the ML replicates. However, thisevidence, although suggestive, lacks strong statisticalsupport. Previous studies based in 18S rRNA failed tojoin the two species in a single clade (Canapa et al.2000a).

Zygochlamys patagonica

This species has been traditionally considered as achlamydinid (Beu 1985; Waller 1991). It appearsalways in the Chlamidynae branch of our trees, some-times close to the putative members of the subfamilyPalliolinae (Fig. 1), but without statistical support.Beu (1985) suggested that Zygochlamys could haveoriginated in the Miocene. However, our results pointto a relatively old lineage, with roots at an early stageof the diversiWcation of the Chlamydinae. This is inline with Waller’s opinion (1991) that the origin ofZygochlamys could be dated at the Eocene or Oligo-cene period.

The position of Argopecten and the deep phylogeny of the Pectinidae

Waller (1993) proposed that the “Aequipecten group”was a branch of the subfamily Chlamydinae. However,after examining new evidences from a previously not

considered fossil group, namely the Pecten perplanusgroup, he revised the phylogenetic positions of Aequi-pecten, Flexopecten and Argopecten, which he nowplaces in the Aequipectinini tribe, at the base of thelineage conducing to the Pectinini and Amusiini tribes,in the subfamily Pectininae (Waller 2006). This pointof view has obtained some support from molecularstudies. Barucca et al. (2004) found the three above-mentioned genera forming a clade within the Pectini-nae subfamily. However, their results could be aVectedby the fact that they could not obtain the 12S sequenceof A. irradians. Moreover, some branches in their trees,including the one leading to the Argopecten/Aequipec-ten/Flexopecten clade, showed low statistical support(< 50%). Matsumoto and Hayami (2000) did notinclude in their study any species of the cited genera,but examined a species of Cryptopecten which is con-sidered very close to Argopecten (Waller 2006). Cryp-topecten appeared in their trees as the sister group ofPecten. Similar results were obtained by Frischer et al.(1998), Giribet and Carranza (1999) and Steiner (1999)for Argopecten and Pecten using the 18S rRNA gene.However, the number of Pectinid species included inthese studies was only seven, which limits the strengthof their conclusions.

Our results contrast with those of previous studies.We have found Aequipecten, Flexopecten and Argopec-ten at the base of the Pectinid phylogenetic trees(Figs. 1, 2). The clade joining Aequipecten opercularisand Flexopecten glaber was found by Barucca et al.(2004) as well. However, we note that these two taxaexhibit considerably longer branches than the remain-ing taxa, suggesting a fast evolutionary rate, and there-fore, the possibility that they clustered together as aresult of “long branch attraction”. The species of Argo-pecten fall into a separate, neighbor clade in both trees,although with low bootstrap support.

The “Aequipecten group” and the previously dis-cussed subfamily Palliolinae are cases in which thepresent data set shows a lack of power to resolve thephylogenetic position of a clade. There is a remarkabledecrease in bootstrap support as we move from the tipsto the deepest nodes of our trees. The values of thehomoplasy statistics of the MP trees (Fig. 1) suggestthat there is some degree of homoplasy in the data setthat impedes the reconstruction of the phylogeneticevents at the base of the Pectinidae with the 16S and12S gene fragments. Enlarging the size of the ribo-somal region sequenced, or sequencing regions ofhigher potential resolution, such as the COI gene(Matsumoto and Hayami 2000), seems necessary toovercome these problems, and resolve the branchingorder of the most basal pectinid clades.

123

118 Mar Biol (2006) 150:111–119

Acknowledgments We are extremely grateful to the followingpersons who provided scallop samples: Miguel Avendaño (Univ.Antofagasta, Chile); Claudia Bremec (INIDEP, Argentina); Lor-raine Hamilton (DFO, Canada); Mike Heasman (NSW Fisheries,Australia); Henry Kaspar (Cawthron Institute, New Zealand);Ellen Kenchington (DFO, Canada); César Lodeiros (Univ. Ori-ente, Venezuela); Alfonso Maeda (CIBNOR, México); SamuelPeña (Harvard Univ., USA); Jay Parsons (DFO, Canada); SamiaSarkis (BBSR, Bermuda); Nick Savva (Spring Bay Seafoods,Australia); Laura Schejter (INIDEP, Argentina); and James Wil-liams (Univ. Auckland, New Zealand). This research has beenWnanced with grants from the Spanish Ministry of Education andScience and the Generalitat Valenciana (Spain). C.S. enjoys a“Ramón y Cajal” contract funded by the Spanish Ministry ofEducation and Science.

Reference

Baco AR, Smith CR, Peek AS, Roderick GK, Vrijenhoek RC(1999) The phylogenetic relationships of whale-fall vesyco-mid clams based on mitochondrial COI DNA sequences.Mar Ecol Progr Ser 182:137–147

Barucca M, Olmo E, Schiaparelli S, Canapa A (2004) Molecularphylogeny of the family Pectinidae (Mollusca: Bivalvia)based on mitochondrial 16S and 12 S rRNA genes. Mol PhylEvol 31:89–95

Bernard FR (1986) Crassadoma gen. nov. for “Hinnites” gigan-teus (Gray, 1825) from the North-eastern PaciWc Ocean (Biv-alvia: Pectinidae). Venus 45:70–74

Beu AG (1985) Pleistocene Chlamys patagonica delicatula (Biv-alvia: Pectinidae) oV southeastern Tasmania, and history ofits species group in the Southern Ocean. Spec Publ S AustDept Mines Energy 5:1–11

Beu AG, Darragh TA (2001) Revision of southern AustralianCenozoic fossil Pectinidae (Mollusca:Bivalvia). Proc R SocVictoria 113:1–205

Canapa A, Barucca M, Caputo V, Marinelli A, Nisi Cerioni T,Olmo E (2000a) A molecular analysis of the systematics ofthree Antarctic bivalves. Int J Zool Suppl 1:127–132

Canapa A, Barucca V, Marinelli A, Olmo E (2000b) Moleculardata from the 16S rRNA gene for the phylogeny of Pectini-dae (Mollusca: Bivalvia). J Mol Evol 50:93–97

Campbell DC (2000) Molecular evidence on the evolution of theBivalvia. In: Harper EM, Taylor JD, Crame JA (eds) Theevolutionary biology of the bivalvia. Geological Society ofLondon Special Publication no. 177. The Geological Society,London, pp 31–46

Coan EV, Scott PV, Bernard FR (2000) Bivalve seashells of west-ern North America. Santa Barbara Museum of Natural His-tory, Santa Barbara

Distel DL (2000) Phylogenetic relationships among Mytilidae(Bivalvia): 18S rRNA data suggest convergence in mytilidbody plans. Mol Phyl Evol 15:25–33

Felsenstein J (1985) ConWdence limits on phylogenies: an ap-proach using the bootstrap. Evolution 39:783–791

Forcelli DO (2000) Moluscos Magallánicos. Vazquez Mazzini,Buenos Aires

Frischer ME, Williams J, Kenchington E (1998) A molecular phy-logeny of somemajor groups of Pectinidae inferred from 18SrRNA gene sequences. In: Jonston PA, Haggart JW (eds)Bivalves: and eon of evolution. University of Calgary Press,Calgary, pp 213–221

Giribet G, Carranza S (1999) What can 18S rDNA do for bivalvephylogeny? J Mol Evol 48:256–258

Grant US IV, Gale HR (1931) Catalogue of the marine Plioceneand Pleistocene Mollusca of California and adjacent regions.San Diego Soc Nat Hist Memoir 1:1–1036

Habe T (1977) Systematics of Mollusca in Japan: bivalvia andScaphopoda. Hokuryukan, Tokyo (in japanese)

Hall TA (1999) BioEdit: a user-friendly biological sequencealignment editor and analysis program for Windows 95/98/NT. Nucleic Acids Symp Ser 41:95–98

Harper EM (1991) The evolution of the cemented habit in thebivalved molluscs. Unpublished Ph.D. dissertation, Depart-ment of Earth Sciences, The Open Univesity, Milton Key-nes, UK

Hertlein LG (1969) Family Pectinidae RaWnesque, 1815. In:Moore RC (ed) Tratise on invertebrate paleontology, PartN, Vol. 1, Mollusca 6, Bivalvia. Geological Society of Amer-ica and Universtity of Kansas, Lawrence, Kansas. pp N348–N373

Keen AM (1971) Sea shells of tropical west America. StanfordUniversity Press, Stanford

Kenchington E, Roddick DL (1994) Molecular evolution withinthe phylum Mollusca with emphasis on the class Bivalvia. In:Bourne NF, Bunting BL, Townsend LD (eds) Proceedings ofthe 9th international Pectinid workshop, Nanaimo (Canada),Vol I. Canadian Technical Report of Fisheries and AquaticSciences pp 206–213

Korobkov IA (1960) Family Pectinidae Lamarck, 1801. In: OrlovUA (eds) Osnovy Paleontologii, Mollusca-Loricata, Bivalviaand Scaphopoda. Acedmy Nauk USSR, Moscow, pp 82–85

Levinton JS (2001) Marine biology: function, biodiversity, ecol-ogy, 2nd edn. Oxford University Press, New York

Lodeiros C, Marín B, Prieto A (1999) Catálogo de los moluscosmarinos de las costas nororientales de Venezuela: Clase Biv-alvia. Edición APUDONS, Cumaná

Matsumoto M, Hayami I (2000) Phylogenetic analysis of the fam-ily Pectinidae (Bivalvia) based on mitochondrial cytochromeC oxidase subunit I. J Moll Stud 66:477–488

Mikkelsen PM (2004) Western Atlantic Bivalves, ver. 1. Avail-able at http://www.peet.amnh.org/Western_Atlantic_bival-ves.html

Ó Foighil D, Taylor DJ (2000) Evolution of parental care andovulation behaviour in oysters. Mol Phyl Evol 15:301–313

Palumbi S (1996). Nucleic acids II: the polymerase chain reaction.In: Hillis DM, Moritz C, Mable BK (eds) Molecular system-atics. 2nd edn. Sinauer, Sunderland, pp 205–247

Peña JB (2003) Taxonomía, Morfología, distribución y hábitat delos pectínidos iberoamericanos. In: Maeda-Martínez AN(ed) Los moluscos pectínidos de Latinoamérica: ciencia yacuicultura. Limusa, México

Posada D, Buckley TR (2004) Model selection and model averag-ing in phylogenetics: advantages of Akaike information cri-terion and bayesian approaches over likelihood ratio tests.Syst Biol 53:793–808

Posada D, Crandall KA (1998) Modeltest: testing the model ofDNA substitution. Bioinformatics 14:817–818

Rice EL, Roddick D, Singh RK (1993) A comparison of mollus-can Bivalvia phylogenies based on palaeontological andmolecular data. Mol Mar Biol Biotechnol 2:137–146

Saavedra C, Peña JB (2004) Phylogenetic realtionships of com-mercial European and Australasian king scallops (Pectensp.) based on partial 16S ribosomal RNA gene sequences.Aquaculture 235:153–156

Saavedra C, Peña JB (2005) Nucleotide diversity and Pleistocenepopulation expansion in Atlantic and Mediterranean scal-lops (Pecten maximus and P. jacobaeus) as revealed by themitochondrial 16S ribosomal RNA gene. J Exp Mar BiolEcol 323:138–150

123

Mar Biol (2006) 150:111–119 119

Shumway SE (ed) (1991) Scallops: biology, ecology and aquacul-ture. Elsevier, Amsterdam

Smith JT (1991) Cenozoic giant pectinids fom California and theTertiary Caribbean province: Nodipecten, “Macrochlamys”,Vertipecten, and Nodipecten species. U. S. Dept. of the Inte-rior Gelogical ProVessional paper 139. Washington, Denver

Steiner G (1999) What can 18S rDNA do for bivalve phylogeny?Response. J Mol Evol 48:258–261

Steiner G, Müller M (1996) What can 18S rDNA do for bivalvephylogeny? J Mol Evol 43:58–70

Steiner G, Hammer S (2000) Molecular phylogeny of the Bivalviainferred from 18S rDNA sequences with particular referenceto the Pteriomorphia. In: Harper EM, Taylor JD, Crame JA(eds) The evolutionary biology of the bivalvia. GeologicalSociety of London Special Publication no. 177. The Geolog-ical Society, London. pp 11–29

SwoVord DL (1998) PAUP*. Phylogenetic analysis using parsi-mony (* and other methods) version 4. Sinauer, Sunderland

Tamura K, Nei M (1993) Estimation of the number of nucleotidesubstitutions in the control region of mitochondrial DNA inhumans and chimpanzees. Mol Biol Evol 10:512–526

Thiele J (1935) Handbuch der systematichen Weichtierkunde.Gustav Fischer, Jena

Thompson JD, Higgins DG, Gibson TJ (1994) CLUSTAL W:improving the sensitivity of progressive multiple sequence

alignment through sequence weighting, positions-speciWcgap penalties and weight matrix choice. Nucleic Acids Res22:4673–4680

Waller TR (1969) The evolution of the Argopecten gibbus stock(Mollusca: Bivalvia), with emphasis on the Tertiary andQuaternary species of eastern North America. Paleont SocMemoir 3. J Paleont 43(suppl 5):1–125

Waller TR (1972) The functional signiWcance of some shell micro-structures of the Pectinacea (Mollusca: Bivalvia). In: Inter-national geological congress, Montreal, 24th Session, Section7, Paleontology, pp 48–56

Waller TR (1986) A new genus and species of scallop (Bivalvia:Pectinidae) from oV Somalia, and the deWnition of a newtribe Decatopectinini. Nautilus 100:34–46

Waller TR (1991) Evolutionary relationships among commercialscallops (Mollusca: Bivalvia: Pectinidae). In: Shumway SE(eds) Scallops: biology, ecology and aquaculture. Elsevier,Amsterdam, pp 1–73

Waller TR (1993) The evolution of “Chlamys” (Mollusca: Bival-via: Pectinidae) in the tropical western Atlantic and easternPaciWc. Am Malacol Bull 10:195–249

Waller TR (2006) New phylogenies of the Pectinidae (Mollusca:Bivalvia): reconciling morphological and molecular ap-proaches. In: Shumway SE, Parsons J (eds) Scallops: biology,ecology and aquaculture II. Elsevier, Amsterdam, pp 1–44

123