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Revista de biología marina y oceanografía

versión On-line ISSN 0718-1957

Rev. biol. mar. oceanogr. vol.49 no.2 Valparaíso ago. 2014

http://dx.doi.org/10.4067/S0718-19572014000200003 

 

ARTÍCULO

Calibrating the chitons (Mollusca: Polyplacophora) molecular clock with the mitochondrial DNA cytochrome C oxidase I gene

Calibrando un reloj molecular para quitones (Mollusca: Polyplacophora) utilizando el gen mitocondrial que codifica para el citocromo oxidasa I

 

Cedar I. García-Ríos1, Nivette M. Pérez-Pérez1, Joanne Fernández-López1 and Francisco A. Fuentes1

1Departamento de Biología, Universidad de Puerto Rico en Humacao, Avenida José E. Aguiar Aramburu, Carretera 908 km 1,2, Humacao, Puerto Rico. cedar.uprh@gmail.com


RESUMEN

Especies hermanas separadas por el Istmo de Panamá se han utilizado para estimar tasas de evolución molecular. Un reloj molecular calibrado con un evento geológico bien estudiado puede ser de utilidad para proponer hipótesis sobre la historia evolutiva. El Istmo de Panamá emergió completamente hace 3,1 millones de años (m.a.), pero se ha evidenciado que comunidades asociadas a costas rocosas y arrecifes quedaron aisladas en fechas más tempranas (5,1-10,1 m.a.). Se determinó la divergencia de segmentos del gen mitocondrial citocromo oxidasa I (COI) para 2 pares de especies hermanas de quitones. El par Stenoplax purpurascens del Caribe y S. limaciformis del Pacífico oriental tropical, son especies muy similares fenotípicamente. Las especies del segundo par, Acanthochitona rhodea del sur del Caribe y A. ferreirai de las costas pacíficas de Panamá y Costa Rica, fueron tratadas como la misma especie hasta la reciente descripción de A. ferreirai. Utilizando ejemplares recolectados en ambas costas de Costa Rica se compararon secuencias homólogas del gen COI. Para el par de especies del género Stenoplax se determinó una divergencia total de 11,4% pares de bases (bp). Para el par de especies del género Acanthochitona, se encontró una divergencia de 10,4-10,5% bp. Estos valores de divergencia del COI permiten calcular razones de divergencia de 3,7% / m.a. (aislación hace 3,1 m.a.) hasta 1,1% / m.a. (aislación temprana). Esta es la primera calibración del gen COI como reloj molecular para los poliplacóforos.

Palabras clave: Istmo de Panamá, reloj molecular, COI, Stenoplax, Acanthochitona


ABSTRACT

Sister species separated by the Isthmus of Panama have been used to estimate the rate of molecular evolution. A molecular clock calibrated with a well studied geological event can be useful to propose evolutionary history hypotheses. The Isthmus of Panama emerged completely 3.1 million years ago (mya), but fossil evidence has shown that rocky shore and reef mollusks communities were isolated early (5.1-10.1 mya). The divergence of segments of the mitochondrial gene cytochrome oxidase I (COI) was determined for 2 pairs of sister species of chitons. The pair Stenoplax purpurascens of the Caribbean and S. limaciformis of the tropical Eastern Pacific consists of phenotypically similar species. The species of the second pair, Acanthochitona rhodea from the south of the Caribbean and A. ferreirai of the Panama and Costa Rica Pacific coast, were treated as the same species until the recent description of A. ferreirai. Homologous fragments of the COI gene were compared using specimens collected on both coasts of Costa Rica. For the Stenoplax genus pair, a total divergence of 11.4% base pairs (bp) was determined. The Acanthochitona pair showed a divergence of 10.4-10.5% bp. These values allow calculated divergence rates from 3.7% / mya (3.1 mya isolation) to 1.1% / mya (early isolation). This is the first calibration of the COI as a molecular clock for the polyplacophora.

Key words: Isthmus of Panama, molecular clock, COI, Stenoplax, Acanthochitona


INTRODUCTION

Molecular clocks are useful for determining the time during which phylogenetic events occur. A molecular divergence analysis, used to calibrate a molecular clock, requires the knowledge of 2 facts: first, the time at which species is genetically isolated to give rise to 2 sister species and secondly, the certainty that the sister species used for the calibration were isolated at that time, and not previously, through other speciation processes.

The Isthmus of Panama is a recent barrier that separated multiple populations of marine organisms, in what is today the Caribbean and tropical eastern Pacific (Knowlton & Weigt 1998). It was assumed that the last moment of genetic exchange between populations of eastern tropical Pacific and Caribbean populations corresponds with the onset of the final closure of the Isthmus of Panama, which geological evidence situate at 3.1 million years ago (mya). The assumption that the sister species were isolated at the time when the exchange of seawater stopped completely (3.1 mya), was the hypothesis used by many researchers (Bermingham & Lessios 1993, Schubart et al. 1998, Hellberg & Vacquier 1999, McCartney et al. 2000, Haye et al. 2002, Collin 2003, Lessios et al. 2003, Morrison et al. 2004, Williams & Reid 2004, Duda & Kohn 2005, Duda & Rolán 2005). However, other researchers propose dates prior to this one (Cronin & Dowsett 1996, Knowlton & Weigt 1998, Marko 2002) arguing that because the lifting of the isthmus was gradual and the early stages of the water flow constriction could be barriers for some taxa, among them the deep-water fishes and the invertebrates intolerant to silt in their respiratory or filter structures (Coates & Obando 1996, Collins 1996). Several researchers provide evidence suggesting that the sister species associated with reef habitat diverged earlier, as compared to the mangrove habitat species (Knowlton et al. 1993, Knowlton & Weigt 1998, Lessios 2008, Miura et al. 2010). Considering gastropods (Tegula geminate pair) and bivalves (arcid) with fossil record for the calibration, Marko (2002) suggests that the divergence time for sister pairs of mollusks associated with rocky shore occurred between 5.1 to 10.1 mya.

The cytochrome oxidase I (COI) gene, can be used as a molecular chronometer, which is an appropriate tool to estimate the time at which gene flow was interrupted between 2 populations, given that the vicariant event occurred within a few million years (Knowlton & Weigt 1998). Nuclear protein coding gene Histone H3 suffers substitutions less frequently, so it's not appropriate for the analysis of divergence for time periods of less than 30 mya (Marko 2002). The same applies to other nuclear ribosomal genes (18S rRNA, 28S rRNA) used for phylogenetic analyses (Clabaut et al. 2005). Thus, the rapidly evolving mitochondrial COI gene is robust for the interpretation of evolutionary relationships within genera.

Among mollusks, the percentage of divergence of COI gene across the Panamic Isthmus has been studied in gastropods sister species (Hellberg 1998, Collin 2003, Latiolais et al. 2006, Miura et al. 2010), also among bivalves (Marko 2002, Lee & Foighil 2005). However, COI has not been used yet to specifically compare sister species in chitons.

According to Mayr (1969), sister species are pairs or groups of closely related species that are reproductively isolated, but are morphologically identical or nearly identical. Prior to biochemical comparisons, the criterion for proposing that a pair of chiton species were likely sister species was the demonstration of strong morphological similarity (Ferreira 1978, Bullock 1985, Lyons 1998). The putative sister species of chitons, Stenoplax purpurascens (Adams, 1845) (Caribbean) and S. limaciformis (Sowerby, 1832) (Pacific Ocean), are so similar in general form, size, shape and proportions of the valves, tegmentum sculpture and colors, shape and size of girdle scales that until recently they were classified as the same species (Ferreira 1978, Kaas & Van Belle 1980) (Fig. 1). Bullock (1985) documented subtle differences in the sculpture of the valves, the morphology of the radula, and in esthetes pore densities, which justified treating S. purpurascens as distinct from the older species named, S. limaciformis.

 

 

Figure 1. Dorsal view. A) Stenoplax limaciformis (Sowerby, 1832), CBUPRH #2870; B) Stenoplax purpurascens (CB Adams, 1845), CBUPRH# 2867

Figura 1. Vista dorsal. A) Stenoplax limaciformis (Sowerby, 1832), CBUPRH #2870; B) Stenoplax purpurascens (CB Adams, 1845), CBUPRH# 2867

 

In a taxonomic revision of the polyplacophora of the Caribbean region, Kaas (1972) recognized that Acanthochitona hemphilli and A. rhodea as similar species in size (4-5 cm), coloration, 18 tufs with around 50 large spicules in the girdle and not rounded tegmental pustules. Acanthochitona hemphilli (Pilsbry, 1893) with a North Caribbean distribution and A. rhodea (Pilsbry, 1893) in the South (Kaas 1972). Both are so similar that Watters (1981) declared A. rhodea to the synonymy of A. hemphilli. Ferreira (1985) proposed A. hemphilli as a synonym of A. rhodea (invert the synonymy). Both researchers did not recognize morphological differences that would justify their classification as different species. A. rhodea was also present in the eastern tropical Pacific (Smith, 1961, Keen 1971). Ferreira (1985) considered the species complex as one, with distribution from Florida to Brazil in the Western Atlantic, and from Mexico to Peru in the Pacific. Lyons (1988) using scanning electron microscope (SEM) analysis of the valves and girdle elements, sort out their consistent morphological differences of the two Caribbean species and describe a third species: A. ferreirai (Lyons, 1988) as the Pacific member of the complex (Fig. 2). Lyons (1988) expressed the hypothesis of greater closeness between the A. rhodea and A. ferreirai pair.

 

 

Figure 2. Dorsal view. A) Acanthochitona ferreirai (Lyons, 1988), CBUPRH# 3148; B) Acanthochitona rhodea (Pilsbry, 1893), CBUPRH# 2694; C) Acanthochitona hemphilli (Pilsbry, 1893), CBUPRH# 2500

Figura 2. Vista dorsal. A) Acanthochitona ferreirai (Lyons, 1988), CBUPRH# 3148; B) Acanthochitona rhodea (Pilsbry, 1893), CBUPRH# 2694; C) Acanthochitona hemphilli (Pilsbry, 1893), CBUPRH# 2500

 

The objective of this study was to use partial COI sequences of two pairs of chitons sister species to determine how many base substitutions have accumulated after the Panamic Isthmus separated them. With the rate of divergence of these two pairs, this gene can be calibrated as a molecular clock. Knowledge of this divergence ratio will allow us to estimate divergence times for other chitons species.

 

MATERIALS AND METHODS

Specimen's collection

The specimens used to compare the sister species were collected in Costa Rica. Specimens of the Pacific coast were obtained in Samará Bay (9°52'29"N, 85°31'57.24"W), Guanacaste, in April 4, 2009. The specimens from the Caribbean coast were collected in Punta Puerto Vargas (9°44'40''N, 82°48'32''W), Cahuita National Park, Limón, in April 9, 2009. The description of the collection stations and other associated species appear in Jörger et al. (2008), for the Pacific Coast, and in García-Ríos & Alvarez-Ruiz (2011), for the Caribbean coast. To evaluate the intraspecific variability of the gene, we included samples obtained in Puerto Rico and Colombia. Other species of the 2 compared genera collected from Bonaire, Puerto Rico and Florida were also included. Sequences from 3 Pacific species of the genus Stenoplax, deposited in the GenBank by other authors, were used to corroborate the close genetic affinities between putative sister species (Table 1).

For the evaluation of the genetic similarity of the acanthochitonids we included other 6 species of the family: Acanthochitona balesae Abbot, 1954; A. hemphilli (Pilsbry, 1893), A. lineata Lyons, 1988; A. pygmaea (Reeve, 1847); Choneplax lata (Guilding, 1829) and Cryptoconchus floridanus (Dall, 1889). The specimens collected for this study were relaxed with ice, tied flat on a slide and preserved by total immersion in 95% ethanol. Species were identified according to Bullock (1985) and Lyons (1988). The specimens for each DNA sequence were deposited in the Colección Biológica de la Universidad de Puerto Rico en Humacao (CBUPRH). The collection numbers, location and the access number of to the sequence in GenBank are shown in Table 1. More collection data is available in the GenBank specimen information field.

 

Molecular data collection

DNA extraction was performed using the QIAGEN DNeasy Blood & Tissue Kit. From each specimen about 10 to 15 mg of foot muscles were extracted. The DNA extracted was diluted in sterile water (200 µl) and kept at 4ºC. An average of 700 ng µl-1 of DNA was obtained from each specimen.

The polymerase chain reaction (PCR) was performed using the extracted DNA and the COI universal primers: LCO 1490 (5'-GGTCAACAAATCATAAAGATATTGG-3') and HCOI 2198 (5'-TAAACTTCAGGGTGACCA AAAAATCA-3') (Folmer et al. 1994). PCR reactions (25 µl) included 0.5 µl of the template DNA, 0.5 µl of each primer (5 pmol µl-1), 2 µl of dNTP blend (2.5 mM each), 5 µl PCR buffer (5X) (Promega), 2.5 µl of 2.5 mM MgCl2 (Promega), and 0.25 µl of Taq DNA polymerase (5U µl-1) (Promega). The PCR reactions were carried out using an Applied Biosystems thermal cycler model 2720. Thermal cycling conditions involved an initial denaturation of 5 min at 95°C, followed by 35 cycles including denaturation at 95°C for 30 s, annealing ranging from 45 to 50°C for 30 s, and extension at 72°C for 1 min, with a final extension step at 72°C for 1 min.

The PCR products were purified by precipitation with polyethylene glycol (PEG) precipitation. DNA sequencing was performed with a multicapillary sequencer (16) ABI PRISM 3130XL (Applied Biosystems) at the Sequencing and Genotyping Facility (SGF) of the University of Puerto Rico. The sequencing cycling conditions were: 96°C for 5 min, followed by 45 cycles of 96°C for 10 s, 50°C for 5 s, 60°C for 4 min, and indefinite hold at 4°C.

 

Editing and analysis of the nucleotide sequences

Consensus sequences were determined by aligning at least two sequences from opposite directions, using the program BioEdit 7.0.9.0 (Hall 1999). All the substitutions were corroborated using the chromatograms. The resulting sequences were aligned using ClustalW 1.4 (Thompson et al. 1994), included in the program BioEdit. Alignments were corrected manually and ambiguously aligned regions were not used in the analysis. Sequence comparisons between pairs were performed using the program MEGA4 (Tamura et al. 2007). For the determination of distance between species, the percent of difference was determined using all the available bases in equal length sequences. This was the basis for subsequent phylogenetic analyzes.

DNA saturation analysis was determined graphically by plotting the transition rate / transversion rate ratio as a function of percentage divergence (Roe & Sperling 2007). Phylogenetic analyzes and the resulting trees were prepared using the Neighbor-Joining method (Saitou & Nei 1987) with bootstrap replications (1000 replicates) (Felsenstein 1985), as implemented in MEGA4.

 

Editing and analysis of the amino acid sequences

The program MEGA4 was used to determine the amino acid sequences resulting from the translation of the gene fragments, using the invertebrate mitochondrial genetic code.

 

RESULTS

The sequences of the COI gene among the sister species Stenoplax limaciformis and S. purpurascens showed 102 variable positions in the pairwise array of 642 aligned sites (about 16%). The total number of variable positions in the pairwise array of 642 aligned sites was 102 (about 16%). The comparison includes 3 specimens of S. purpurascens collected from three separated localities within the Caribbean: Costa Rica, Colombia and Puerto Rico. The minimum differences between the specimens of Puerto Rico and the ones from Colombia and Costa Rica are 6.2 and 6.7%, respectively. Among the specimens from Colombia and Costa Rica there is a divergence of 2.2%. The closest genetic similarity of S. limaciformis was with the specimens of S. purpurascens from Costa Rica, 11.4% divergence (0.013 SE) (Table 2, Fig. 3).

 

Figure 3. Phylogenetic tree for populations of S. purpurascens from the Caribbean coast of Costa Rica (CR), Colombia (COL), and Puerto Rico (PR), and S. limaciformis from the Pacific Coast of Costa Rica; based on 642 bp of the COI gene (initial position in the 1516 bp), using nucleotide percent distance. The evolutionary relationship was inferred using the Neighbor-Joining method. Values at nodes represent the percentage of 1000 bootstrap replicates. The scale bar represent the number of substitutions per nucleotide position. Tree is rooted by Chiton tuberculatus. CR= Costa Rica, COL= Colombia, PR= Puerto Rico

Figura 3. Árbol filogenético de las poblaciones de S. purpurascens de la costa caribeña de Costa Rica (CR), Colombia (COL), Puerto Rico (PR) y S. limaciformis de la costa del Océano Pacífico de Costa Rica; basado en el porcentaje de distancia de 642 bp del gen mitocondrial COI (iniciando en la base 1516). La relación evolutiva se infiere usando el método vecino más cercano. El porcentaje de réplicas de las asociaciones entre taxones, a partir de un análisis de bootstrap de 1000, aparece cercano a las ramas. Chiton tuberculatus se utiliza como raíz. CR= Costa Rica, COL= Colombia, PR= Puerto Rico

 

The comparison of 6 species of the genus Stenoplax, 4 from the Pacific and 2 from the Caribbean, showed 162 variable positions in a 515 bp matrix (31.5%). The lowest values of divergence between different species (11%) correspond to S. purpurascens from Costa Rica and S limaciformis (Table 3). On average, the base thymine is more abundant, with 36.7%, followed by 26.8% adenine, guanine with 20.3%, and cytosine with 16.2%. A phylogenetic tree was inferred from the data on Table 3 (Fig. 4).

 

Figure 4. Phylogenetic tree of Pacific and Caribbean Stenoplax spp. based on 515 bp of the COI gene (initial position in the 1574 bp), using nucleotide percent distance. The evolutionary relationship was inferred using the Neighbor-Joining method. Values at nodes represent the percentage of 1000 bootstrap replicates. The scale bar represent the number of substitutions per nucleotide position. Tree is rooted by Chiton tuberculatus. COL= Colombia, CR= Costa Rica, PR= Puerto Rico, FL= Florida

Figura 4. Árbol filogenético de poblaciones de Stenoplax spp. del Pacífico y Caribe, basado en el porcentaje de distancia de 515 bp del gen mitocondrial COI (iniciando en la base 1574). La relación evolutiva se infiere usando el método vecino más cercano. El porcentaje de réplicas de las asociaciones entre taxones, a partir de un análisis de
bootstrap de 1000, aparece cercano a las ramas. Chiton tuberculatus se utiliza como raíz. COL= Colombia, CR= Costa Rica, PR= Puerto Rico, FL= Florida

 

The graph of percent genetic distance against the ratio of transitions: transversions, made with COI sequences of all Stenoplax specimens considered here suggests poor saturation before reaching 14% of divergence (Fig. 5).

 

Figure 5. COI gene saturation analysis using percent distance versus transition/transversion ratio of Stenoplax spp.

Figura 5. Análisis de saturación del gen COI, utilizando el porcentaje de distancia versus la razón de transición/transversión de Stenoplax spp.

 

An analysis of the amino acid sequence encoded by the compared gene fragments revealed that from a total of 171 amino acids, there were five variations among the 6 species compared. No differences were found in the pair S. purpurascens and S. limaciformis (all substitutions are silent). On the other hand, S. floridana differs by one amino acid (position 158 of our sequence) of the previous mentioned pair. The substitution of isoleucine (position 158) to valine in S. floridana (northern Caribbean species) was shared by S. alata (Indonesia) and S. mariposa (Eastern Pacific). Stenoplax conspicua differed from the pair of putative sister species in one amino acid (position 134). The amino acid at that position, phenylalanine, was shared by the other two species of the Pacific, but not with S. limaciformis (Table 4).

The comparison of the sequence divergence of 551 base pairs of COI in 8 species of 3 genus of the family Acanthochitonidae, shown in Fig. 6. The total number of variable positions in the comparison matrix was also large, 162 (29.9%). On average, the base thymine is more abundant, with 42.3%, followed by guanine (23.0%), adenine (20.5%), and cytosine with 14.2%.

 

Figure 6. Phylogenetic tree for selected Acanthochitonidae species from the Caribbean and A. ferreirai from the Pacific coast of Costa Rica; based on 551 bp of the COI gene (initial position in the 1562 bp), using nucleotide percent distance. Values at nodes represent the percentage of 1000 bootstrap replicates. The scale bar represent the number of substitutions per nucleotide position. Tree is rooted by Chiton tuberculatus. CR= Costa Rica, BON= Bonaire, PR= Puerto Rico

Figura 6. Árbol filogenético de especies caribeñas de la familia Acanthochitonidae y A. ferreirai de la costa del Pacífico de Costa Rica, basado en el porcentaje de distancia de 551 bp del gen mitocondrial COI (iniciando en la base 1574). La relación evolutiva se infiere usando el método vecino más cercano. El porcentaje de réplicas de las asociaciones entre taxones, a partir de un análisis de bootstrap de 1000, aparece cercano a las ramas. Chiton tuberculatus se utiliza como raíz. CR= Costa Rica, BON= Bonaire, PR= Puerto Rico

 

The acanthochitonids COI sequences compared encoded a total of 183 amino acids. There were only 2 variations in the amino acid sequence among the 9 species compared. Crytoconchus floridanus differs by one amino acid (position 162 of our sequence), Acanthochitona pygmaea and A. balesae differs in the 143 amino acid. The other 6 species showed identical amino acid sequence fragments (silent nucleotide substitutions) (Table 5).

A comparison of divergence of the 590 pb COI sequence in 8 specimens of the 3 morphologically and genetically similar Acanthochitona species was done. Three of the specimens compared belong to the species A. ferreirai (Pacific coast of Costa Rica), 4 specimens of A. hemphilli (Pilsbry 1893), 3 from Puerto Rico and one of Bonaire, and a specimen of A. rhodea from the Caribbean coast of Costa Rica. Sequence comparisons between the 3 species of Acanthochitona, revealed that the smallest distance found between A. rhodea (Caribbean Costa Rica) was with the specimens of A. ferreirai of the Costa Rica Pacific coast (10.5 to 10.4%, 0.002 SE). While the distances to specimens of A. hemphilli of Bonaire and Puerto Rico were 12.4 and 13.2-13.6%, respectively. Among the specimens of A. hemphilli of Bonaire and Puerto Rico we found a distance of 4.8-5.1% (Table 6 and Fig. 6).

 

DISCUSSION

The phylogenetic comparison between species of the genus Stenoplax (Fig. 4) and the comparison of the 171 amino acids sequence (Table 4) is consistent with the hypothesis that S. purpurascens and S. limaciformis are sister's species. Equally, the comparison of the sister species Acanthochitona hemphilli, A. rhodea and A. ferreirai (Fig. 6) identifies the species A. rhodea (Southern Caribbean) and A. ferreirai (Tropical Eastern Pacific) to be genetically closer (Table 6). This supports the hypothesis expressed by Lyons (1988: 86) of greater closeness between this pair.

The graph of genetic percent distance against the transition/transversion ratio, shows a saturation of about 14% of divergence (Fig. 5). This is similar to the 11% reported for red algae (Sherwood et al. 2010) and much less than 20% (K2p distance) for species of shrimps of the genus Macrobrachium (Zhang et al. 2009). This limits the use of the COI gene as a molecular clock in chitons to divergences of less than 14%, margin that allows the calibration of the sister species compared in this study.

Using as separation time of the populations 3.1 mya, based on the minimum estimated time that the Caribbean and Pacific faunas are thought to have been isolated by the Panamic Isthmus, then this assumed vicariance date would lead to a calculated divergence ratio of 3.7% / mya in Stenoplax (Table 2) and 3.4% / mya in Acanthochitona (Table 6). These values correspond to 4.1 K2p in Stenoplax and 3.6 K2p in Acanthochitona (Kimura 1980 method, as implemented by MEGA4).

Divergence values of 3.4 to 3.7% bp / mya calculated for these chiton species are of similar magnitude to those reported for shrimps of the genus Penaeus (3%, Baldwin et al. 1998), Diadema echinoderms (2.9 to 4.2%, Lessios et al. 2001) Tripneustes, Diadema and Meoma (3.5%, Lessios et al. 2003) and higher than the 2.6% calculated for the gastropod Echinolittorina (Williams & Reid 2004) and 1.8 to 2.2% in Eucidaris and Echinometra urchins (Bermingham & Lessios 1993). The heterogeneous molecular evolutionary rates among taxa are well documented (e.g., Kumar 2005, Ward 2009, Morrison et al. 2004). The differences in generation time of each species could explain some of those differences. However, some of these divergence rates have been qualified as overestimates (Marko 2002), because not all marine organisms might be able to keep genetic connectivity through a shallower seaway, where conditions of high sedimentation characteristic of mangroves existed prior to the end of the closing (Knowlton & Weigt 1998).

The use of fossil record data for the calibration of molecular clocks in transisthmian sister species, has shown that times of separation for the mollusks associated with reefs and rocky coasts took place between 5.1-10.1 mya (Marko 2002). If we assume that the chitons populations were also isolated with the interruption of environments with hard-bottom and low sedimentation rates, the adjusted calibration for Stenoplax will be 2.2 to 1.1% / mya and 2.0 to 1.0% for Acanthochitona. These values of divergence ratio are similar to the 1.2% calculated for clams in the family Arcidae (Marko 2002) and 2.4% for gastropods of the genus Tegula (Hellberg & Vacquier 1999). Both estimates belong to mollusks that inhabit rocky bottoms, as the chitons. For multiple pairs of shrimps in the genus Alpheus a rate of 1.4% / mya was inferred by Knowlton & Weigt (1998).

The genetic distances between individuals of the same species in different localities of the Caribbean showed values up to 8.0% in S. purpurascens and 4.9% in A. hemphilli. The specimens compared are not the most distant species within the genus; therefore higher intraspecific differences can be expected (Tables 2 and 4). These divergence values exceed the distance used to define some species (Avise 2000). Distances greater than those are found among many chitons species, according to Kelly & Eernisse (2008). These divergence values suggest a reduced gene flow between the populations compared here, and support the early separation hypothesis.

Although rafting on floating objects as adults is possible, it is more likely that most of the dispersion depends on distances traveled during the pelagic larval stages, for at least free spawning species. Because chiton larvae are lecithotrophic (i.e., non-feeding), even those from free spawned gametes probably have a relatively short pelagic stage. Overall, the pelagic phase of the larval stage can last from hours to more than a week (Strathmann & Eernisse 1987). In culture, Mopalia spp. larvae were observed to settle in 5 to 10 days (Leise 1984, Eernisse 1988). It is assumed that the development of larvae in tropical waters is faster, which should further reduce the population connectivity (Kelly & Eernisse 2007). Reported larval settling time for Chiton tuberculatus is 3 days, and 2 days for Acanthopleura granulata (Lewis 1960). Bullock (1985) postulated that the pattern of distribution of chiton species in the Caribbean suggests that larvae must only remain in the plankton for a short time. Heath (1899 in Bullock 1985) reported that larvae of Stenoplax heathiana complete their free-swimming stage within 15 min to 3 h after hatching, but this species does not free spawn gametes, as most common chitons do. Instead, the female lays sticky benthic egg masses, and larvae develop within these, only emerging at a late larval stage already capable of remaining benthic without any pelagic stage. A short pelagic stage could be consistent with the high values of divergence between populations that we found, but reproduction has not been studied in any of these species. In order to calibrate molecular clocks with sister species, specimens should be collected on the coasts of the Isthmus, thus to avoid the error introduced by the variability of the gene within the geographic distribution of the species.

Using the phylogenetic trees for Stenoplax species (Fig. 3) and Acanthochitonidae family species (Fig. 6) we can establish that the distances between sister's species are very small. The morphological similarity between these species is not the product of convergence evolution of distantly related species. However, the low values of the bootstraps test for higher clades do not allow inferences on the evolutionary relationship among all the species represented. Such analysis would require the inclusion of other taxonomic gene and a more complete representation of these genera.

The rise of the Isthmus of Panama altered the chemical and physical conditions of the oceans that were separated (Glynn 1972). This event detoured the North Atlantic Equatorial Current to the north, intensifying the flow of currents in the Caribbean and the Gulf of Mexico (Berggren & Hollister 1974, Lessios 2008), creating warmer and saltier conditions throughout the Caribbean and western Atlantic (Jackson et al. 1993). The change created a favorable scenario for the northward movement of tropical marine species. For species with limited dispersal and movement, such as chitons, the closure of the isthmus should have promoted colonization and speciation processes. As this is a recent geologic event, we expect that these speciation processes are not always reflected in distinctive morphological adaptations. The use of the COI gene can aid to identify possible cryptic species, and our calibration can estimate divergence times for these new species.

 

ACKNOWLEDGMENTS

The Research Fund of the University of Puerto Rico at Humacao, allowed the first author to complete this manuscript. We appreciate the guidance and recommendations about the sampling places in Costa Rica to Dr. Ingo S. Wehrtmann, Universidad de Costa Rica. We acknowledge the assistance of Migdalia Álvarez-Ruiz (Universidad de Puerto Rico at Ponce) with the field work in Costa Rica. We appreciate the comments on early versions of the manuscript provided by Douglas Eernisse, of the University of California, Santa Cruz and Héctor L. Ayala del Río, of the Universidad de Puerto Rico at Humacao. The sequencing facilities of UPR-RP, which are supported by NCRR Grant # P20 RR16470 AABRE, NIH-SCORE Grant # S06GM08102, Universidad de Puerto Rico at Río Piedras, Biology Department, NSF-CREST Grant # 0206200, NINDS U54 NS39405-SNRP. We appreciate the funding provided, for material and financial aid to students who participated in this research, by the programs `Minority Access to Research Careers' (MARC) and `Research Initiative for Scientific Enhancement' (RISE), NIH Grant # 5R25GM07534803, both from UPR-Humacao.

 

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Received 19 June 2013 and accepted 1 April 2014
Associate Editor: Gabriela Muñoz C.