The role of the Sunda shelf biogeographic barrier in the cryptic differentiation of Conus litteratus (Gastropoda: Conidae) across the Indo-Pacific region

Introduction

The Arabian Sea in the Indian Ocean harbors a wide range of marine ecosystems and its endemic biodiversity is strongly linked to many unique ecological and biogeographical features (Laxmilatha et al., 2021). Studies on biogeographical factors reveal the discontinuities in species distribution and genetic diversity of species. Major barriers which restrict larval dispersion and gene flow between oceanic macro ecosystems have previously been identified (Ekman, 1953; Briggs & Bowen, 2012). The mode of larval development is the road map of evolutionary studies in the marine realm and plays an important role in connectivity (Palumbi, 1994; Servedio & Noor, 2003; Beger et al., 2014; Tiego et al., 2017).

The west coast of India is one of the most productive seas in the Indian Ocean, with strong seasonal upwelling resulting in high levels of primary productivity (Kumar et al., 2000) that shapes unique marine biodiversity in the eastern Arabian Sea, including the Lakshadweep archipelago (Ameri et al., 2022). The genetic structure of marine benthic invertebrate populations and their larval dispersal are known to be greatly influenced by oceanographic and geographical processes (Palumbi, 1994; Cunha et al., 2005; Whittaker & Fernández-Palacios, 2007). Several mitochondrial-based studies have revealed the role that biogeography and barriers have played in the dispersal and divergence of coral reef fish lineages (Fessler & Westneat, 2007). Recent phylogeographic studies revealed the existence of cryptic differentiation within South Pacific coral reef fish species due to the influence of soft biogeographic barriers (Drew & Barber, 2012). Due to a biogeographic break located at 30°S in the southeast Pacific, several marine benthic invertebrate taxa exhibit strong genetic differentiation in this area. This indicates that biogeographic barriers often limit gene flow, promoting divergence and speciation (Haye et al., 2014, 2019). Genetic diversity in Chilean barnacles also reflects the role of a biogeographic transition zone (Zakas et al., 2009). The Sunda shelf barrier (SSB), is a shallow continental shelf among Indonesian islands that restricts exchange between the tropical Indian Ocean and the western Pacific during low sea level stands (Ekman, 1953; Randall, 1998). Significant genetic structure due to the influence of the SSB has been detected in pelagic coral reef fishes (Bay et al., 2004; Bowen et al., 2016).

Cone snails have separate sexes, and produce eggs that typically adhere to a hard surface and undergo internal fertilization (Kohn, 1959). The type of larval development has a significant impact on the dispersal abilities of these venomous marine snails (Cunha et al., 2005). Larvae exhibiting nonplanktonic lecithotrophic developmental mode have no pelagic phase, while planktotrophic larvae may remain in the water column for large periods (Jablonski, 1986). The present study aims to analyze the genetic diversity of Conus litteratus Linnaeus, 1758 (Gastropoda, Conidae) from the Lakshadweep archipelagos in the Indian Ocean.

Materials and Methods

Sample collection and morphological analyses

Five specimens were collected from the lagoon of Agatti, Andrott and Kalpeni Islands of Lakshadweep archipelago, Arabian Sea (Fig. 1) by snorkeling after obtaining field permit from the Lakshadweep administration. Tissue samples from the foot muscle were preserved in 95% ethanol. Taxonomy based on the following morphological characters of the shell (color, standard length (SL); shell width (SW); aperture height (AH); relative spire height (RSH = (SL − AH)/SL); position of the maximum diameter (PMD) = HMD/AH) followed Röckel, Korn & Kohn (1995).

Results

Morphological analyses

The morphological variation within the collected C. litteratus samples (Fig. 2) was the following: standard length from 51 to 59 mm; shell width from 31 to 32 mm; aperture height from 50 to 58 mm; relative diameter from 0.53 to 0.62; relative spire height of 0.01; position of the maximum diameter from 0.82 to 0.86 mm.

Phylogenetic analyses

Analyses of the partial COI sequences revealed two well-differentiated clades within C. litteratus, one including specimens of the Arabian Sea and the other with samples from the Pacific Ocean (Fig. 3). The partial 16S rRNA gene sequences yielded an identical phylogenetic tree (BS 98) (Fig. S1). The dating analysis reveals that C. litteratus of Arabian Sea specimens diverged at 2.14 MYA (Fig. 4).

Population genetic analyses

The intraspecific K2P genetic distance between the Arabian Sea and the Pacific Ocean C. litteratus specimens ranged from 7.4% to 7.6% and from 2.4% to 2.8% based on COI (Table 2) and 16S rRNA (Table 3), respectively. The AMOVA based on COI sequences shows high genetic diversity within C. litteratus (ϕ_ST = 0.99409; P < 0.001) and revealed 46 segregating and 39 segregating sites differentiating Lakshadweep and Pacific populations, respectively (Fig. 5). The AMOVA based on 16S rRNA sequences (ϕ_ST 0.98703; P < 0.001), shows 13 and 11 segregating sites differentiating Lakshadweep and Pacific populations, respectively (Fig. S2).

Table 2:

Kimura 2 parameter (K2P) genetic distances based on COI.

Conus_litteratus_LD008.1
Conus_litteratus_LD008.2	0.000
Conus_litteratus_LD008.3	0.002	0.002
Conus_litteratus_LD008.4	0.002	0.002	0.000
Conus_litteratus_LD008.5	0.000	0.000	0.002	0.002
KJ550338_Conus_litteratus	0.074	0.074	0.072	0.072	0.074
KJ549941_Conus_litteratus	0.076	0.076	0.074	0.074	0.076	0.005
KJ550337_Conus_litteratus	0.078	0.078	0.076	0.076	0.078	0.007	0.005
KJ550336_Conus_litteratus	0.078	0.078	0.076	0.076	0.078	0.007	0.002	0.007
MG786077_Conus_eburneus	0.151	0.151	0.149	0.149	0.151	0.158	0.160	0.167	0.158
MG786074_Conus_eburneus	0.151	0.151	0.149	0.149	0.151	0.158	0.156	0.162	0.158	0.022
KJ550217_Conus_eburneus	0.151	0.151	0.149	0.149	0.151	0.151	0.149	0.156	0.151	0.028	0.019
MG786107_Conus_tessulatus	0.149	0.149	0.151	0.151	0.149	0.147	0.145	0.147	0.147	0.087	0.077	0.075
MG786106_Conus_tessulatus	0.151	0.151	0.153	0.153	0.151	0.149	0.147	0.149	0.149	0.085	0.075	0.073	0.005

DOI: 10.7717/peerj.15534/table-2

Table 3:

Kimura 2 parameter (K2P) genetic distances based on 16s rRNA.

Conus_litteratus_LD008.1
Conus_litteratus_LD008.2	0.000
Conus_litteratus_LD008.3	0.000	0.000
Conus_litteratus_LD008.4	0.000	0.000	0.000
Conus_litteratus_LD008.5	0.000	0.000	0.000	0.000
EU794329_Conus_litteratus	0.024	0.024	0.024	0.024	0.024
KJ550867_Conus_litteratus	0.028	0.028	0.028	0.028	0.028	0.004
HM212495_Conus_litteratus	0.024	0.024	0.024	0.024	0.024	0.000	0.004
KJ550652_Conus_litteratus	0.024	0.024	0.024	0.024	0.024	0.000	0.004	0.000
KJ550825_Conus_eburneus	0.068	0.068	0.068	0.068	0.068	0.061	0.061	0.061	0.061
KJ550607_Conus_eburneus	0.071	0.071	0.071	0.071	0.071	0.064	0.064	0.064	0.064	0.002
KJ550608_Conus_eburneus	0.068	0.068	0.068	0.068	0.068	0.061	0.061	0.061	0.061	0.004	0.006
KJ550735_Conus_tessulatus	0.078	0.078	0.078	0.078	0.078	0.068	0.071	0.068	0.068	0.026	0.024	0.028

DOI: 10.7717/peerj.15534/table-3

Discussion

An especially interesting land barrier, the Sunda Shelf Barrier (SSB) between the Indian and Pacific Oceans shapes unique marine biodiversity in tropical Indian and Pacific Ocean regions (Randall, 1998). This Indo-Pacific barrier formed during glacial periods when the sea level drops expose a complete land wall between the Indian and western Pacific Oceans (Ekman, 1953; Randall, 1998). Recent phylogeographic studies have revealed cryptic diversity within South Pacific coral reef fishes promoted by soft biogeographic barriers (Fessler & Westneat, 2007; Drew & Barber, 2012). Bullethead parrotfish and seahorses from the Indo-Pacific region show significant genetic differences among locations separated by the Indo-Pacific SSB (Bay et al., 2004; Lourie & Vincent, 2004). The present study provides new insights into the role of the SSB in the divergence of a gastropod species, C. litteratus. Our analysis showed two distinct lineages of C. literatus, one from the Pacific and the other in the Arabian Sea. The two lineages did not exhibit any significant differences at the morphological level. The intraspecific genetic distance of C. litteratus between the Arabian Sea and Pacific Ocean lineages is larger than the threshold limit for different Conidae species (>6% in COI and >2% in 16S rRNA) (Puillandre et al., 2014). There is an increasing number of molecular-based studies showing the existence of cryptic diversity, in the absence of diagnostic morphological characters, particularly in gastropods (Duda et al., 2008; Puillandre et al., 2014).

The dispersal range and rate of speciation of cone snails may be significantly influenced by the different developmental modes that may range from planktotrophy (pelagic larval stage) to lecithotrophy (absence of pelagic larval stage), particularly in the archipelagic ecosystem (Kohn & Perron, 1994). For instance, in the Cape Verde archipelago all endemic species of Conus exhibit a nonplanktonic lecithotrophic developmental mode (Kohn & Perron, 1994; Cunha et al., 2005), which limits gene flow between populations because of the reduced capacity of the larvae to disperse. It is not possible from our data to determine whether the ancestors of C. litteratus of the Lakshadweep archipelago had planktotrophic larval development or non-planktonic lecithotrophy. However, insights from previous studies in cone snails confirm that most isolated oceanic islands show high levels of endemism promoted by nonplanktonic lecithotrophy (Kohn & Perron, 1994; Cunha et al., 2005, 2008).

Our dating analysis revealed that the two lineages of C. litteratus diverged during the Pleistocene at 2.14 MYA (Fig. 5). Earlier research indicated that the isolation of marginal ocean basins during the Pleistocene low sea-level stands may have promoted speciation within the Indo-West Pacific islands (McManus, 1985; Potts, 1985). The exchange of water between the west Pacific and Indian Oceans was averted during the Pleistocene due to the formation of a biogeographic barrier, the SSB, when the land area was maximal (Voris, 2000; Bird, Taylor & Hunt, 2005). The role of other biogeographic barriers in lineages’ divergence has been described in the Isthmus of Panama where the exchange of water between the tropical Atlantic and Pacific Oceans ceased about 3 million years ago promoting diversification of the existing fauna (Leigh, O’Dea & Vermeij, 2013). The broad distribution of C. litteratus throughout the Indian and Pacific Oceans makes this species an excellent model organism for studying patterns of species diversification. Our phylogenetic and phylogeographic analyses detected high levels of genetic differentiation between the two-ocean basis most likely due to the influence of SSB on C. litteratus dispersal during Pleistocene low sea level stands.

Conclusion

The present study provides evidence for the existence of cryptic diversification within C. litteratus from the Lakshadweep archipelago. Molecular sequence analyses revealed the existence of two well-differentiated clades in specimens of C. litteratus from the Indian and Pacific Oceans. The intraspecific genetic distance between specimens of C. litteratus from the two ocean basins is larger than the threshold limit for interspecific differentiation, nonetheless further analyses based on nuclear markers are needed to confirm the taxonomic status of the species. Dating analyses revealed that the divergence of the two lineages occurred during the Pleistocene and was promoted by the SSB oceanographic barrier, when the land area was maximal.

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