Cryptic Divergence of Rochia nilotica (Gastropoda: Tegulidae) from Chuuk Lagoon, Federated States of Micronesia, Revealed by Morphological and Mitochondrial Genome Analyses

Abstract

Rochia nilotica is a tropical Pacific gastropod inhabiting shallow coral reef habitats and supporting important marine resources in Pacific island nations. In this study, we analyzed specimens collected from Weno Island, Chuuk Atoll, Federation States of Micronesia (FSM), using an integrative approach that combined morphological characteristics, molecular phylogenetics (COX1 and 16S rRNA), and complete mitochondrial genome analysis. While the Chuuk population exhibited morphological features consistent with R. nilotica, molecular data revealed substantial genetic divergence. Phylogenetic analyses based on the complete mitochondrial genome (17,664 bp) clustered the Chuuk specimen with Rochia virgata. Phylogenies inferred from concatenated COX1 and 16S rRNA gene sequences yielded congruent topologies, placing the Chuuk lineage within the Rochia clade but clearly separated from other R. nilotica populations in New Caledonia and Mo'orea Island, French Polynesia. This genetic divergence is likely driven by the long-term geographic isolation of Chuuk Atoll. The lagoon's fringing reefs descend rapidly into waters exceeding 4000 m, which may act as a barrier to restricting larval dispersal. Combined with the extremely short planktonic larval duration of R. nilotica (approximately four days), such environmental isolation may promote the formation of a distinct gene pool. Despite morphological uniformity, the observed genetic divergence suggests that the Chuuk population may represent a cryptic species. Our study provides a complete mitochondrial genome and offers robust phylogenetic framework that provides an understanding of species boundaries within Rochia. These findings underscore the importance of integrating genomic and morphological data for accurate species identification and have implications for conservation and sustainable aquaculture practices in geographically isolated reef ecosystems.

Keywords: Chuuk Atoll; Trochoidea; cryptic species; marine resource; phylogenetics.

Figure 1

(A) Map of the study area showing Weno Island within Chuuk Atoll. The red marker indicates the sampling site. (B,C) Specimen of Rochia nilotica inhabiting coral reefs near Weno Island.

Figure 2

Images of Rochia nilotica shells. Adult: (A) dorsal view with aperture visible; (B) ventral view without aperture. (C) apical view; (D) abapical view. Juvenile (1-year-old): (E) dorsal view; (F) ventral view; (G) apical view without aperture; (H) abapical view with aperture visible. Scale bar: (A,D) 5 cm; (E–H) 2 cm.

Figure 3

Radula morphology of Rochia nilotica from Chuuk Atoll. (A) Entire radula showing the rhipidoglossan arrangement. (B) Central region of the radula, including the central and lateral teeth. (C) Left marginal teeth in high magnification. (D) Right marginal teeth in high magnification. Scale bar: 200 μm.

Figure 4

(A) Mitochondrial genome map of Rochia nilotica inhabiting Chuuk Atoll. The innermost blue bar plot represents GC content. The middle purple circle shows sequencing read depth. The outermost circle depicts gene order: red for COX genes, blue for ATP synthase, purple for cytochrome b (CYTB), and green for ND genes. The two rRNAs are in apricot, and the 22 tRNAs are in gray. (B) Phylogenetic tree constructed using 13 protein-coding genes (PCGs) from the mitochondrial genome. Vertical lines next to the species names indicate their family affiliations. The synteny of the 13 mitochondrial PCGs is aligned in a row to the right of the phylogenetic tree. Posterior probabilities (left) of Bayesian inference (BI) and bootstrap supports (right) for the maximum likelihood (ML) are indicated on the branches. The scale bar indicates 0.50 substitutions per site. Bootstrap supports <50% and posterior probabilities <80% are not shown.

Figure 5

Phylogenetic trees of the superfamily Trochoidea based on concatenated COX1 and 16S rRNA sequences. Bayesian inference (BI) and maximum likelihood (ML) analyses were performed using the concatenated alignment of partial COX1 and 16S rRNA gene sequences. Specimens of Rochia nilotica from this study are indicated in bold. The best-fit substitution model (GTR + I + G) was applied, and node support values are shown as Bayesian posterior probabilities (0–1) and ML bootstrap values (0–100) based on 1000 replicates. GenBank accession numbers for all sequences used in the tree are listed in Table S1.