# Morphological and Molecular Evidence Revealed New Species in Mactra antiquata Sensu Lato

## TL;DR

A new clam species, Mactra haiboensis, was discovered in Chinese coastal waters using shell and genetic data, revealing hidden biodiversity.

## Contribution

The discovery of a new cryptic species within Mactra antiquata sensu lato using combined morphological and molecular evidence.

## Key findings

- Two distinct groups (N-group and S-group) were identified based on shell morphology and genetic data.
- The S-group was classified as a new species, Mactra haiboensis sp. nov., with clear morphological and genetic differentiation.
- Genetic distances between the two groups exceeded intraspecific thresholds, confirming their separation.

## Abstract

This study aimed to clarify the taxonomic status of Mactra antiquata sensu lato, which has long been controversial due to morphological and molecular variations. Samples newly collected from four Chinese coastal provinces were analyzed using integrated morphological and molecular methods. Results showed two distinct groups (N-group and S-group) with significant differences in shell ratios, pallial sinus orientation, and escutcheon shape. Molecular analyses (phylogeny, haplotype networks, genetic distances, ASAP, and ABGD) confirmed obvious genetic differentiation exceeding intraspecific thresholds. The S-group was identified as a new species, Mactra haiboensis sp. nov., distributed in the East and South China Seas. This study provides diagnostic criteria for the new species and lays a foundation for its targeted conservation and further research.

Mactra antiquata sensu lato, a commercially important clam species in China, exhibits remarkable morphological and molecular diversity, which has led to the proposal of cryptic species within this complex. In the present study, specimens of M. antiquata sensu lato were collected from four coastal provinces (Shandong, Guangdong, Guangxi, and Hainan) of China, and an integrated comparative analysis was performed based on morphological traits and partial sequences of two mitochondrial genes (cytochrome c oxidase subunit I, COI; and 16S rRNA). Our results revealed that M. antiquata sensu lato could be clearly delineated into two distinct clades: the N-group (comprising specimens collected from Shandong in this study) and the S-group (including specimens collected from Guangdong, Guangxi, and Hainan), with significant intergroup differences. Morphologically, S-group individuals possessed relatively narrower shells (mean shell width-to-length ratio = 0.465) and shorter shells (mean shell height-to-length ratio = 0.781) compared to N-group conspecifics. Additionally, the pallial sinus of S-group clams extended directly toward the anterior adductor muscle, whereas that of N-group clams pointed to the region below the anterior adductor muscle. Furthermore, the escutcheon of N-group individuals was considerably more slender than that of the S-group. Phylogenetic trees and haplotype networks constructed based on both partial COI and 16S rRNA sequences further confirmed a deep genetic divergence between the two groups, with Kimura 2-parameter distances of 0.158 for COI and 0.084 for 16S rRNA. Collectively, these morphological and molecular lines of evidence strongly support the existence of cryptic species within M. antiquata sensu lato. By comparing the morphological characteristics of specimens in this study with the original description of M. antiquata, we herein propose that the S-group represents a new species, which we named M. haiboensis sp. nov. Our findings provide a scientific basis for the targeted conservation and further research of both M. antiquata and M. haiboensis sp. nov.