# Amplicon sequencing reveals the cryptic diversity in the dicyemid parasites of coleoid cephalopods sampled from the Atlantic and Pacific Oceans

## TL;DR

Using sequencing, researchers found more dicyemid parasite diversity in cephalopods than previously known, revealing hidden species and complex host-parasite relationships.

## Contribution

This study reveals cryptic diversity in dicyemid parasites using amplicon sequencing, uncovering more genetic types than traditional taxonomy.

## Key findings

- Identified 95 genetic types of dicyemids, far exceeding morphology-based records.
- Found 46 dicyemid types in the common cuttlefish, contrasting with prior reports of only four species.
- Eight new cephalopod hosts in the Pacific were identified, and community diversity varied by geography and host.

## Abstract

Dicyemids (phylum Dicyemida), primarily found in the renal organs of coleoid cephalopods, are a unique group of morphologically simple parasites with global distribution. Here, we investigated the diversity and prevalence of dicyemid communities in a wide range of cephalopod hosts across four geographic zones (the North East Atlantic, Mediterranean Sea, China Sea in the Western North Pacific, and Australia in the South Pacific) using Illumina sequencing of the 18S rDNA amplicons. Across 227 host samples, we identified 482 amplicon sequence variants, which clustered into 95 genetic types. The results indicated a higher number of distinct genetic types within Dicyemida than those currently identified through morphology-based taxonomy. Our finding of 46 dicyemid types in the common cuttlefish (Sepia officinalis) contrasts sharply with the previous records of a maximum of four species in this host. Furthermore, only a few host species exhibited a single dicyemid type, while most harbored multiple types; several types were distributed worldwide. Additionally, we identified eight new cephalopod hosts in the Pacific. Analyses of community (α) diversity suggested the unique character of certain geographical areas, such as the Bass Strait (Australia). β-diversity analyses confirmed that geographic location and host species were significant determinants of the dicyemid community composition. These results suggest that current species classifications may underestimate the true diversity of dicyemids. They emphasize the intricate interplay between geography, host specificity, and dicyemid community diversity.

The online version contains supplementary material available at 10.1007/s42995-026-00353-w.