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

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 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 alpha diversity suggested the unique character of certain geographical areas, such as the Bass Strait (Australia). beta 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.