# Rediscovery and morpho-molecular characterization of three astome ciliates, with new insights into eco-evolutionary associations of astomes with their annelid hosts

## TL;DR

This study rediscovered and analyzed three astome ciliates, revealing their evolutionary history and long-term association with annelid hosts.

## Contribution

The paper provides new morphological and molecular insights into astome ciliates and their eco-evolutionary relationships with annelids.

## Key findings

- Mouthless ciliates from annelids form a monophyletic group within Oligohymenophorea.
- Astomes originated in marine polychaetes during the Paleozoic era.
- Multiple independent transfers of astomes occurred from endogeic to other earthworm lifestyles.

## Abstract

Astome ciliates live in the digestive tract of a broad spectrum of marine, freshwater, and terricolous annelids. In aquatic lumbriculid and criodrilid oligochaetes collected in Central Europe, we rediscovered three insufficiently known astomes: Hoplitophrya secans, Mesnilella clavata, and Buchneriella criodrili. Their morphology was studied using in vivo observation, protargol, and dry silver nitrate impregnation. Multiple nuclear and mitochondrial molecular markers were used to determine their phylogenetic positions and reconstruct their evolutionary history. According to our phylogenetic analyses: (1) mouthless ciliates isolated from annelids form a robustly supported monophylum within the class Oligohymenophorea, (2) the progenitor of astomes invaded the digestive tract of marine polychaetes during the Paleozoic era, (3) lumbricid earthworms likely served as a source of astomes for criodrilid, almid, and megascolecid earthworms, (4) the ancestral host of the earthworm-dwelling astome clade led an endogeic lifestyle, and (5) there were multiple independent transfers of astomes from endogeic to epigeic and anecic earthworms. These findings support previous views of the annelid phylogeny, suggesting that astomes reside and evolve in tandem with annelids for several hundred million years.

The online version contains supplementary material available at 10.1007/s42995-024-00275-5.