# Ectoplana limuli, a Parasite of the Horseshoe Crab (Tachypleus tridentatus): A New Record in China

## TL;DR

This paper reports the first discovery of the parasite Ectoplana limuli in China, attached to horseshoe crabs, which may contribute to their high mortality during molting.

## Contribution

The first report of Ectoplana limuli in China, using both morphological and molecular methods for identification.

## Key findings

- Ectoplana limuli was identified on adult Tachypleus tridentatus in China using 18S rDNA analysis.
- The parasite showed over 99% genetic homology with Ectoplana limuli and closer relations to Nerpa fistulata.
- This finding provides insights into disease-related mortality in cultured horseshoe crabs.

## Abstract

Disease can contribute to the mortality of the horseshoe crab during molting. We discovered an ectoparasite on the body surface and mouthparts of adult Tachypleus tridentatus. To identify the species and its taxonomic status, we conducted a study that combined morphological methods with 18S rDNA molecular markers. The results showed that the parasite is highly homologous to Ectoplana limuli. This study firstly reports this parasite in China, providing important insights for disease control and contributing to the conservation of T. tridentatus aquaculture.

The mortality rate of first- to second-instar horseshoe crabs during molting is extremely high under culture conditions (pH of 7.6 ± 0.1, salinity of 27 ± 2, temperature of 26–32 °C), and we preliminarily speculate that it is related to disease. Our team found that Ectoplana limuli was attached to the ventral limbs of adult horseshoe crabs during culture. Parasite samples were collected from the external appendages and mouthparts of adult Tachypleus tridentatus for classification and identification. The primary objective of this experiment was to identify the species of this parasite and determine its taxonomic status. To this end, the experiment employed a combination of morphological methods and 18S rDNA gene molecular markers. The obtained sequences showed over 99% homology with Ectoplana limuli. Sequence alignment and phylogenetic tree results indicated that Ectoplana limuli showed a closer genetic relationship with Nerpa fistulata, but more distant relationships with Paucumara and Baikalobia. This is the first time that this parasite has been found in China, providing additional information for the study of horseshoe crab diseases.