Abstract
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.