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

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 <i>Ectoplana limuli</i> was attached to the ventral limbs of adult horseshoe crabs during culture. Parasite samples were collected from the external appendages and mouthparts of adult <i>Tachypleus tridentatus</i> 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 <i>Ectoplana limuli</i>. Sequence alignment and phylogenetic tree results indicated that <i>Ectoplana limuli</i> showed a closer genetic relationship with <i>Nerpa fistulata</i>, but more distant relationships with <i>Paucumara</i> and <i>Baikalobia</i>. This is the first time that this parasite has been found in China, providing additional information for the study of horseshoe crab diseases.