Verification of the current monitoring method for the sea spider Nymphonella tapetis by microscopic observation of early larvae infecting the asari clam Ruditapes philippinarum

Infection of the asari clam Ruditapes philippinarum collected along the eastern coast of the Chita Peninsula, Mikawa Bay, central Japan with the sea spider Nymphonella tapetis were detected visually and fluorescent microscopically from April 2015 to March 2016. Visual detection (usual method), which has been applied for periodical monitoring of the sea spider infection in our laboratory, can determine overall infection of larval sea spider conveniently by naked eyes or through loupe. Fluorescent microscopic detection is defined accurately enable to determine minute early larvae of the sea spider using fluorescent staining for chitin. Variation patterns of prevalence of infection (number of infected clams/ number of observed clams) and mean abundance of infection (total number of observed larvae/ total number of infected clams) by the usual method changed nearly with those by the fluorescent method. The growth period of the parasites from initiation of infection to visual size was estimated to be about one month during spring to summer. Survival of the larvae from start of infection to visual size was estimated at least 30%. Usual method can be applied effectively as fluorescent method for the detection of infecting small larvae in epidemical investigation of the sea spider with natural asari clam.