Density and adult ratio of the symbiotic harpacticoid copepod Idomene purpurocincta in the compound ascidian host Aplidium yamazii

Density and adult ratio of the symbiotic harpacticoid copepod, Idomene purpurocincta, living in the colony of the compound ascidian host, Aplidium yamazii, were investigated on the specimens taken from a boulder shore of Tosa Bay, western Japan, to know how these ecological parameters of the symbiont were related to the host size. Field collection of the host ascidian was carried out during the low tide in every spring tide from December 2004 to August 2006. All of the copepods isolated from the hosts using menthol were enumerated according to ontogenetic stages. The ascidian host colonies appeared from January to June 2005 and from December 2005 to August 2006. The symbiotic copepod was found in all of the hosts with few exceptions even at the initial appearance of the hosts in each season. These facts suggest that the copepod entered the host immediately after the colony formation. The density of the copepod in a host was usually 10E3 individuals per gram of host in dry weight. An exponential negative correlation was seen between the copepod density and the host size, and the density varied less as the host size increased. The adult ratio of the copepod also greatly varied in small hosts and tended to converge to about 25% with increase of the host size, and was negatively correlated with the copepod density. The reason for these host size-related copepod density and adult ratio may be explained by recruitment of nauplii in a host and migration of adults among hosts.