Parasitic castration of the green echinoid Strongylocentrotus droebachiensis by the nematode endoparasite Echinomermella matsi: Reduced reproductive potential and reproductive death

The echinoid Strongylocentrotus droebachiensis in the Vestfjorden area of northern. Norway is infected by the epizootic nematode endoparasite Echinomermella matsi. The incidence of E. matsi was 68.8% in a sample taken from Godoystraumen, and 16.4% in a sample from Vaeroy Island. The high-incidence sample was collected in late February prior to the annual spawning season, when the echinoids have sexually mature gonads, and the low-incidence sample was collected in late August, when the echinoids have sexually immature gonads. Infected echinoids in both samples had smaller gonads than non-infected echinoids. The parasite-related reduction in gonad size for adult echinoids greater than or equal to 35 mm in test diameter was approximately 75% in the high-incidence sample, and approximately 63% in the low-incidence sample. Most S. droebachiensis in the high-incidence sample were infected before they reached a size of 20 mm test diameter. The infected individuals had parasite loads ranging from approximately 0.01 to > 5 g. However, lightly infected individuals with loads < 0.1 g were most common, comprising 52% of the infected individuals in the high-incidence sample, and 72% of the infected individuals in the low-incidence sample. The abundance of heavily infected echinoids with parasite loads greater than or equal to 0.1 g peaked in the [30, 35 > mm size group, but dropped abruptly in the next interval of the size-frequency distribution. Although the peak. abundance of heavily infected echinoids coincided with the onset of sexual maturity for non-infected echinoids, the majority of the infected echinoids in this size group were still classified as virtual castrates of unknown sex. These results suggest that a large proportion of infected echinoids never reproduce, but are effectively castrated as juveniles and succumb to parasite-related mortality before they reach a size of 35 to 40 mm.