The clam Eurhomalea lenticularis may be parasitized by digenean trematodes of the family Plagiorchidae, specifically in the gonads (parasitic castration). A quantitative histological analysis of the parasitized gonads demonstrated a significant decrease in gonadal area, in the size of individual acini, and in the numbers of differentiated germ cells compared to unparasitized clams. Castration may be caused by mechanical compression due to trematode sporocyst growth. However, the uniform loss of germ cells in areas without sporocysts suggests that a more generalized mechanism is responsible. We suggest that parasitic castration has a primary effect on the host's neuroendocrine and gametogenic systems that regulate gamete production.
KEY WORDS: Bivalve mollusc · Parasitic castration · Histological analysis · Eurhomalea lenticularis · Digenea · Trematode · Central Chile
Resale or republication not permitted without written consent of the publisherDis Aquat Org 59: [151][152][153][154][155][156][157][158] 2004 of alteration in the neuroendocrinological system of the clam, we quantified the variation between germ cell number in the presence and absence of sporocysts.
MATERIALS AND METHODSSample processing and histology. To determine the prevalence of the parasite, a total of 1702 clams was obtained from June 1995 to February 1998 by diving on a natural bank located at El Algarrobo inlet, central Chile (33°20' S, 71°40' W). They were transported live to the laboratory, and at least 1 random sample of 30 clams was processed each month (1034 in total; the remaining clams were dissected and observed under the stereoscopic microscope to determine the presence of the parasite) using routine histological techniques (Gabe 1968). After 1 h fixation in Bouin Hollande fluid to harden the tissues, serial transverse sections, 5 mm thick, were made of the visceral mass containing the gonad. After fixation in fresh fixative for 48 h, the thick sections were washed in tap water for 24 h, dehydrated in graded ascending concentrations of ethanol to 100%, cleared in butanol and embedded in Paraplast Plus (Oxford ® ). Histological sections of 5 µm thickness were made at different levels through the tissue at 300 µm intervals, deparaffinized, and rehydrated in a decreasing series of ethanol concentrations, and stained by a trichrome staining method. Briefly, the sections were stained in Harris hematoxylin solution for 75 s and rinsed in running tap water for 10 min, followed by a quick rinse in distilled water. Next, they were stained with a mixture of 0.5% Erythrosin-0.5% Orange G for 30 min, and quickly rinsed in distilled water. They were immersed in 0.5% phosphotungstic acid for 10 min, and again quickly rinsed in distilled water. Finally, they were stained in 1% Aniline blue for 75 s and immediately dehydrated in 3 consecutive baths of 95% ethanol followed by 3 baths in 100% ethanol (Arteta trichrome stain; López et al. 1982). After clearing in xylenes, the slides were coverslipped using Canada balsam (López et al. 1982...