180 



Fishery Bulletin 91(1), 1993 



infestation appeared low, with 1-2 larvae observed per 

 cm 2 section of gonadal tissue. Encysted larvae appeared 

 surrounded by a connective tissue capsule of host ori- 

 gin and were located immediately subjacent to the wall 

 of the gut. Each larva measured -600 urn in length. 



The presence of these organisms was accompanied 

 by morphological changes in the intestinal wall tissue 

 of A. gibbus in the vicinity of the cyst. The wall of the 

 intestine normally consisted of a columnar ciliated epi- 

 thelium separated by a basement membrane from a 

 thin underlying layer of connective tissue (Fig. 2). De- 

 bris and sand particles were prominent features within 

 the lumen of the intestine. Hemocytes were present in 

 the sinus surrounding the intestinal loop, but were 

 not normally present in great numbers between the 

 gonadal acini at this stage of gametogenesis. 



In contrast, the intestinal epithelium of parasitized 

 scallops appeared to have undergone cellular change 

 in which the normal ciliated columnar epithelium had 

 been replaced by a stratified squamous epithelium, or 

 undifferentiated cells. The lumen of the intestine con- 

 tained sloughed epithelial cells and what appeared to 

 be cellular debris and intact hemocytes (Fig. 3). In 

 addition, hemocytes were abundant within the connec- 

 tive tissues of the gut wall and surrounding the acini 

 of the gonads, although the germinal epithelium ap- 

 peared identical to non-parasitized tissue and eggs did 

 not differ significantly in diameter. 



Differences in the gonadal dry weight indices were 

 also observed among parasitized and non-parasitized 





;ov;f- 



Q T • 

 ;  > 







I E 



r 



 



I L 



» K 



1 6V pnrij 



Figure 2 



Cross-section of intestinal loop through gonad of unparasitized 

 Argopecten gibbus. Note the ciliated columnar epithelium (IE). 

 Few macrophagous hemocytes are present in the hemolymph 

 sinus (S) surrounding the intestine, and the lumen (ID con- 

 tains only a slight amount of particulate matter. OV = ovar- 

 ian acini, C = cilia, CT = connective tissue. Mallory's trichrome 

 stain. 



NM 





h . vSti' its • '^^^; 



Sir. 



Figure 3 



Cross-section of intestinal loop through gonad of a parasit- 

 ized Argopecten gibbus. Note that the epithelium (IE) shows 

 a loss of integrity, and the cells are more squamous than 

 columnar. Ciliation has disappeared. The sinus surrounding 

 the intestine contains numerous macrophagous hemocytes 

 (M). The lumen (ID is filled with non-cellular debris (NM). 

 T = testicular acini. CT = connective tissue. Mallory's 

 trichrome stain. 



A. gibbus. The gonadal indices of non-parasitized A. 

 gibbus during late gametogenesis and spawning were 

 significantly higher (ANOVA, p=0.0001) than those of 

 parasitized individuals in the same gametogenic state 

 (Table 1). 



Discussion 



Past reports of larval Echeneibothrium hosts have been 

 limited to species ofVenerupis staminea, inhabiting the 

 west coast of North America (Sparks & Chew 1966). 

 Therefore, this report presents the first evidence for the 

 occurrence of this genus in a bivalve from the east coast 

 of North America, and the first indication of a scallop 

 host. It is presently unclear how Echeneibothrium has 



Table 1 



Tukey's Studentized Range Test of mean gonadal indices (GI) 

 among uninfected (January-April 19901 and infected (No- 

 vember 1990 and February 19911 Argopecten gibbus in the 

 same reproductive state. Means not significantly different 

 are underlined. 



Pairwise comparisons of GI 



Feb '90 Mar Jan Apr Feb '91 Nov 



Mean 

 SD 



21.15 

 ±6.10 



17.13 

 ±2.96 



15.93 

 ±3.05 



14.74 

 ±4.28 



9.93 6.11 

 ±1.97 ±2.08 



