of brackish water and is inhibited by an increased 

 salinity. In cases of heavy infestation, the 

 gonads and digestive diverticula are almost 

 completely replaced by cercariae and by the long 

 germ tubes of the sporocysts, which after their 

 liberation infest Menidia, other small fishes, and 

 lylosurus marinus. Destruction of the gonad is 

 the most obvious pathological effect caused by 

 B-ucephalus. So far this trematode has not been 

 suspected of causing mortalities in oyster 

 populations. 



The parasitic copepod Mytilicola intestinalis is 

 common among mussels of the Mediterranean. 

 Another species, M. orientals, infests C. gigas and 

 Mytilus crassitesta of the Inland Sea of Japan. 

 The parasitic copepod is found in the intestinal 

 tract of bivalves and is easily recognized by its 

 red color and relatively large size which makes it 

 visible to the naked eye. In the United States 

 Mytilicola orientalis is widespread in lower Puget 

 Sound, occurring in 0. lurida and C. gigas, Mytilus 

 edulis, Paphia staminea, and Crepidula jornicata. 

 Infection is heaviest in the common mussels, 

 often reaching 100 percent in some areas (Odlaug, 

 1946). A single specimen of Mytilicola intestinalis 

 was found by Pearse and Wharton (1938) in C. 

 virginica on the Gulf coast of Florida. The 

 presence of M. orientalis in 0. lurida in the lower 

 Puget Sound area interferes with their fatness, 

 but apparently inflicts no serious injuries to 

 oyster stocks. In C. gigas the copepod produces 

 metaplastic changes in the gut, completely de- 

 stroys the cihated epithehum, and penetrates the 

 underlying connective tissue (Sparks, 1962). 



The presence of parasites in adult oysters 

 makes them unmarketable for esthetic reasons 

 and, therefore, detracts from the commercial 

 productivity of oyster bottoms. 



Any disease factor, regardless of the identity of 

 the pathogen, can be evaluated by determining 

 the percentage of the infected oysters, the intensity 

 of infection, the loss caused by the mortalities, 

 and the decrease in yield of marketable oysters. 



COMMENSALS AND COMPETITORS 



The shell and body of the oyster are the natural 

 abodes for many plants and sedentary animals 

 which attach themselves to the shell surface or 

 bore through it to make for themselves a well- 

 protected residence; some settle on the soft body 

 without penetrating its tissues while others invade 

 the inner organs. The difference between the 



420 



commensals, i.e., organisms which share the food 



gathered by the host, and the parasites, which 



live at the expense of their hosts and sometimes 



inflict serious injuries, is not very sharp. Some 



commensals may cause injury to the host and 



become parasites. 



Competitors are those organisms which hve in 



close pro.ximity to each other and struggle for the 



space and food available in the habitat. Some 



appear to be innocuous while others by virtue of 



their habits and high reproductive capabilities are 



harmful. 



Boring Sponges 



Small round holes on the surface of moUusk 

 shells indicate the presence of the most common 

 animal associated with the oyster, the boring 

 sponge. There are seven species of the genus 

 Cliona along the Atlantic Coast of the United 

 States. In a case of heax'y infestation the shell 

 becomes brittle, breaks under slight pressure, and 

 reveals conspicuous tunnels and cavities filled with 

 yellow sponge tissue. Microscopic examination 

 shows a typical sponge structure with numerous 

 siliceous spicules from 150 to 250 m long, of the type 

 called tylostyles, and small skeletal elements of 

 different shapes and sizes known as microscleres. 

 Species identification is based on the type of 

 cavities or galleries made by the sponge and the 

 shape and sizes of the spicules (Old, 1941). Small 

 fragments of shell material at the holes by Cliona 

 may suggest mechanical action of the sponge. 

 Warburton (1958) found experimentally that 

 sponge cells in contact with a surface of calcite 

 form a reticulum of fine pseudopodia and filaments. 

 A corresponding pattern of lines is etched into the 

 mineral, and the marked areas are of the same size 

 and shape as the fragments discharged by the 

 sponge. Apparently the cytoplasmic filaments 

 penetrate the calcite by secretion of minute 

 amounts of acid and undercut fragments which 

 are carried out by excurrent canals of the sponge. 

 It is not known whether boring sponges use the 

 organic component (conchiolin) of the shell, but 

 it is obvious that they do not draw their nutrients 

 from the body of the oyster. The sponge touches 

 the surface of the body only in cases of old, heavy 

 infestation. In such instances the holes made by 

 the sponge are rapidly covered by a deposition of 

 conchiolin. The holes made by the sponge are 

 clearly visible on the mner surface of the valve 

 under a newly deposited layer of conchiolin (fig. 

 379) . The race between the sponge and the oyster 



FISH AND WILDLIFE SERVICE 



