THE EFFECT OF THE ECTOPARASITIC PYRAMIDELLID SNAIL, 



BOONEA IMPRESSA, ON THE GROWTH AND HEALTH OF 



OYSTERS, CRASSOSTREA VIRGINICA, UNDER FIELD CONDITIONS 



Elizabeth A. Wilson,^ Eric N. Powell/ and Sammy M. Ray^ 



ABSTRACT 



Boonea (= Odostomia) impressa are contagiously distributed on oyster reefs so that some oysters are 

 parasitized more than others. The parasite's mobihty and the abihty of oysters to recover from snail 

 parasitism may be important in assessing the impact of parasitism on oyster populations. During a 4-week 

 exposure period in the field, B. impressa reduced American oyster, Crassostrea virginica, growth rate 

 and increased the intensity of infection by the protozoan, Perkinsus ( = Dermocystidium) marinus, but 

 produced few changes in the oyster's biochemical composition because, although net productivity was 

 reduced, the oysters retained a net positive energy balance (assimilation > respiration). During a 4-week 

 recovery period, growth rate returned to normal (control) levels, but infection by P. ynarinv^ continued 

 to intensify in previously parasitized oysters kept B. impressa-tree. Most changes in biochemical com- 

 position during recovery, including increased lipid and glycogen contents, could be attributed to the con- 

 tinuing increase in infection intensity of P. marinus. Consequently, the temporal stability and size of 

 snail patches, particularly as they regulate infection by P. marinus, may be the most important factors 

 influencing the impact of B. impressa on oyster reefs. 



Parasitism can be an important factor affecting the 

 population dynamics (Wickham 1986; Brown and 

 Brown 1986; Rabat 1986) and health (Brockelman 

 1978; Mohamed and Ishak 1981; Ford 1986) of host 

 species. Three parasites are known to be especially 

 important in oysters. Perkinsus (= Dermocysti- 

 dium) marinus, Haplosporidium nelsoni (MSX), and 

 Boonea ( = Odostomia) impressa detrimentally af- 

 fect oyster growth, health, and biochemical composi- 

 tion (Mengebier and Wood 1969; Feng et al. 1970; 

 Soniat and Koenig 1982; White et al. 1984; Ford 

 1986; Ward and Langdon 1986; White et al. 1988, 

 in press). 



The pyramidellid gastropod, Boonea impressa, is 

 one of a widely distributed group of parasitic, marine 

 opisthobranchs (Fretter and Graham 1949; Fretter 

 1951; Allen 1958). Boonea impressa removes nutri- 

 ents directly from its host by piercing the flesh with 

 a hollow stylet and sucking the body fluids using a 

 buccal pump (Fretter and Graham 1949; Fretter 

 1951; Allen 1958). The most common host of 5. im- 

 pressa is Crassostrea virginica (Hopkins 1956; Allen 

 1958; Wells 1959) but, like other odostomians, it is 

 not entirely host specific (Wells 1959; Robertson 

 1978; Robertson and Mau-Lastovicka 1979). Found 



'Department of Oceanography, Texas A&M University, College 

 Station, TX 77843. 



^Department of Marine Biology, Texas A&M University at 

 Galveston, Galveston, TX 77550. 



Manuscript accepted March 1988. 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. 



abundantly on oyster reefs from Massachusetts to 

 the Gulf of Mexico, B. impressa has been reported 

 in numbers as high as 100 per oyster (Hopkins 1956). 



Under laboratory conditions, B. impressa reduced 

 oyster growth rates as the result of both direct 

 removal of assimilated carbon from the oyster and 

 direct interference with the oyster's ability to feed 

 (White et al. 1984, 1988; Ward and Langdon 1986). 

 Not surprisingly, parasitism by B. impressa produc- 

 ed changes in the biochemical composition of oyster 

 tissue. Parasitism by 15 snails, a relatively high field 

 density found in dense snail patches decreased car- 

 bohydrate and free amino acid content and increased 

 lipid content of mantle tissue. Reproduction was af- 

 fected as well (White et al. in press). 



Perkinsus marinus is an important cause of mor- 

 tality of oysters in the Gulf of Mexico (Mackin 1962; 

 Ray 1966a; Hofstetter 1977). Sublethal effects in- 

 clude reduced growth (Menzel and Hopkins 1955; 

 Ray et al. 1953) and changes in biochemical com- 

 position (Soniat and Koenig 1982; White et al. in 

 press). Perkinsus marinus can be transmitted from 

 one oyster to another through the water (Ray 1954; 

 Mackin 1962; Andrews 1965) or by B. impressa 

 feeding (White et al. 1987). The intensity of infec- 

 tion by P. marinus is also increased in B. impressa- 

 parasitized oysters (White et al. 1987). 



Usually stress affects organisms by altering nor- 

 mal metabolic activity. Many organisms may be able 



553 



