The effect of peat moss particles on the physiology of the Eastern oyster, Crassostrea virginica

Journal of Shellfisheries Research, Jan, 2005 by Andre L. Mallet, Claire E. Carver, Jean-Yves Daigle

ABSTRACT This study was designed to evaluate whether the addition of peat moss particles to an algal diet influenced the physiology, biochemical composition, reproductive development, and survival of the Eastern oyster (Crassostrea virginica) under laboratory conditions. In two consecutive 5-wk trials (13[degrees]C to 15[degrees]C), 70-mm oysters were fed a mixed algal diet (25 [micro]g Chl [L.sup.-1] or 2.3 mg TPM [L.sup.-1]) supplemented with 3 concentrations of peat particles (2.7, 11.2, or 31.2 mg TPM [L.sup.-1]). The proportion of organic matter (POM:TPM) in the various diets increased from 79% for the algal control to 96% for the highest peat concentration. Following a 5-d acclimation period, oysters in the various treatments exhibited similar clearance rates calculated either on the basis of chlorophyll (2.3 L [h.sup.-1] [g.sup.-1]) or particle removal (2.8 L [h.sup.-1] [g.sup.-1]). Chlorophyll filtration rates were similar among treatments (63 [micro]g Chl [h.sup.-1] [g.sup.-1]), whereas particle filtration rates increased with peat concentration in the diet (9-58 mg TPM [h.sup.-1] [g.sup.-1]). Although pseudofeces production rates increased with peat concentration (2-30 mg TPM [h.sup.-1] [g.sup.-1]), estimates of particle ingestion rate also increased (7-28 mg TPM [h.sup.-1] [g.sup.-1]). Fecal production rates were similar for the various treatments (2 mg TPM [h.sup.-1] [g.sup.-1]) as were respiration rates (0.32 ml [O.sub.2] [h.sup.-1] [g.sup.-1]). Oysters sampled after 3-wk and 5-wk exposure to the four diet treatments exhibited no significant differences in biochemical composition, gonad maturation rate or survival. In general, this study suggested that the presence of peat particles in an excess algal diet had no impact on oyster performance. However, the results of these laboratory trials may not be directly applicable to oysters subjected to similar peat concentrations under field conditions.

KEY WORDS: biochemical composition, Crassostrea virginica, filtration, oyster, peat moss, respiration

INTRODUCTION

Atlantic Canadian oyster growers have repeatedly voiced concerns over the potential environmental impact of commercial peat moss harvesting; specifically, they argue that the influx of peat particles into the marine ecosystem may be detrimental to, or even cause significant mortality in local oyster populations (Lavoie 1995). During the industrial peat extraction process, particles are continually dispersed into the adjacent bays and inlets via wind activity and water drainage (Glooschenko 1990). The recent introduction of sedimentation ponds has served to reduce the influx of water-borne particles; according to government regulations, drainage waters may now contain no more than 25 mg TPM [L.sup.-1] (Thibault 1998). Particle size analyses of pond effluent indicate that peat particles range from >2 mm (approx. 35%) to <0.150 mm (3% to 8%) (Daigle, pets. obs.).

Few studies have focused on the potential impact of peat moss particles on the physiology and performance of marine organisms. A field study in Richibucto, New Brunswick concluded that although peat particles draining into the system had elevated levels of mercury, there was no evidence of bioaccumulation in the marine biota (Surette et al. 2002). A second study demonstrated that sand shrimp (Crangon septemspinosa) ingest peat fibers when they occur in association with food particles; however, peat alone is not perceived as an alternative food source (Ouellette et al. 2003). In a laboratory study, Strychar and MacDonald (1999) concluded that the presence of peat particles in the diet of the Eastern oyster (Crassostrea virginica) would negatively affect the scope for growth. Specifically, C. virginica exhibited a reduction in clearance rate with increasing peat concentration (1-20 mg TPM [L.sup.-1]), and an inability to absorb peat particles efficiently.

In general, bivalves are known for their ability to adapt their feeding strategy and their absorption efficiency to maximize their energy uptake under variable conditions (Newell et al. 1989, Willows 1992, Bayne 1993). Among the various bivalve species, oysters are particularly well adapted to tolerate the high turbidity levels (e.g., 100 mg TPM [L.sup-1]) often encountered in shallow inlets and estuaries (Loosanoff 1962, Newell & Jordan 1983, Berg & Newell 1986). Clearance rates for C. virginica are reputed to decline only at seston concentrations >25 mg [L.sup.-1] (Newell & Langdon 1996). This species is also known for its ability to selectively filter and retain those particles with the highest nutritional value (Newell & Jordan 1983). The purpose of the following study is to determine whether the introduction of peat particles into the diet of C. virginica would negatively affect their physiologic performance, biochemical composition, and reproductive development over a 5-wk period. It should be noted that the algal diet and temperature regimen were consistent with those normally used to induce reproductive maturation of this species.

MATERIALS AND METHODS

Experimental Design

Two consecutive 5-wk trials were undertaken at the Aquarium and Marine Center, Shippagan, New Brunswick, Canada, the first starting on January 14, 1995 and the second on February 18, 1995. Forty oysters (70-80 mm in shell height) were placed in each of four raceway tanks at the beginning of each trial (Fig. 1). Each group was supplied with the same mixed algal diet but a different concentration of peat particles ranging from 0 to ~30 mg [L.sup.-1]. The temperature in the raceways was maintained between 13[degrees]C and 15[degrees]C.

[FIGURE 1 OMITTED]

The mixed algal diet was injected via a metering pump into the main line that provided seawater to each of the four raceway tanks. The flow rate through each tank was set at 1.2 L [min.sup.-1] such that each group of oysters received the same volume of algae; estimates of algal cell concentration in the tanks varied from 65,000-100,000 cells m[L.sup.-1]. The diet was prepared daily and consisted of a minimum of three algal species; depending on availability, these included Isochrysis galbana (TISO), Monochrysis lutheri (MONO), Chaetoceros calcitrans (CCAL) and Chaetoceros mulleri (CHGRA).

Peat particles were obtained by grinding commercially available peat moss with a Micropul (sample mill) pulveriser and screening the particles through a 150-[micro]m mesh stainless steel sieve. Each day, 140 g of peat particles were stirred into 180 L of filtered seawater and maintained in suspension by vigorous air agitation. A double-headed peristaltic pump was used to supply the two lower peat particle concentrations (2 and 10 mg [L.sup.-1]), while a metering pump was used to supply the 30 mg [L.sup.-1] treatment. For convenience, the 4 diet treatments were labeled APO (algae 0 mg peat [L.sup.-1]), AP2 (algae 2 mg peat [L.sup.-1]), AP10 (algae 10 mg peat [L.sup.-1]), and AP30 (algae 30 mg peat [L.sup.-1]). The whole system was drained and cleaned daily such that peat particles and fecal material did not accumulate in the raceways.