Evaluating the use of exhalent siphon area in estimating feeding activity of blue mussels, Mytilus edulis.(Report)

ABSTRACT We evaluated the technique of measuring the exhalant siphon area (ESA) as an indicator of feeding activity in the blue mussel, Mytilus edulis. To accomplish this, we established the relationships between ESA measured using video and image analysis and clearance rate (CR) measured simultaneously in mussels exposed to various concentrations of microalgae as a food source in the laboratory. Two size classes of mussels (30 and 60 mm shell length) were fed 6 and 7 concentrations of the unicellular alga Isochrysis galbana., respectively (0, 0.6, 1.2, 1.8, 2.4, 3.0, 6.0 mg [L.sup.-1]). ESA and the variation in feeding activity associated with individual mussels significantly influenced CR in 60mm mussels, whereas effects of algal concentration were not significant within the ran[TM] tested. Individual variation in feeding activity significantly influenced CR in 30 mm mussels. However, unlike the 60 mm mussels, ESA did not significantly influence CR, whereas algal concentration did have a significant effect in 30 mm mussels. We did observe significant relationships between ESA and CR in some groups of mussels suggesting it may be a useful indicator of feeding activity under certain circumstances. However, the high degree of variation observed in our laboratory-based experiments on the relationship between CR and ESA leads us to conclude that measurements of ESA may be better used as an estimate of general behavior trends in feeding rather than a quantitative measure of clearance rate.

KEY WORDS: exhalent siphon area, Mytilus edulis, clearance rate, feeding activity, individual variability

INTRODUCTION

The bivalve, Mytilus edulis, is an ecologically and economically valuable species distributed throughout the world's cool temperate oceans (Seed & Suchanek 1992). Research has often focused on the feeding activity of this bivalve to understand its requirements for culture, its effects on the surrounding environment and its function as an indicator of ecosystem health (Seed & Suchanek 1992). Mussel feeding rates are usually determined by calculating clearance rate (CR; 1 [h.sup.-1]) and/or filtration rate (FR; mg [h.sup.-1]). Clearance rate is defined as the volume of water cleared of particles per unit time, whereas FR is the product of CR and particle concentration often defined as the number or weight of particulate matter removed from the suspension over time (Bayne 1985). There are often different definitions for these terms throughout the literature so it is often necessary to establish the specific definition for each study.

Several different methods have been used to estimate mussel CR, and thus FR, including: Coughlan static method, the flow-through method, and the bio-deposition method. Each of these methods typically has their own advantages and disadvantages. Coughlan (1969) developed a chamber method, which measures the decrease in food particles over time in a static chamber, thus providing a quantitative measurement of feeding rate. The flow-through method measures the decrease in food particles in a flow-through chamber, containing the bivalve, over time, compared with an empty container (Bayne et al. 1977, Widdows 1985). Both of these indirect methods are practical in a controlled laboratory setting, although the flow-through method may be somewhat more representative of natural conditions (Hildreth & Crisp 1976). The third method (bio-deposition) estimates feeding rate through measurements of suspended particles and the bio-deposit product (Iglesias et al. 1998). Basically, this technique uses inorganic particles as a tracer in the gut of the bivalve; by comparing the POM/PIM ratio of the seston in the water to the POM/PIM ratio in the bivalve feces, or the amount of feces, the feeding rate can be determined (Iglesias et a1. 1998). Two inherent problems with this method are that: (1) it assumes that the POM/PIM ratio is the same for the seston and the ingested particles (there is no preferential selection or difference in retention times), and (2) biodeposit collection must be precise for accurate quantitative analysis (biodeposits from one individual cannot be mixed with another's and the whole sample must be collected, which can be difficult when water currents or sedimentation processes are occurring) (Iglesias et al. 1998). This method can be practical in the laboratory when the diet is properly controlled over time and when the bio-deposition products are isolated. However, as an indicator of feeding in the field, this method is less practical because mussel bio-deposition products cannot be accurately isolated without disturbing the mussel. As well, strict diet control is not possible, thus it is unknown if the bio-deposition product is a true reflection of the environmental conditions at that time or of past conditions. Therefore gut retention times, which may vary according to a multitude of conditions Dame (1996) must be considered when using this method.

These methods for estimating feeding rates have been recently reviewed by Navarro and Velasco (2003) and Petersen et al. (2004). Navarro and Velasco (2003) found that Coughlan's static method and the bio-deposition method gave similar values for FR of the bivalves Mulinia edulis and Mytilus chilensis when measured gravimetrically, but not when measured volumetrically. Petersen et al. (2004) found that CR of Mytilus edulis differed significantly as a function of method used (e.g., flow-through method, bio-deposition method or Coughlan's static method) and with the origin of the mussels (e.g., genetic predisposition for gill surface area and weight). They also reported that CR, as determined gravimetrically by the bio-deposition method, was significantly lower than those measured by the flow-though or static chamber methods. It has also been argued that conditions in the laboratory may not accurately reflect in situ filtration (Cranford 2001, Riisgfird 2001), as a suite of constantly changing environmental attributes may influence basic physiological parameters and thereby render comparisons difficult (Petersen et al. 2004). With the observation that many feeding rates may have been underestimated by the bio-deposition method (Petersen et al. 2004), the potential benefits of a reliable method that could be used in the field, without disturbing the specimens and that would more accurately reflect the mussel's true feeding activity are apparent and are worth pursuing. Yahel et al. (2005) have described an alternative nonintrusive, but technically demanding method for estimating feeding rates of some species of active suspension-feeders in the field, including bivalves, by sampling inhalant and exhalent water masses using fairly precise techniques under water using SCUBA.

Several recent studies have suggested that the opening state of the valves and siphons may be closely related to food availability, and could potentially be useful as an indicator of feeding activity. Dolmer (2000) found that mean valve gap size of M. edulis was correlated with the concentration of chlorophyll a. Newell et al. (2001), and MacDonald and Nodwell (2003), found a significant positive effect of particle concentration on mussel exhalant siphon area (ESA). Both studies found that ESA was a potentially good qualitative indicator of feeding activity in the mussel Mytilus edulis. However, quantitative relationships between ESA and CR have not been established.