Journal of Sea Research 47 (2002) 85–95 www.elsevier.com/locate/seares The influence of sediment grain size on burrowing, growth and metabolism of Donax trunculus L. (Bivalvia: Donacidae) R. de la Huz, M. Lastra*,J.Lo´pez Departamento de Ecologı´a y Biologı´a Animal, Facultad de Ciencias, Universidad de Vigo, 36200 Vigo, Spain Received 24 September 2001; accepted 17 January 2002 Abstract The bivalve Donax trunculus L. commonly inhabits the intertidal and shallow subtidal zone of exposed sandy beaches along European and North African coasts. We examined the responses of burrowing time, respiration rate and growth of this species to sediment grain size. Burrowing time was examined for individuals from 5 to 45 mm shell length in artificially prepared sorted sediments varying from fine sand (125–250 Am) to medium sand (250–500 Am), coarse sand (500–1000 Am), very coarse sand (1000–2000 Am), gravel ( > 2000 Am), and native sediment (432 Am mean grain size). The shortest burrowing time for individuals between 5 and 25 mm was observed in medium and coarse sands. For individuals of 25–45 mm shell length, the shortest burrowing time occurred in fine and medium sands. The longest burrowing time for all size classes was observed in gravel. These results were analysed within the framework of the swash exclusion hypothesis. According to this hypothesis, burrowing time of D. trunculus in relation to wave period determines to what extent individuals are drifted from the zone where they preferentially live. The shorter burrowing time of smaller clams, and their ability to burrow faster than larger individuals in coarse sands suggest that juveniles may be able to withstand more reflective morphodynamic conditions than adult individuals. Effects of sediment grain size on metabolic activity and growth rates were compared by maintaining D. trunculus (26–28 mm shell length) in different sediment grades. Metabolic activity and growth rate were highest in medium and coarse grades of sand. In gravel and very coarse sand, growth rates and metabolic rates were lower by as much as 43%. Our data indicated that a small decline in metabolism was accompanied by a greater decrease in growth. Thus, our results agree with the swash exclusion hypothesis: the coarsest gra- nulometries, predominant on the reflective type of beaches, reduce the ecological efficiency of individuals by decreasing either their burrowing rate, and/or growth and metabolism. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Donax trunculus; Grain size; Burrowing; Respiration; Growth rate 1. Introduction action. For bivalve molluscs inhabiting such environ- ments, burrowing behaviour is an important adaptation Many of the characteristics of the macroinfauna of (Stanley, 1970; Trueman, 1971; Brown and McLa- exposed beaches (life cycle, rhythms of activity, repro- chlan, 1990; McLachlan et al., 1995). Bivalves inhab- duction, motility, etc.) are adaptations to an environ- iting all types of soft substrates are capable of bur- ment dominated by sediment instability and wave rowing, but bivalves living on exposed beaches must burrow rapidly and efficiently to avoid physical exclu- * Corresponding author. sion of individuals by waves or current action (Brown E-mail address: [email protected] (M. Lastra). and McLachlan, 1990; McLachlan et al., 1995). Phys- 1385-1101/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S1385-1101(02)00108-9 86 R. de la Huz et al. / Journal of Sea Research 47 (2002) 85–95 ical properties of sediment, such as particle size, grain have been undertaken by Trueman et al. (1966), Ansell shape, water content and shear strength, affect the and Trevallion (1969), Trueman (1971), Moue¨za suitability of a substrate as a habitat, by influencing (1972), McLachlan and Young (1982), Donn and Els the burrowing behaviour and life habits of benthic spe- (1990), and Ansell et al. (1998). These authors focus on cies (Sanders, 1958; Stanley, 1970; Trueman, 1971). the effects of temperature and environmental stimuli on Over the past few decades, numerous studies have burrowing activity, without considering the influence attempted to relate life habits of infaunal bivalves to of potentially important sedimentary factors, such as sedimentary characteristics (e.g., Alexander et al., grain size. 1993; Snelgrove and Butman, 1994). Results obtained Besides having a direct effect on burrowing behav- in laboratory experiments have shown that sediment iour, grain size may cause physiological stress at a grain size affects burrowing behaviour of a number of sublethal level, reducing metabolic activity, growth rate species as a whole (Trueman and Ansell, 1969; Stanley, and reproduction. Therefore, the viability of popula- 1970; McLachlan and Young, 1982; Phelps, 1989; tions in habitats with more unfavorable sediments can Donn and Els, 1990; Brown and Trueman, 1991; be reduced (Pratt and Campbell, 1956). Metabolic Alexander et al., 1993; Henderson and Richardson, activity, determined by respiratory rate, is little known 1994). However, few studies have analysed the effect for most marine invertebrate species. Thus, few studies of grain size on the burrowing behaviour of different have been made on respiratory rates in the benthic size classes of infaunal bivalves (Dugan et al., 2000), or macrofauna (Dye, 1981; Asmus, 1982; Schwinghamer the effect of grain size on other biotic parameters such et al., 1991), though such measurements are useful for as metabolism or growth rate. calculating respiratory quotients (CO2 production rate/ On sandy beaches spatial distributions of some O2 consumption rate, Hatcher, 1989) and regressions species of macrofauna are correlated with grain size between this parameter and energy expenditure or (McLachlan et al., 1995; Nel et al., 1999; Dugan et al., production (Asmus, 1982; Schwinghamer et al., 1991). 2000). However, most of these results provide little In this study, we tested the hypothesis that the additional insight into variation associated with biotic burrowing behaviour and metabolism of D. trunculus and abiotic processes. On exposed beaches from are affected by sediment size. We examined the pre- temperate to tropical latitudes, the genus Donax is diction that D. trunculus can burrow more rapidly in the predominant group of bivalves (Brown and McLa- sediments characteristic of morphodynamically inter- chlan, 1990). All the species within this genus are mediate and dissipative sandy beaches, where this spe- rapid burrowers, with a slender and elongated shell to cies is most common, as opposed to reflective beaches, accommodate the large foot, thick valves for stability, where physical stress by wave exposure excludes most and a smooth external shell for streamlining (Stanley, of macroinfaunal species (McLachlan et al., 1993). 1970). In eastern Atlantic waters, D. trunculus is one Therefore we investigated the responses of individuals of the better-known species. It occurs primarily in the to sediment size by comparing the burrowing rate, intertidal zone, with differential distribution by size or growth and respiration of a range of sizes of D. trun- age. The youngest individuals occur highest on the culus under laboratory conditions. shore, and the oldest close to the level of low water of spring tides (Ansell and Lagardere, 1980; Bayed and Guillou, 1985). Ecological preferences of this species 2. Methods have been studied by Degiovanni and Moue¨za (1972), Moue¨za (1972), Ansell and Lagardere (1980) and 2.1. Burrowing Guillou and Bayed (1991). The reproductive cycle and population dynamics have been examined by We collected a wide size range (5–45 mm) of Guillou (1982), Maze´ and Laborda (1988), Maze´ individuals of Donax trunculus from the intertidal zone (1990), and Tirado and Salas (1998). Detailed anatom- of Playa America (42j08VN, 08j49VW). This is an ical studies were performed by Moue¨za and Frenkiel intermediate sandy beach on the northwest coast of the (1974, 1976, 1978) and by Moue¨za (1976). Studies on Iberian Peninsula with a mean wave height of about 1 the burrowing behaviour of different species of Donax m, mean wave period of about 10 s, mean slope of R. de la Huz et al. / Journal of Sea Research 47 (2002) 85–95 87 Table 1 Characteristics of the sediment used in the burrowing trials Grade Sorting Water content % Shear strength (kPa) % in native sediment Fine 1.30 (very well sorted) 22.40 1.90 30.81 Medium 1.27 (very well sorted) 23.36 1.43 28.93 Coarse 1.23 (very well sorted) 22.14 1.00 22.18 Very coarse 1.21 (very well sorted) 20.99 0.73 15.60 Gravel 1.14 (very well sorted) 23.51 0.46 2.48 Native Sediment 1.71 (well sorted) 20.26 1.33 100.00 Percentage in native sediment is the share of each grade of sand occurring in the native sediment where individuals to be used in the experiments were collected. about 1/21, a modal value of Dean’s parameter (Dean, Individuals were maintained in tanks with 10 cm of 1973) of 2.7, and a Beach State Index of 3.1 (De la Huz, native sediment and 15 cm water until the experiments 1999). These values indicate an intermediate wave- started. Burrowing time in the different types of sorted dominated type of beach (McLachlan et al., 1993). sands was measured individually in tanks with 10 cm The maximum length of the valves of each individual of sorted sediment and 15 cm of water. Individuals was measured with a digital caliper to the nearest were timed in seconds, from the initiation of burrow- 0.01 mm. Four size classes were used: 5–15 mm, ing (effective penetration of the foot into the substrate) 15–25 mm, 25–35 mm and 35–45 mm. to the disappearance of the valves under the sediment We collected sand from the zone where the speci- surface. Bivalves were removed from the sorted sands mens were collected. This native sediment was sorted immediately after the burrowing test and replaced in with sieves into five grades, following the Wentworth the native sediment.