Canadian Journal of Zoology Living on the edge: population parameters of the wedge clam Donax hanleyanus in its southernmost limit of distribution range, Southwest Atlantic, Argentina Journal: Canadian Journal of Zoology Manuscript ID cjz-2021-0040.R1 Manuscript Type: Article Date Submitted by the 30-Mar-2021 Author: Complete List of Authors: Risoli, M. Cielo; Instituto de Investigaciones Marinas y Costeras; Universidad Nacional de Mar del Plata Facultad de Ciencias Exactas y Naturales; Consejo Nacional de Investigaciones Cientificas y Tecnicas Defeo, Omar;Draft Laboratorio de Ciencias del Mar (UNDECIMAR) Lomovasky, Betina; Instituto de Investigaciones Marinas y Costeras; Universidad Nacional de Mar del Plata Facultad de Ciencias Exactas y Naturales; Consejo Nacional de Investigaciones Cientificas y Tecnicas Is your manuscript invited for consideration in a Special Not applicable (regular submission) Issue?: bivalve, wedge clam, <i>Donax hanleyanus</i>, sandy beach, GROWTH Keyword: < Discipline, production, mortality © The Author(s) or their Institution(s) Page 1 of 34 Canadian Journal of Zoology Living on the edge: population parameters of the wedge clam Donax hanleyanus in its southernmost limit of distribution range, Southwest Atlantic, Argentina M.C. Risoli1*, O. Defeo2, B.J. Lomovasky1 1Instituto de Investigaciones Marinas y Costeras (IIMyC), Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata (UNMdP) - Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), CC 1260 (7600), Mar del Plata, Argentina. Tel: (+54) 223-4734635, Fax: (+54) 223-4753150. *E- mail: [email protected]. E-mail: [email protected] 2Laboratorio de Ciencias del Mar (UNDECIMAR), Facultad de Ciencias, Montevideo, Uruguay. E-mail: [email protected] Draft 1 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 2 of 34 Living on the edge: population parameters of the wedge clam Donax hanleyanus in its southernmost limit of distribution range, Southwest Atlantic, Argentina M.C. Risoli1*, O. Defeo2, B.J. Lomovasky1 Abstract The wedge clam Donax hanleyanus (Philippi, 1845) is a conspicuous member of the sandy beach macrofauna along hundreds of km in the northern coasts of Argentina. Age, growth, mortality and productivity of this species were assessed in its southernmost limit of distribution range (Mar del Plata, Buenos Aires Province, Argentina). The von BertalanffyDraft growth function parameters were SL∞ -1 = 23.23 mm, K = 1.62 year , and t0 = 0.08 years. The maximum individual production amounted 7.9 x 10-7 g shell-free wet mass (SFWM)·year-1 at a shell length of 14-16 mm. Individuals between 10- mm and 12- mm contributed the most to the secondary production P of 0.01 g SFWM·m-2·year-1. The mean annual biomass B was of 0.002 g SFWM·m-2·year-1. Annual P/B ratio and mortality coefficient Z were 4.48 year-1 and 4.78 year-1, respectively. Low production and high mortality could be caused by a sharp decrease in habitat quality, given by suboptimal temperatures at the southernmost edge of the species distribution. These results strongly limit its potential in a future scenario of commercial exploitation. Keywords: bivalve, wedge clam, Donax hanleyanus, sandy beach, growth, production, mortality. 2 © The Author(s) or their Institution(s) Page 3 of 34 Canadian Journal of Zoology Introduction Exposed sandy beaches are high energy environments dominated by physical variables such as grain size, wave energy and tidal regime, whose interaction define their morphodynamic state, ranging from microtidal-reflective to macrotidal-dissipative systems (Short 1996, 1999; McLachlan et al. 2018). These physical factors have been suggested as the main drivers of macrofaunal responses (McLachlan et al. 1995), shaping processes such as recruitment, growth, mortality and connectivity between populations (McLachlan and Defeo 2018). These physical factors determine species distribution boundaries, in conjunction with other ecological and evolutionary features (McLachlan and Defeo 2018). Thus, population structure, life-history and genetic traits, which geographically reflect the range Draftlimits of a species, become especially relevant to be assessed in peripheral populations (Gaston 2003; Fenberg and Rivadeneira 2011). Bivalve mollusks are one of the most conspicuous members of exposed sandy beaches, where they can reach high population densities (McLachlan et al. 1996). Among them, Donax species are the most abundant group adapted to inhabit these high-energy environments, especially in tropical and subtropical beaches. Donax hanleyanus (Philippi, 1845) is a common wedge clam found along the South American Atlantic coast, from tropical (17°43'S, Caravelas, Brazil) to temperate sandy beaches (38°04'S, Mar del Plata, Argentina) (Penchaszadeh and Olivier 1975; Cardoso and Veloso 2003). Life-history traits of the wedge clam had been studied in the recent decades in Argentina (Penchaszadeh and Olivier 1975; Marcomini et al. 2002; Thompson and Sánchez de Bock 2009; Herrmann 3 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 4 of 34 et al. 2009b, 2010), Uruguay (Defeo and de Álava 1995; Delgado and Defeo 2007) and Brazil (Cardoso and Veloso 2003; Gil and Thomé 2004a, 2004b). D. hanleyanus became dominant in many beaches of the Southwestern Atlantic after the drastic population decline of the yellow clam (Mesodesma mactroides) due to the combined effect of several stressors, including recreational and artisanal harvesting, urbanization (Dadon 1999; Marozzi and Dadon 2000) and climate change effects that generated extensive mass mortalities throughout its geographic range, including beaches of Brazil, Uruguay and Argentina (Ortega et al. 2016). Thus, the wedge clam plays an increasing and pivotal role in the food web, linking the primary production with higher trophic levels (McLachlan et al. 1981; Gianuca 1985; Soares etDraft al. 1997), being the main primary consumer in most Argentinian sandy beaches and the prey of many invertebrates, fish, birds, and even mammals (Penchaszadeh and Olivier 1975; Ansell 1983; Salas et al. 2001). Several Donax species are commercially exploited worldwide (McLachlan et al. 1996; Ferguson et al. 2021), and D. hanleyanus is considered as a potentially harvestable resource in Argentina. In this context, its biology and reproductive strategies are critical for fisheries development and management. Therefore, the objective of this study is to assess population parameters and secondary production of a D. hanleyanus population in the southernmost limit of its distribution range (38°04'S, Mar del Plata, Argentina). Materials and methods Beach characterization, sampling and laboratory procedures Samplings were carried out at “Punta Mogotes” beach (38°04'S, 57°32'W), Mar del Plata, Buenos Aires province, Argentina (Fig. 1), from January 2018 to 4 © The Author(s) or their Institution(s) Page 5 of 34 Canadian Journal of Zoology February 2019. “Punta Mogotes” is a sandy beach located in a 4 km arc bay to the south of the city (Bertola 2006), and constitutes the southernmost limit of the wedge clam distribution range (Penchaszadeh and Olivier 1975). The tidal regime is mixed semi-diurnal, with a mean amplitude of 0.8 m and a maximum of 1.7 m (Bertola 2006). In order to characterize the beach, 6 equally-spaced sediment samples covering the entire intertidal area were collected every two months using a 6 cm diameter cylinder, for laboratory determination of mean grain size and organic matter content. Sediment was sieved through a 10- sieve column ranging from 1.4 to 0.006 mm mesh, and then sediment retained on each sieve was weighed. The mean grain size was obtained Draftby the Folk's method using the GRADISTAT software (Blott and Pye 2001). The organic matter content was determined in 10 g subsamples as the difference between dry weight (80ºC to constant weight) and ash weight (450ºC for 8 hours). Beach slope was estimated using the Emery’s profiling technique (Emery 1961), and beach morphodynamics was assessed by Dean's parameter Ω (Short 1996): (1) Ω = (Hb·100) / (Ws·T) where Hb is breaker height (m), Ws is sand fall velocity at 20ºC (m·s–1) and T is wave period (s). Tables from Gibbs et al. (1971) were used to calculate settling velocities based on particle size. D. hanleyanus was sampled through a systematic sampling design that consisted in 3 transects (perpendicular to the shoreline, 50 m apart) covering the whole across-shore wedge clam distribution (Defeo et al. 1992). Along each transect, sampling units (SUs) were taken at 3-m intervals from the upper intertidal to lower 5 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 6 of 34 intertidal where 2 successive SUs without wedge clams were recorded. SUs were obtained by excavating quadrats of 0.09 m2 to a depth of 35 cm with shovels, and the sediment was sieved through a 2 mm mesh to retain wedge clams. Shell length (SL, anterior-posterior axis) was measured (precision 0.1 mm) and shell-free wet mass (SFWM) was determined (precision 0.01 g) in all individuals collected. In each SU, a subsample of 15 wedge clams was obtained; the shell- free tissue of each wedge clam was dried at 70ºC to constant weight and then at 450ºC for 6 hours to obtain the ash-free dry mass (AFDM). Both SFWM (N = 938) and AFDM (N = 324) data sets were used to assess the length-mass relationships, following: (2) M = a·SLb Draft where M is the mass (SFWM, AFDM), a is the intercept, and b is the slope. Linear regression analyses were carried out with the log-transformed data. Shell growth patterns and age Subsamples for growth analysis (N = 165) were obtained monthly, selecting 6 to 16 wedge clams per month with sizes from 6.15 to 27.07 mm SL. The individual age was inferred from internal shell growth bands (see Richardson 2001). Each right valve was embedded in polyester resin and sectioned in shell height with a Bosch® circular saw with a diamond blade.