POPULATION BIOLOGY AND SECONDARY PRODUCTION OF OLIVANCILLARIA VESICA VESICA (GMELIN, 1791) (GASTROPODA: OLIVIDAE) ON A SANDY BEACH IN SOUTHEASTERN BRAZIL CARLOS HENRIQUE SOARES CAETANO, VALÉRIA GOMES VELOSO AND RICARDO SILVA CARDOSO Laboratório de Dinâmica de Populações Marinhas, Departamento de Ciências Naturais, Universidade do Rio de Janeiro (UNIRIO), Av. Pasteur, no. 458, Sala 411, Urca, CEP 22290 – 240, Rio de Janeiro, Brazil ABSTRACT Olivancillaria vesica vesica (Gastropoda: Olividae) is a common inhabitant of exposed sandy beaches in southern and southeastern Brazil. The population biology and secondary production of this species was studied from June 1998 to July 2000 on Restinga da Marambaia beach, state of Rio de Janeiro, Brazil. Specimens were hand-collected in two sectors (10 ϫ 25 m), and shell length was measured in the field. Growth and mortality were estimated from length-frequency data, using computer-based methods. Life span was estimated by an inverse von Bertalanffy growth equation, considering tmax as 95% of the asymptotic length. Mass-specific growth-rate and size-frequency methods were used to calculate secondary production. Highest population densities were observed during winter (September 1998 and 1999) and autumn (June 2000). The growth rate varied seasonally, with the slowest growth rates occurring in late summer (1st year) and spring (2nd year). Mortality rates (Z ϭ 3.12 and 2.56 year–1, for the first and second year, respectively) did not differ significantly between years. Life span was nearly 5 years. Estimated secondary production varied between 0.142 and 0.213 g AFDW m–2 year–1, while annual P/B ratio varied between 1.029 and 1.111. INTRODUCTION from June 1998 through July 2000, always at low tide. Two rect- angular sectors (500 m apart) were established, each 10 m wide Gastropods and bivalves are among the most conspicuous with a 25-m base parallel to the waterline, located from the members of the macrofauna of exposed sandy beaches (Brown swash zone to a 50-cm water depth in the sublittoral. In each sec- & McLachlan, 1990), being generally more abundant on dissi- tor, specimens were hand-collected by two collectors until every pative and intermediate beaches than on reflective beaches snail had been removed. Shell length was measured in the field (Dexter, 1984). using a vernier caliper (0.1 mm accuracy). Afterwards, all speci- Gastropods of the family Olividae inhabit sandy shores in mens were returned to their respective sectors, except those tropical and subtropical regions of the world (Petuch & Sargent, utilized to determine the length–weight relationship (n ϭ 95). 1986), and a few species are also found in the cooler seas of In the laboratory, shells were cracked in a vice and the animals southern Australia and New Zealand (Smith, 1998). The genus removed. These were dried at 70°C for 24 h, weighed, ashed in a Olivancillaria includes about 10 species, seven of them occurring muffle furnace for 4 h at 600°C and reweighed. in Brazil (Rios, 1994), where some are important for subsistence The study period was arbitrarily divided into two sampling fishing. Nevertheless, studies of this genus are sparse and mainly years (from June 1998 to June 1999 and from July 1999 to July concern systematics (Klappenbach, 1964, 1965, 1966; Thomé, 2000) in order to analyse temporal variations in population 1966) and anatomy (Marcus & Marcus, 1959; Jurberg, 1970; parameters. Lopes, 1991; Borzone, 1995; Borzone & Vargas, 1999). Olivan- cillaria vesica (Gmelin, 1791) is a common inhabitant of beaches in southern and southeastern Brazil, living semi-buried in Environmental parameters sandy substrates from the intertidal zone to deeper areas in the Sediment samples for particle-size analysis were taken monthly sublitteral (Gianuca, 1985). According to Klappenbach (1966), with a plastic corer of 3.5 cm diameter to a depth of 10 cm, in the O. vesica occurs in two geographic forms or subspecies. One sublittoral zone. Samples were oven-dried at 70°C and sieved form with a long narrow shell, O. vesica vesica, occurs from the through graded screens in order to determine mean particle state of Rio de Janeiro south to Paraná, while another with a size (Folk & Ward, 1957). The slope of the beach was deter- smaller wider shell, O. vesica auricularia (Lamarck), ranges from mined by Emery’s profiling technique (Emery, 1961). Dean’s Santa Catarina in Brazil to the Mar del Plata in Argentina. In the dimensionless parameter (⍀; Short & Wright, 1983) was calcu- present study we describe the growth, mortality, life span and lated for each month as a measure of beach morphodynamic secondary production patterns of O. vesica vesica, based on a two- ⍀ϭ state: Hb/Ws.T, where Hb is breaker height in cm, Ws the sand year study. settling velocity in cm s–1 (obtained from the particle size and Gibbs, Mathews & Link, 1971) and T the wave period in seconds. MATERIAL AND METHODS Water temperature and salinity in the surf zone were measured using a thermometer and a salinometer, respectively. Sampling and laboratory procedures The population of Olivancillaria vesica vesica at Restinga da Population parameters Marambaia beach (23°03Ј S; 43°36Ј W) was sampled monthly Growth and mortality functions were estimated from the monthly Correspondence: C. H. S. Caetano; e-mail: [email protected] length measurements in the population (26 length-frequency J. Moll. Stud. (2003) 69: 67–73 © The Malacological Society of London 2003 C. H. S. CAETANO, V. G. VELOSO & R. S. CARDOSO samples with individual lengths grouped into 2-mm size classes). Statistical analysis The routine Electronic Length Frequency Analysis (ELEFAN) of the package FAO-ICLARM Stock Assessment Tools (FISAT) Simple linear regressions were performed to assess the relation- was used to fit a seasonal von Bertalanffy model (Gayanilo, ships between the density of O. vesica vesica and the density of Sparre & Pauly, 1996) to the set of restructured length-frequency their main food item, the intertidal mole crab Emerita brasiliensis samples. This curve has the form: Schmitt (data obtained from Cardoso, Veloso & Caetano, 2002); sand particle size; beach slope; and water temperature π π π π ϭ [–K(t–t0) + (KC/2 )sin 2 (t–ts)–(KC/2 )sin 2 (t0–ts)] Lt Lϱ[1–e ] and salinity. Both O. vesica vesica and E. brasiliensis were collected during the same sampling period. An analysis of covariance where Lt is length (mm) at time t; Lϱ is the theoretical maximum (ANCOVA) was used to compare mortality rates between years, length that the species would reach if it lived indefinitely; K is using age as the covariate. The Mann–Whitney U-test was used to the curvature parameter; C is a constant for the amplitude of compare biomass and secondary production between years. In oscillation in seasonal growth; to is age at zero length; and ts is the all statistical analyses, a significance level of 5% was adopted. initial point of seasonal oscillation in relation to t ϭ 0 and WP (winter-point, i.e. the period of growth reduction, expressed as a decimal fraction of the year). The graphical representation of this equation produces a curve that is evaluated through the RESULTS goodness of fit index Rn (Gayanilo et al., 1996). The instantaneous mortality rate (Z) was calculated by the Environmental parameters single negative exponential model, using the length-converted Restinga da Marambaia beach showed a modal intermediate catch curve method (Pauly, Moreau & Abad, 1995) of the FISAT morphodynamic state with a mean omega (⍀) value of 1.8 program (Gayanilo et al., 1996). (SD Ϯ 0.6). The beach had a gentle slope (1/11.73–1/85.71). Life span was estimated by an inverse von Bertalanffy growth The sediment was composed of fine to medium sands, ranging equation, considering maximum length as 95% of the asymp- from 0.20 to 0.32 mm (x¯ ϭ 0.23; SD Ϯ 0.03). The mean tem- totic length (King, 1995). perature in the water of the surf zone was 22.7ºC (SD Ϯ 2.6), with the lowest temperatures of 17.5 and 16.6ºC occurring in Length–weight relationships spring (November 1998 and 1999, respectively), and the highest temperature of 28.7ºC in February 1999. The salinity of the surf The relationship between shell length and ash-free dry weight waters was nearly constant (x¯ ϭ 34.3; SD Ϯ 0.5). of the soft tissue was estimated by linear regression analysis, with the data converted to natural logarithms in the equation: lnW ϭ ln a ϩ b ln L, where W is the mean ash-free dry weight per Population abundance individual (g); L is the length of the size class (mm); and a is the intercept on the Y axis and b is the slope of the regression line. Densities of O. vesica vesica varied considerably with the greatest densities observed during winter (September 1998 and 1999) and autumn (June 2000; Fig. 1). No significant correlations Secondary production between the density of O. vesica vesica and the abiotic (sand par- Secondary production was calculated by two methods. The ticle size, water temperature and salinity) and biotic (density first was the mass specific growth rate (MSGR) of Crisp (1984). of Emerita brasiliensis) factors analysed were observed, except for ϭ Ͻ In this method production is given by the equation: beach slope (r 0.45; P 0.05). ϭ⌺⌺ ⌬ P fi *G i *wi * t, where fi is the mean number of individuals in size class i, Gi is the mass specific growth rate of size class i, ⌬ Growth, life span and mortality wi the mean weight of the size class i and t is the interval of time. Mass specific growth rate (Gi) can be obtained by During the study period, 1594 snails were collected and meas- ϭ ⌬ Gi ln wi + 1 – lnwi/ t(i + 1 – i).