Museo Argentino de Ciencias Naturales, Argentina.
Certain species of the Volutidae family in the South Atlantic Sea have been object of accidental extraction during trawling fishing, but in the last years a new market demand appeared, which involved specific fishing of few species. It represents a food source and employment and generates important incomes to small-scale artisanal fishers. This new vulnerability and the risk of extinction for this species is high due fundamentally to the fact that all South American volutids have direct development without planktonic dispersal stage, a characteristic that allows dispersion and massive renovation of stocks.
Adelomelon beckii is the biggest species that inhabits sandy bottoms of the western coast of the southern Atlantic Ocean, from the south of Brazil to Tierra del Fuego, Argentina. Despite its widespread distribution and the recent perspective of this resource in the markets, there is very little information about the population dynamics. Actually the fishery of this group has not been regulated so it is important to make studies to develop a suitable management plan in order to protect reproductive capacity of stocks and assure the continuity of this natural resource.
The age of the individuals could be determined in many ways depending of the organisms and the structure that we are analyzing. In this case, we are going to obtain the age of the individuals by the use of the stable oxygen isotope ratios ( δ16 O/ δ18 O) and validation of annual growth mark formation by internal shell microgrowth banding patterns. The application of this technique is possible because the calcium carbonate of the shell is in isotopic equilibrium with the seawater. Measuring the isotopic composition along the growth direction allows us to evaluate the variation of water temperature in time and at the same time the age of the studied specimen.
The aim of this study is to analyze individual growth and age in this specie using stable oxygen isotopes in order to provide parameters of the population dynamics, which will be useful to develop a suitable management plan in order to assure the continuity of this natural resource.
Materials and Methods
About 10 specimens of Adelomelon beckii were collected during commercial bottom trawling off Mar del Plata, Buenos Aires, Argentina (38°20´S, 57°37´W) in 50 m water depth between January and May 2007. The whole animal and soft parts were weighed to the nearest 1 gr and shell length (SL), shell width (SW) and spiral growth trajectory (SG) were measured with a vernier caliper to the nearest 1 mm. Annual growth bands in A. beckii shells were identified by internal growth band pattern and validation of annual formation by stable isotopes analysis. In the laboratory of the Argentine Museum of Natural Sciences “Bernardino Rivadavia” (Buenos Aires, Argentina) shells were cut along the whorls following the spiral growth trajectory from the apex to the posterior end of the aperture. The cut surface were polished with different fine grained sand paper and viewed under a stereomicroscope to examine the internal structure and then checked for growth marks in the three-layered shell. Growth marks were numbered subsequently, and the distance from the shell apex to each mark was measured. Mark number and corresponding spiral growth trajectory were interpreted as size at age data. Growth parameters were estimated with a nonlinear fitting algorithm such as the Newton algorithm using the SOLVER tool of Microsoft EXCEL program. Different entry value parameters were tried and those that offered the better results were chosen.
In order to assess if internal marks in the shell corresponded to annual growth marks stable oxygen isotopes ratios were analyzed. Two representative specimens (individual I: SL: 383 mm; SW: 170 mm and SG: 805 mm, individual II: SL: 370 mm;SW: 164 mm and SG: 763 mm) were selected for the stable isotope analysis. From individual I about 435 samples and from individual II about 317 samples (50 mg each) were taken with a small dental drill from the central shell layer, at 1-4 mm distance from each other, along the spiral growth trajectory. The stable oxygen isotope composition was determined in the Stable Isotope Laboratory of the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven, Germany) with a Finnigan MAT251 mass spectrometer coupled to an automatic carbonate preparation device. The results were reported in -notation versus PDB (Vienna Pee Dee belemnite) standard calibrated via NIST 19 (National Institute of Standards and Technology isotopic reference material 19). Counting 18O troughs/peaks (i.e. summers/winters) from apex towards aperture provided the age of each trough and the distance of the corresponding sample from the apex provided the corresponding shell length at this age.
The 308 size-at-age data pairs obtained from the 10 specimens analyzed were fitted best by the specialized von Bertalanffy growth model such that:
St = 842.41 * (1 - e -0.049 * (t – 2.451))
Where 842.41 is the size reached after an infinite time of growth (asymptotic size), 0.049 is the instantaneous growth coefficient which defines the speed of growth and 2.451 is the point at which the von Bertalanffy curve intersects the x-axis (age at which size would be zero) and St is the size at age t.
Figure 1. Oxygen stable isotope profile along the spiral growth trajectory of Adelomelon beckii of 380 mm shell length. Vertical lines represent internal shell growth marks and corresponding to low sea water temperature (winter). This animal was 28 years.
The stable isotope profile showed a distinct pattern that reflects a cyclical environmental temperature, according to the paelotemperature equation of Epstein and Lowestam´s (1953) (Figure 1). The position of the growth marks visible on the internal shell structure coincide with the position of the 18O peaks. According to this patterns individual I (SL 383 mm) was 28 year and individual II (SL 370 mm) was 26 year.
Potential lifespan of Adelomelon beckii in Mar del Plata area is 28 years as the largest individual recorded had a shell length of 390 mm. This specie appears to be rather long lived compared with other gastropods such as Adelomelon brasiliana (20 years, Cledón et al. 2005), Zidona dufresnei (17 years, Giménez et al. 2004), Concholepas concholepas (10 years, Stotz 2000), Odontocymbiola magellanica (12 years, Bigatti et al. 2006). The life span of several South Western Atlantic volutids seems to be twice compared with other large gastropods, probably reflecting a phylogenetic difference between families.
Bigatti G, Penchaszadeh PE, Cledón M (2006) Age and growth in Odontocymbiola magellanica (Gastropoda: Volutidae) from Golfo Nuevo, Patagonia, Argentina. Marine Biology 150: 1199-1204
Cledón M, Brey T, Penchaszadeh PE, Arntz W (2005) Individual growth and somatic production in Adelomelon brasiliana (Gastropoda; Volutidae) off Argentina. Marine Biology 147: 447-452
Epstein S, Lowenstam HA (1953) Temperature shell-growth relations of recent and interglacial Pleistocene shoal-water biota from Bermuda. J Geol 61:424–438
Giménez J, Brey T, Mackensen A, P.E. P (2004) Age, growth and mortality of the prosobranch snail Zidona dufresnei (Donovan, 1823) in the Mar del Plata area, SW Atlantic Ocean. Marine Biology 145: 707-712
Stotz W (2000) Informe Final Proyecto 97-36. Formulación de una metodología para el estudio de edad y crecimiento en el recurso loco. Universidad Católica del Norte, Facultad de Ciencias del Mar, Dpto. de Biología Marina, Santiago de Chile, pp 1–152