Annual growth line formation in the deep water Patagonian scallop Zygochlamys patagonica

BETINA JUDITH LOMOVASKY; OSCAR IRIBARNE,; THOMAS BREY,; MACKENSEN, A.; BALDONI, A.; MARIO LASTA,; SILVANA CAMPODÓNICO

Halifax, Canada

Congreso; 16th International Pectinid Workshop; 2007

The Patagonian scallop Zygochlamys patagonica occurs in beds at depths around 100 m on the SW Atlantic shelf of South America between 36ºS and 55ºS; and it is an important economic resource in the SW Atlantic. Up to four factory trawlers capture up to 43.000 tons of scallops per year, which correspond to about 6.000 tons of adductor muscle. Present size-at-age keys for Z. patagonica are quite different and thus inconclusive, most likely owing to differences in the methods used, such as growth line identification on shell surface, internal growth bands and growth line validation using monthly sampling of terminal growth stage or stable isotope analysis. The purpose of our study is to provide reliable estimates of Z. patagonica age and growth to facilitate the management of the Atlantic populations. We combine up-to-date techniques of ageing and age validation with oceanography data to analyze and validate individual age and growth patterns in this species from four large beds across its SW Atlantic distributional range. Stable oxygen and carbon isotope ratio analysis in scallops from the following beds: Uruguay, 36º17’S, Reclutas, 39°20’S, Tango B, 42º30’S, and Beagle, 55º10’S, in combination with condition indexes and oceanography date strongly suggest that shell growth lines in this species are formed annually. Most growth lines coincide with low values of both d13C and d18O (i.e., they are formed at times of high remineralization activity and of higher water temperature). This pattern is consistent throughout the distributional range (Fig. 1). Owing to the specific Argentinean shelf oceanography, higher temperatures at this depth occur during austral autumn – beginning winter in the region of Reclutas and Tango B but during summer - autumn at the northern and southern limit of the distributional range. Besides temperature, alimentation and differential energy allocation are the major determinants of growth. Seasonal oscillation in food supply can cause seasonal oscillation in growth with very slow or even no growth at times of lowest food availability. Differential energy allocation in somatic (including shell) and gonad production is another common source of seasonal oscillation in shell growth of bivalves. The out-of-phase annual cycles of both gonad and adductor muscle relative condition index in Z. patagonica at Reclutas point in this direction. Soma is built in summer and early autumn (November to March), whereas gonads are produced in late autumn and winter (May to September), as confirmed by monthly histological analysis, showing that shell growth line coincide with a pause in somatic growth during austral winter, when energy investment is shifted from somatic growth to gonad development. The observed interconnections between hydrography, primary production and pectinid seasonal growth cycles as observed at Reclutas will apply to Tango B, too, as this scallop bed is governed by the same hydrography. Things may be different at the northern Uruguay site and the southern Beagle site which both are situated in different, albeit opposite hydrographic regimes. Uruguay bed is represented in part by a higher sea-temperatures, strongest thermocline and different temperature seasonality by the influence of the Subtropical Shelf Front and the Rio de Plata discharge. The opposite process occurs in Beagle bed which is characterized by a nearly homogenous water column by continuous vertically mixing and with higher sea bottom temperature amplitude (4.5ºC). However, no differences were observed in the interannual variability of oxygen isotopes between beds (corresponding to 1º to 3ºC), suggesting that organims from Beagle bed did not deposit shell material through the full range of temperature observed; thus, not growing at higher temperatures. Thus, the mechanism that produces the annual shell growth line at these sites remains unclear so far. The annual periodicity of external growth lines of Z. patagonica validated by oxygen isotopes analyses in combination with condition indexes and oceanography data give the possibility to be used as age approximation in future studies. However, the season of formation of shell growth lines across different beds could be different depending of mechanism acting in the regulation of the reproductive process which is considered as the trigger of shell growth cessation (i.e., indirect mediated by food availability).