Globally, the Manila clam (Ruditapes philippinarum) stands as the second most important bivalve species in fisheries and aquaculture. Native to the Pacific coast of Asia, it is now well-established in North America and Europe, where its on-going management reflects local economic interests. The historic record of transfers spans the 20^th century and suggests sequential movement from Japan to North America, as a hitch-hiker on oysters, and then intentional introduction in Europe, but global genetic data are missing. We have studied mitochondrial DNA and microsatellite markers in nine populations from Asia, North America and Europe. The results from the two types of markers indicated a good concordance of present-day genetic structure with the reported history of clam transfers across continents, and no evidence of relevant concealed introductions from continental Asia in Europe and North America. However, European populations showed a loss of genetic variability and significant genetic differentiation as compared to their American counterparts. Our study shows that in spite of the increasing ease for species to spread out of their native range, in the case of the Manila clam this has not resulted in new invasion waves in the two studied continents.

13 páginas, 3 tablas, 3 figuras, 7 apéndicesThis study evaluates the influence of different states of energy balance, positive and\ud negative, on the accumulation and utilisation of energy reserves during the period of gonad development\ud in the clam Ruditapes decussatus (L.). Our results show a clear sexual differentiation in the\ud biochemical composition of the clams. The differences were associated with gonad development and\ud were particularly evident in proteins and lipids. These findings imply that separation of the sexes for\ud studying variations in the biochemical composition of sexually developed specimens is essential, and\ud thus questions the results of previous studies in which both sexes were analysed simultaneously.\ud Gonad development occurs under conditions of both positive and negative energy balance, and permits\ud the development of a model of the utilisation of reserves during gonad development, which is\ud closely linked to the clam’s energy balance. Under conditions of positive energy balance, the amount\ud of food ingested provides sufficient energy for both metabolic consumption and the accumulation of\ud reserves. The absolute value of all biochemical components increases, in particular for lipids in the\ud case of female clams, with no evidence of consumption of glycogen and other carbohydrates or their\ud conversion into lipid reserves. Conditions of negative energy balance lead to a moderate degree of\ud organic weight loss in the clams. Glycogen and other carbohydrates are rapidly consumed, falling to\ud almost a quarter of initial values, whilst protein and lipid levels remain constant in females. Lipid values\ud decrease slightly in males. Finally, under situations of extreme nutritive stress (when energy\ud reserves are scarce) there is a loss of all biochemical components in a specific order of priority. Most\ud of the glycogen and other carbohydrates are consumed initially (a decrease of 61 and 74%, respectively),\ud followed to a lesser extent by lipids in males (40%) and then proteins (between 20 and 25%).\ud In females, there was no lipid loss until the majority of the carbohydrates and part of the proteins had\ud been consumed, indicating that lipids constitute the reproductive reserve par excellence in this species.This study\ud was financed by the project PGIDT-99MAR60401 and MAR-\ud 99024002. M.D. was supported by a research personnel training\ud grant from the European Social Fund—Spanish Oceanographic\ud Institute (1998–1999) — and a grant from the Consello Regulador\ud do Mexillón de Galicia (Board of Control of the Galician\ud Mussel) (2000–2001) whilst working on this study.Peer reviewe

Laboratory experiments were used to study the influence of temperature on the reproductive behaviour of two species of clam, Ruditapes decussatus and Ruditapes philippinarum, during their adaptation to the temperature conditions of Galician coastal waters. In both species the rate of gonadal development was directly related to the increase in temperature. At 14°C the reproductive behaviour was similar, both species needing over 2 months to mature. At 18°C, the rate of gonadal development of R. philippinarum was greater than that of R. decussatus. Nevertheless, the results of this study confirm that both species have adapted perfectly to the temperature conditions of the Galician Rias, though certain differences between the reproductive behaviour of these species were detected. R. philippinarum accumulates oocytes prior to their partial or total emission, while in the case of R. decussatus gametes are liberated continuously. In the early phases of its development, the gonad of R. philippinarum is highly heterogeneous in nature, with up to 3 reproductive states being present at the same time, while in R. decussatus gonadal development is much more uniform. Another major difference concerns the phenomenon of reabsorption, common in R. philippinarum but very rare in R. decussatus. These characteristics may well result in a greater reproductive activity in R. philippinarum than in R. decussatus, and a longer reproduction period in the former. They may also represent a certain advantage for the adaptation of the foreign species (R. philippinarum) over the native species (R. decussatus) to the temperature conditions of the Galician Rias.