The San Francisco Bay/Delta Estuary hosts more nonindigenous species than are known for any other estuary, with 212 established species, 15 species too recently arrived to determine whether they have become established, and 125 cryptogenic species (species which could be either native or introduced). Nonindigenous organisms dominate many habitats, accounting for 40% to 100% of the common species in benthic and fouling communities. On average one new species has become established every 36 weeks since 1850, increasing to one new species every 24 weeks since 1970. Nonidigenous species have altered habitats, disrupted food webs, and contributed to the eradication or reduction of native populations. Some intentionally introduced fish and two accidentally introduced clams supported commercial fisheries that have since declined or closed. Most economic effects have been negative, including over $600 million damage from a wood-boring clam, reduced vessel speed and increased fuel consumption from hull fouling, and costs of controlling nonindigenous plants and fish. Nonindigneous species, by contributing to the endangerment of native species, and by destabilizing and making the ecosystem less manageable, are a factor in increasing restrictions on water diversions, levee maintenance, channel dredging and other economic activities in and near the Estuary.

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Nonindigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and delta

Several non-indigenous bivalve species have been colonising aquatic ecosystems worldwide, in some cases with great ecological and economic impacts. In this paper, we focus on the ecosystem engineering attributes of non-indigenous invasive bivalves (i.e., the capacities of these organisms to directly or indirectly affect the availability of resources to other species by physically modifying the environment). By reviewing the ecology of several invasive bivalves we identify a variety of mechanisms via which they modify, maintain and/or create habitats. Given the usually high densities and broad spatial distributions of such bivalves, their engineering activities can significantly alter ecosystem structure and functioning (e.g., changes in sediment chemistry, grain size, and organic matter content via bioturbation, increased light penetration into the water column due to filter feeding, changes in near bed flows and shear stress due to the presence of shells, provision of colonisable substrate and refuges by shells). In addition, changes in ecosystem structure and functioning due to engineering by invasive bivalves often have very large economic impacts. Given the worldwide spread of non-indigenous bivalves and the varied ways in which they physically modify habitats, their engineering effects should receive more serious consideration in restoration and management initiatives.

Non-indigenous invasive bivalves as ecosystem engineers

The Asian clam Corbicula fluminea is one of the most invasive species in freshwater aquatic ecosystems. The rapid growth, earlier sexual maturity, short life span, high fecundity and its association with human activities makes C. fluminea a non-indigenous invasive species likely to colonize new environments. This species, originally distributed in Asiatic ecosystems, is now a common inhabitant of American and European freshwater habitats. The present paper reviews the information related to the life cycle, ecology and potential ecological and economic impacts caused by C. fluminea in the invaded habitats. Furthermore, this paper also proposed future works that may be implemented in order to increase our general knowledge about the ecology of this bivalve.

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Ecology of the invasive Asian clam Corbicula fluminea (Müller, 1774) in aquatic ecosystems: an overview

The globalization of economies and trade have facilitated the spread of exotic species including the five most important freshwater suspension feeding invaders Dreissena polymorpha, D. bugensis, Corbicula fluminea, C. fluminalis, and Limnoperna fortunei. We suggest that the spread of these exotic species has not been a continuous process, but rather punctuated by periods of rapid long distance spread (jump), during which species greatly expanded their geographic ranges. Each jump has been associated with changes in the tempo of some human activity, such as the construction of shipping canals for trade, building of reservoirs for water storage and power production, political boundary changes or changes in political systems, which affected the position or permeability of national borders, human migration, changes in the mode and volume of international trade, or recent industrial practices and environmental laws. We hypothesize that the rate of spread of exotic species depends on the spatial scale of spread and may be accelerated or slowed by various human activities. In general, aquatic exotic species may quickly spread along connected waterways in a new continent they invade and soon reach their maximum range (continental scale). However, it will take much longer to colonize all isolated regions (regional scale) and longer still to spread to all isolated lakes and river systems (local scale). The difference in the rate of colonization across scales may be several orders of magnitude.

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Changes in Global Economies and Trade: the Potential Spread of Exotic Freshwater Bivalves

Since its invasion of Europe in the early 1980s, the Asian clam Corbicula fluminea has become very abundant in nearly all western river systems. Today this species is one of the most important biomass producers in the River Rhine. Monitoring the valve movements of C. fluminea over a period of 2 years revealed a circadian rhythm in summer, with extended periods (10-12 h) of valve closure, predominantly in the morning hours. Altogether valve movements were very scarce, frequently fewer than four movements per individual per day. Simultaneous measurements of heat dissipation and oxygen consumption (calorespirometry) revealed an intermittent metabolism in the clam. With the onset of valve closure, C. fluminea reduced its metabolic rate to 10% of the standard metabolic rate (SMR) measured when the valves were open. Nevertheless, this depressed metabolism remained aerobic for several hours, enabling the clam to save energy and substrates compared to the requirements of the tenfold higher SMR. Only during long-lasting periods of valve closure (more than 5-10 h) did the clams become anaerobic and accumulate succinate within their tissues (2 micromol g(-1) fresh mass). Succinate is transported into the mantle cavity fluid, where it reaches concentrations of 4-6 mmol l(-1). Because this succinate-enriched fluid must pass the gills when the valves open again, we suggest that this anaerobic end product is at least partly reabsorbed, thus reducing the loss of valuable substrates during anaerobiosis. Propionate was also produced, but only during experimental N2-incubation, under near-anoxic conditions. The intermittent metabolism of C. fluminea is discussed as an adaption to efficiently exploit the rare food supply, saving substrates by the pronounced metabolic depression during valve closure.

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Energy metabolism and valve closure behaviour in the Asian clam Corbicula fluminea.

This work received financial support from the Fauna Iberica Projects (DGICYT n° PB870397 and PB89 0081).

The Asiatic clam Corbicula fluminea (Müller, 1774) (Bivalvia: Corbiculidae) in Europe

Two populations of Corbicula fluminea were found in the Iberian Peninsula; one in Spain and the other in Portugal. A detailed description in terms of ecology shell morphology and microstructure, morphometrics and anatomy is given for the Spanish population from the Mino River. Lectotypes for Tellina fluminea and T. fluminalis, and a neotype for T. fluviatilis are designated and illustrated. Distribuuon and spread of C. fluminea in Europe are revised. Comparisons among some European populations and the populations from Canton, China, and the Mino River are made. Results suggest that, except for one doubtful population, all records of Corbicula in Europe are attributable to C. fluminea.

M. A. Ramos

Papers

The Asiatic clam Corbicula fluminea (Müller, 1774) (Bivalvia: Corbiculidae) in Europe

(Bivalvia: Corbiculidae) in Europe