Corbicula fluminea


Corbicula fluminea is a species of freshwater clam native to eastern Asia which has become a successful invasive species throughout North America, South America, and Europe. C. fluminea is commonly known in the west as the Asian clam or Asiatic clam. In Southeast Asia, C. fluminea is known as the golden clam, prosperity clam, or good luck clam.

Overview

Corbicula has enjoyed global success as an aquatic invasive species, having been introduced to a novel range including South America, North America and Europe. Human industrial activity, such as transport of larvae via ballast water in container ships, has been noted in the literature as a chief invasion vector. A market exists for Asian clams for human consumption in Japan, China, and other countries in the region. According to the United States Geological Survey, C. fluminea is likely to continue to expand its North American range until it reaches the maximum extent of its low temperature tolerance. The periostracum of the shell is normally yellow-green, with concentric growth rings of the prismatic layer visible through the proteinous outer layer. The periostracum can flake, allowing the white prismatic layer to show through. The shells exhibit a light purple nacre on the inside.

Life Cycle

Right after reaching maturity, these clams produce eggs, followed by sperm. Throughout adult life, Corbicula is a self-fertile simultaneous hermaphrodite which can broadcast spawn up to 570 mucoid larvae per day per individual, and more than 68,000 per year per individual. Larvae are ~200 microns in length when discharged from an adult and dispersed through water until becoming sessile adults. Adults can reach a length of about 5 cm.

Feeding

Corbicula fluminea is an active suspension feeder, and in the process of feeding by pumping water through its body. They feed primarily on phytoplankton, which they actively filter out the water, but may pedal feed on organic matter in the sandy or muddy bottoms of streams, lakes, or canals where the clam establishes a population.

Range

As a native species

This clam originally occurs in freshwater environments of Eastern Asia, including Russia, Thailand, the Philippines, China, Taiwan, Korea, and Japan. C. fluminea also occurs naturally in freshwater environments of Africa.

As an invasive species

Many coastal rivers with a heavy industrial shipping presence in the invaded range of C. fluminea sustain Asian clam populations. Various non-indigenous populations of C. fluminea include:

Human Vectors

Human activities are the chief reason for the wide dispersal of C. fluminea as an invasive aquatic organism. There is some evidence suggesting that Asian clams followed immigrating Asian communities to North America circa 1924 as a food source. C. fluminea, along with other exotic bivalve larvae, may be transported via ballast water to novel ecosystems and establish populations. The spread of Asian clams has been further exacerbated by other human activities such as the shellfish trade and accidental overland transport by recreational boaters bringing the clam into interior freshwater systems.

Life History Advantages

C. fluminea enjoys several physiological capabilities which are advantageous in promoting its invasion of novel lentic/lotic environments including:
Corbicula fluminea is a self-fertilizing, simultaneous hermaphrodite which can asexually produce internally-brooded, semi-buoyant planktonic larvae when spermiogenesis is induced at temperatures above 10°C. This allows C. fluminea to colonize novel habitats at an advanced rate.

Habitat Association

Studies on which abiotic habitat characteristics are most strongly associated with Asian clam population abundance have produced varying results. Brazilian habitats have been found to have support the largest abundances of invasive Corbicula spp. in areas with coarser dominant sediment fractions, while negatively correlated with increasing levels of organic matter. Others studies have shown abiotic habitat characteristics such as water redox potential, inorganic nutrient content, hardness, and organic matter content in tandem with the amount of very course sand combine to explain 59.3% of Corbicula population habitat association via statistical tests. Asian clam invasions seem to be limited by elevation, latitude as well as the minimum winter temperature of the ecosystem.

Impacts on Invaded Ecosystems

C. fluminea reworks the sediments it resides on through the process of bioturbation. Asian clams are considered biodiffusors similar to marine clam species due to their observed bioturbation activity which may negatively affect biogeochemical affect other members of the benthic community in invaded areas.
Corbicula has also been shown to profoundly influence community dynamics within the macrobenthos of invaded systems. Corbicula has been shown to remove as much as 70% of phytoplankton biomass in reaches of invaded rivers with a robust clam population. The clam has also been reported as causing a decline of dissolved oxygen in the water of the same river system with wide ranging second-order effects.
The primary economic and social impact of the invasion of C. fluminea has been billions of dollars in costs associated with clogged plumbing and, heat exchangers, or other human-created infrastructure. Ecologically, C. fluminea contributes to declines and replacement of highly vulnerable, already threatened native clams.

Global Invasion Pattern

The first recorded instance of Corbicula presence in the scientific literature in the Western Hemisphere was of its introduction into British Columbia circa 1924, followed by a spread throughout the Pacific Northwest and across the American south through South America. Invasions in Europe and Central America were more recent, first appearing in Caribbean countries in 1998.

Mitigation

They have been blamed for algal blooms and concerns exist they will outcompete and displace native species such as the montane pea clam and the ramshorn snail.Efforts are underway at Lake Tahoe to smother the clams on the bottom with rubber mats.

Taxonomy

Corbicula fluminea is a species of freshwater clam, an aquatic bivalve mollusk in the family Cyrenidae. C. fluminea is often confused with Corbicula fluminalis due to the two species' similar color and texture. Two species may be present in some introduced populations: C. fluminea and C. fluminalis. The names themselves are similar in the literature.

The ratio of width and height in C. fluminea is on average 1.1. In C. fluminalis it is smaller ; still, there is much variation and considerable overlap in shape. Most easily, they can be distinguished by the number of ribs on the shell; C. fluminea has 7 to 14 ribs per cm, C. fluminalis 13 to 28. This character is already clearly recognizable in very small specimens. In addition, when viewed from the ventral side, C. fluminalis is rounder, almost heart-shaped, while C. fluminea has a slightly flatter shape like a teardrop with a notched broad end. Small specimens of C. fluminalis are almost spherical, while those of C. fluminea are decidedly flattened. All these differences except the rib number are a consequence of C. fluminalis'' having a markedly more swollen, pointed and protruding umbo.