In recent decades the oceanographer's "tool box" has expanded in several directions: upward into space with advanced satellite imagery, downward with autonomous underwater vehicles, and we might say "inward" with DNA technology. Use of molecular genetic techniques is common in terrestrial biology, but their application to oceanographic questions is a more recent development.
In the aquatic environment introduction of foreign species to an ecosystem due to human activity poses a threat to natural biodiversity. Introduced species are responsible for millions of dollars per year in economic impacts to fisheries and biofouling of coastal industrial facilities.
In the marine environment, the main source of biological introductions is ballast water discharge. Ballast water is seawater pumped into large tanks built into the hulls of commercial vessels. The water provides stability for crossing ocean basins and is normally dumped when a ship reaches its destination. In many cases planktonic organisms are picked up from their native range, transported, and delivered to a new locale. In some cases the organism persists and may become dominant due to a lack of natural population controls such as predators and diseases.
In early February 1990, an exotic mollusc was discovered on the Port Aransas Jetty, at the mouth of Corpus Christi Bay, Texas. The mussel was identified as either Perna perna, the brown mussel from the Atlantic coasts of South America and Africa, or Perna viridis, the green mussel from the Indo-Pacific. Although this genus can be readily identified, there are problems in separating green and brown mussels because shell shape and color vary depending on ecological conditions.
Initially, little was known about this new resident on the Texas coast, including its precise identity, its geographic origin, or how it was delivered. I decided to address these questions using genetic techniques. To identify the mussel, I isolated, stained, and photographed its chromosomes. I confirmed its identity as Perna perna, since the cells contained 28 chromosomes, a diagnostic feature for this species.
Also in 1990, the green mussel appeared in Trinidad, and spread to Venezuela. The Texas mussels had not been firmly identified yet, and there was speculation that the exotic mussels on the Texas coast and in the Caribbean were the same species and that the introductions might be related. This study confirms that there were two unrelated introductions of two mussels belonging to the genus Perna in the western Atlantic Ocean in 1990.
Prior to running further genetic tests I obtained Port of Corpus Christi shipping records for the period preceding the discovery of Perna outside Corpus Christi Bay. Most cargo imported through the Port of Corpus Christi is crude oil, the majority of which comes from Venezuela. Shipping records showed that no vessel had entered the port from any other area within the natural range of the brown mussel. Therefore, if the mussel was delivered by a ship, the ship originated in Venezuela.
I obtained samples of brown mussels from natural and introduced populations. Analysis of their DNA sequences showed that the Texas brown mussels were closely related to the Venezuelan brown-mussel populations. We can be 86% certain that the Texas P. perna came from Venezuela.
Further analyses showed that natural populations of the brown mussel have high genetic variability, and detected no reduction of genetic diversity in the introduced populations. This indicates that a large number of larval colonists from Venezuela initiated the invasion in the gulf. High genetic variability allows an invading organism to adapt and persist in an environment outside of its natural range. It also provides yet another strong indication that ballast water probably brought Perna to the Gulf of Mexico. Ballast water discharge is the only mechanism that delivers large volumes of seawater capable of carrying thousands of individuals. The ability of ballast water to pick up and deliver planktonic organisms in sufficient numbers to counteract any genetically detectable bottleneck can be termed "gene pool capture."
The 28 chromosomes shown above belong to the species Perna perna.
Perna viridis has 30 chromosomes.
[22K] Map of Perna species distributions.
