Invasive Species of Concern
What is Caulerpa, and What is Caulerpa taxifolia?
Caulerpa is a genus of green algae that includes about 75 different species. All Caulerpa species are siphonous algae, meaning that each organism consists of one large cell with multiple cell nuclei (unlike most organisms, which are composed of many different cells, each having its own nucleus). Nonetheless, Caulerpa species have structures that resemble roots, stems, and leaves:
- Their "roots", which are actually rhizoids, grow into the substrata or ocean floor, absorbing water and nutrients.
- Their leaf-like, photosynthetic structures are called fronds; each frond has many feather-like projections called pinnules.
- The fronds are interconnected by stolons, stem-like structures that grow above the ocean floor, and from which both rhizoids and new fronds emerge.
Caulerpa taxifolia is native to warm tropical waters around the world, such as:
- the West Indies and Africa (Atlantic Ocean)
- the coastal waters of Sri Lanka, Pakistan and western Australia (Indian Ocean)
- the Philippines, Indonesia, Japan, New Caledonia, and northern Australia (Pacific Ocean)
In areas where it is native, Caulerpa taxifolia is not invasive; its growth is controlled by factors such as grazing marine predators (for example, mollusks) that co-evolved with the alga.
It has been observed that Caulerpa species can reproduce both sexually and asexually, but sexual reproduction seems to be rare, occurring most often at warm temperatures. More commonly, Caulerpa species spread asexually by fragmentation: a small piece of the alga breaks off and is reestablished at another location. This is easy for them to do because of their being single-celled organisms. The invasive clone, or strain, of Caulerpa taxifolia shows little genetic variation, indicating that it propagates by fragmentation.
Like many other species of this genus, Caulerpa taxifolia has some unique growth characteristics, besides being siphonous, that promote its invasiveness in habitats where it is not native:
- It can "heal" wounds almost immediately after being damaged by producing a material that "plugs" the wound;
- it tolerates shade and cooler temperatures well, so it can grow during colder times of year (winter) and at greater depths than many other algae;
- it grows well in both sheltered and unsheltered environments;
- it produces a chemical that is quite toxic to grazing marine animals, especially animals that did not co-evolve with it.
Once a dense carpet of Caulerpa taxifolia forms, it drastically changes the ecology of the underwater area where it has spread. This has already been observed in the Mediterranean Sea, and is of great concern in the U.S. where the alga has been found in at least two sites in California.