Cryptic invasion

Baseodiscus sp. (taken from http://www.ucmp.berkeley.edu)

Cryptic invasion occurs when native and non-native populations are difficult to distinguish from one another. This presents a challenging set of circumstances, where a non-native invader remains undetected or underestimated. Cryptic invaders can also displace native populations, often forcing difficult management decisions as to which populations to control. (taken from Albert Carbo's course website on Phragmites)

Doesn't that sound like a problem that has been designed for DNA Barcoding? Not convinved? Well, then let's talk about ribbon worms. These animals have their own phylum, Nemertea. They are usually less than 20 cm long,  but you can find several references to a find of one specimen that has been estimated at 54 m! Interesting animals, most of which are marine, but there are a few freshwater species, and even a few species that live in moist tropical habitats on land. Ribbon worms were once classified close to the flatworms, which they superficially resemble but more modern molecular analyses place them more closer to annelids and molluscs .The phylum comprises about 1000 species and they are difficult to tell apart:

Morphology-based nemertean taxonomy is a highly specialised discipline: proper fixation and histological procedures are essential for the correct morphological identification in several groups. Most of the material collected during previous marine expeditions was of poor quality for histological studies, due to differences in fixation and anaesthetisation protocols, and thus, the correct identification of those specimens has been impeded. Within the Nemertea, the taxon Palaeonemertea is a group that often lacks external morphologically discriminant characters; thus, an anatomical analysis is usually essential. Identification of members of the genus Cephalothrix, composed of 28 named species, which we analysed here, is based on very subtle and hard-to-interpret characters.

This is the introductory paragraph of a paper published earlier this year by researchers from the Museo Nacional de Ciencias Naturales in Madrid. Their work which was a combination of morphology and DNA Barcoding unveiled an ongoing invasion of a pacific species, Cephalothrix simula into the Mediterranean Sea:

We consider Cephalothrix simula to be an alien invader whose larvae could have been introduced to their distribution area in the ballast waters of ships. The presence of the same haplotypes in different invaded areas and in its natural distribution range suggests several invasion events. The Mediterranean Sea is one of the world’s regions most affected by biological invasions, mainly since the opening of the Suez Canal. However, the presence of Cephalothrix simula from the eastern Mediterranean Sea has not been reported, and the invasion of Atlantic localities cannot be explained by the hypothesis of a Lessepsian migration to the Mediterranean Sea. In this context, it is possible that environmental changes produced by climate change are currently facilitating the settlement of this species. The presence of developed gonads in one specimen and the presence of juvenile individuals reveal that reproduction is occurring in the invaded areas.

The authors also state that the utility of DNA Barcodes is currently hampered by the fact that there aren't many barcode sequences available for nemerteans. At the time they were writing the paper only about 6% of the species were barcoded. A quick look today on BOLD tells me that we are now at about 13%.