What crab is it?

Originally from Asia, the brush-clawed shore crab appeared in Europe in 1993, likely transported through hull fouling or ballast water. The first specimens were found on a ship’s hull of a car-carrier in the harbor of Bremerhaven, Germany. However, no established reproductive population could be found at that time. A year later they started a real invasion at the Atlantic coast near La Rochelle quickly expanding their range north and south along French and Spanish Atlantic coasts. By the early 2000's the crab was encountered in the North Sea, e.g. in the intertidal alien Crassostrea-reefs in the Wadden Sea. Pacific oysters (Crassostrea gigas) have been invading the central Wadden Sea since 1998, predominantly settling on intertidal blue mussel (Mytilus edulis) beds which are increasingly transformed into Crassostrea-reefs. This habitat change is already considered to be a threat for waterbirds losing important feeding sites in the intertidal of the Wadden Sea and now it seems that this new habitat provides an ideal home for another invader.

Until 2005, researchers assumed that they were looking at one particular species, the brush-clawed shore crab Hemigrapsus penicillatus. Then two researchers from Japan described a new sibling species as Hemigrapsus takanoi. That left especially European scientists with a question as both species occur sympatrically in Japan and may thus also coexist in European waters because of their similar ecology or the invader is only one species, but which one?

The distinction of both species using morphological traits is extremely difficult and unreliable. That's were a new study from Germany comes in:

To clarify the identity of the alien species, two mitochondrial (partial COI, partial 16S rDNA) and two nuclear genes (partial sodium-potassium ATPase α-subunit, complete 18S rDNA) of several German and Japanese specimens were analysed. In addition to molecular analyses, key morphological characteristics were assessed. As such this is the first integrative approach providing a specimen-specific analysis and a comparative description of both native and invasive specimens.

The colleagues identified their samples from the Central Wadden Sea as Hemigrapsus takanoi. As it turns out most of the specimens from Japan and a set of GenBank sequences of brush-clawed crabs from Japan, Korea and China which were traditionally classified as Hemigrapsus penicillatus in Asia turned out to be Hemigrapsus takanoi. I leave the conclusion of this study to the authors:

The study underlines the difficulty in distinguishing H. takanoi from H. penicillatus on the basis of morphological characteristics alone. We therefore recommend additional molecular identification of the alien species along the coasts of the Northeast Atlantic because several independent introductions may have resulted in regionally displaced invasions of H. takanoi and/or H. penicillatus. Besides natural dispersal, range expansion of the crabs via Pacific oyster transportations between aquaculture facilities is assumed to have facilitated the spread of the species in Europe. Enhanced alertness in identifying the invader is thus required in regions where intensive aquaculture or mussel transfer is practiced. Special attention should also be paid in areas where port operations are expected to increase in the future.