the occurrence of three-node motifs and present their normalized Z score, a measure for their abundance in a network (Fig. 4). Strikingly, the three fleets have practically the same motif distribution. In fact, the Z scores closely resemble those found in the World Wide Web and different social networks which were conjectured to form a superfamily of networks (Milo et al. 2004). This superfamily displays many transitive triplet interactions (i.e. if X → Y and Y → Z , then X → Z ); for example, the overrepresented motif 13 in Fig. 4, has six such interactions. Intransitive motifs, like motif 6, are comparably infrequent. The abundance of transitive interactions in the ship networks indicates that cargo can be transported both directly between ports as well as via several intermediate ports. Thus, the high clustering and redundancy of links (robustness to link failures) appears not only in the GCSN but also in the three subnetworks. The similarity of the motif distributions to other humanly optimized networks underlines that cargo trade, like social networks and the World Wide Web, depends crucially on human interactions and information exchange. While advantageous for the robustness of trade, the clustering of links as triplets also has an unwanted side effect: in general, the more clustered a network, the more vulnerable it becomes to the global spread of alien species, even for low invasion probabilities (Newman 2003b). 

## V. NETWORK TRAJECTORIES 

Going beyond the network perspective, the data base also provides information about the movement characteristics per individual ship (Table I). The average number of distinct ports per ship 〈 N 〉 does not differ much between different ship classes, but container ships call much more frequently at ports than bulk dry carriers and oil tankers. This difference is explained by the characteristics and operational mode of these ships. Normally, container ships are fast (between 20 and 25 knots) and spend less time (1 . 9 days on average in our data) in the port for cargo operations. By contrast, bulk dry carriers and oil tankers move more slowly (between 13 and 17 knots) and stay longer in the ports (on average 5 . 6 days for bulk dry carriers, 4 . 6 days for oil tankers). 

The speed at sea and of cargo handling, however, is not the only operational difference. The topology of the trajectories also differs substantially. Characteristic sample trajectories for each ship type are presented in Fig. 5a-c. The container ship (Fig. 5a) travels on some of 

