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The nematode Anguillicoloides crassus is one of the many threats hanging over anguillid eels, now known to infect six Anguilla species worldwide. It was first described in Japan, in 1974, and is commonly thought to natively stem from East Asia. Here our primary objective was to critically evaluate this long-held statement. We first retraced the factual history of this global invader, to later investigate the pros and cons for an East Asian origin. After exploring the alternative scenarios for the joint origin of the two anguillicolid parasites occurring in this area, we concluded that the geographic zone covering the natural range of the local eel A. japonica is still the most probable origin (in the absence of another identified candidate host and area). However, we cannot exclude that A. crassus may have been previously introduced along with exotic eel species, at some early stages of aquaculture in Japan. We call for caution when dealing with the native origin of this and other parasitic invaders in provenance of East Asia, a region to be regarded as a major crossroads for fish and parasites of the world. We finally identified the need for a possible resolution of the question, which includes a deeper sampling effort in the Indo-Pacific zone and the further development of molecular phylogeographic studies of all five anguillicolid species and their hosts.

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Macroecological relationships provide insights into rules that govern ecological systems. Bergmann's rule posits that members of the same clade are larger at colder temperatures. Whether temperature drives this relationship is debated because several other potential drivers covary with temperature. We conducted a near‐global comparative analysis on marine copepods (97 830 samples, 388 taxa) to test Bergmann's rule, considering other potential drivers. Supporting Bergmann's rule, we found temperature better predicted size than did latitude or oxygen, with body size decreasing by 43.9% across the temperature range (‐1.7 to 30ºC). Body size also decreased by 26.9% across the range in food availability. Our results provide strong support for Bergman's rule in copepods, but emphasises the importance of other drivers in modifying this pattern. As the world warms, smaller copepod species are likely to emerge as ‘winners', potentially reducing rates of fisheries production and carbon sequestration.