Know your limits; miniCOI metabarcoding fails with key marine zooplankton taxa

Eleven years after the publication of the first work applying deoxyribonucleic acid (DNA) metabarcoding to zooplankton communities, the commonly known "miniCOI" barcode is widely used, becoming the marker of choice. However, several primer combinations co-exist for this barcode and a critical evaluation of their performance is needed. This article reviews the misperformance of miniCOI metabarcoding with marine zooplankton communities, comparing them to microscopy and/or other universal markers. In total, misperformances were reported for 26 zooplankton taxa, including 18 copepods and five tunicates. We report a detection failure with Class Appendicularia and contrasting performances for Oithona similis (from good correspondence to detection failure), two worldwide abundant taxa with a crucial role in the marine pelagic realm. A combination of forward primer mismatches, the presence of long poly-T inserts and a low number of reference sequences would explain the failure to detect appendicularians. However, the contrasting performance with O. similis would correspond to distinct numbers of mismatches in the forward primer in different lineages within this cryptic taxon. This is reinforced by the report of similar patterns with other locally abundant zooplankton taxa. Therefore, we strongly call for the use of miniCOI in combination with alternative methods capable of addressing these limitations.