Backdating first records of non-indigenous species in the Mediterranean: are initial findings underestimated?

Περίληψη

This work lists 75 cases of backdated non-indigenous species (NIS) first records in the Mediterranean Sea which constitute 7.6% of the total marine NIS diversity of the region. Of the backdated first records, approximately one-third belong to species of Mollusca, with the second most important group being Macrophyta, followed by Polychaeta and Crustacea. The backdating time spans in our dataset vary between 1 and 52 years with an average of approximately 8 years but considerable differences exist among taxonomic groups. Among reasons for NIS backdating delays in publishing due to a lack of taxonomic expertise emerges as a prominent factor. Backdating in Mollusca is partly attributed to shell collectors, who may delay in publishing their findings. In fish species backdating is primarily attributed to unreported findings that were only accidentally noticed. Misidentifications of  species, often due to confusion with native or cryptogenic species, has also contributed to backdating, particularly in polychaetes. This challenge can be addressed by combining morphological identification with molecular analyses. A third reason for backdating
is the volume of overlooked species in museums and/or private collections. In many cases, multiple factors such as misidentification
and overlooked specimens in museum collections may contribute to large backdating intervals. It is expected that the number
of backdated NIS will increase as overlooked specimens in museum drawers or private collections are re-examined and as new
identification techniques are applied. Re-examination of older samples, using both morphological and molecular methods, may
reveal even earlier entries of NIS.