Occurrences of Callinectes sapidus in native and invaded habitats were retrieved on July 22nd, 2020 from the Global Biodiversity Information Facility (GBIF, www.gbif.org), Biodiversity Information Serving Our Nation (BISON, https://bison.usgs.gov), and the Ocean Biogeographic Information System (OBIS, www.obis.org). GBIF and BISON data comprised records from the citizen science initiative iNaturalist (https://www.inaturalist.org/). Citizen science, i.e. the involvement of volunteers in science, is making substantial contributions to large-scale international biodiversity monitoring41,42 and, as rapid flow of information on the occurrence of species is critical to implement effective monitoring actions, it also provides a precious opportunity to improve the information available on the distribution of NIS4345. Acknowledging this view, we retrieved iNaturalist occurrences with no copyright for any use and without any restriction recorded between July 10th and 22nd, 2020, as the most recent iNaturalist records included in GBIF and BISON dated July 9th, 2020 and July 21st, 2018 respectively.

The procedure adopted for the collation of records from the different online sources was implemented in the R environment46 and it is summarized in Fig. 1. A total of 55,815, 50,395, 52,216, and 124 records were retrieved from GBIF, BISON, OBIS, and iNaturalist respectively, using the function occ in the package spocc47.

Flow chart of literature search and data extraction. If not stated otherwise, numbers in the chart generally indicate the number of records. The asterisk indicates that after quality control, in iNaturalist occurrences were included 82 “research-grade” records plus one “needs-ID” record (Source ID = 1595170075) identified as Callinectes sapidus by the authors after examination of the information and photographic documentation provided in iNaturalist.

A quality check was performed on the four datasets to remove entries providing no year of record. Subsequently, invalid occurrences having no, or identical, or 0,0 coordinates were excluded using the functions cc_equ, cc_val, and cc_zero in the package Coordinate Cleaner48. Even though records of specimens preserved in museums or in other biodiversity facilities were generally eliminated together with entries having no coordinates, the function cc_inst was run to identify occurrences located in a radium of 100 m around each institution. One record referring to a specimen preserved since 2015 at the Northeastern University Marine Science Center in Nahant, Massachusetts, USA, was identified in the OBIS dataset (ID# a87a5c8d-11ec-4fc4-81ee-eba9959bafc4, 42.418698°N, −70.9074°E) and excluded.

Occurrences recorded with an inadequate spatial resolution (e.g., those reported in the field “coordinatePrecision” in GBIF with an accuracy of 0.01 decimal degrees or lower) were successively eliminated by removing entries whose coordinates were reported with less than three decimals, thus with an accepted indeterminacy of approximately 100 m. In addition, iNaturalist records included in both GBIF and BISON have exclusively a “research grade” status (RG hereafter), i.e., they are subjected to a Data Quality Assessment and are georeferenced with an advanced precision. Moreover, photographs are typically included in observations in order to be validated49. Consensus among at least two thirds of identifiers ultimately elevates a record to “research grade”. We adopted an identical selection criterion, thus only RG iNaturalist records were selected, with one exception: the record 53602467 (dating 19/07/2020, and located 40.28892°N, 16.777°E in the Basilicata Region, Southern Italy) was classified as “needs_id”, i.e., lacking a final agreement by at least 2/3 of the members of the iNaturalist community on its identification. The record was ultimately incorporated after a thorough comparison with other records in the area and examination of associated images.

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