Depends on what you mean by “link”. I currently use a saved query that I manually export as a .csv to update an ArcGIS dashboard I maintain with occurrence records from our database. But this is only a 1 way transfer of information, I don’t use ArcGIS to make changes to the Specify database, I just use it to visualize data.
The native ability to run spatial queries in specify is limited at present, (you can limit results of a query to be within a bounding box of lat and longs if you specify the corners.)
I think, at a basic level (for example, you just want to find records in Specify that match a complex spatial query you run in GIS), if you run the GIS query on a point layer that includes the specify table IDs of the locality records you’re interested in, you would be able to match those records against the Specify database by querying that table’s ID field using the “IN” operator and pasting the comma separated list of IDs (or using the API to do so)
You can create, update, and delete queries using the Specify API, and if you have your database configured to have an anonymous user, you don’t need a login token / credential to authenticate API requests. So it’s possible that you could set up a one or two-way system that linked a GIS point layer and the specify locality table. @mcruz @Grant and I had a discussion about this last year and it seems like there’s a lot of untapped potential for integrations between GIS databases and Specify, but as far as I know nobody has made such a link yet.