The World Register of Marine Species (WoRMS) is a species information backbone, providing an authoritative classification and catalogue of names of marine organisms. WoRMS contains the names, synonyms, and vernacular names, as well as references, images, biogeographic data, distributions, and traits, where those information are available. The information in WoRMS is curated and maintained at VLIZ, and a number of services to query for information in WoRMS are provided via its website (https://www.marinespecies.org/). The data system behind WoRMS is called Aphia. All the names in WoRMS are tagged with an unique and persistent ID: the AphiaID. Using the WoRMS RESTful services, one can obtain the information held in Aphia using the AphiaID as key, or using the names, external identifiers, classification and trait keywords, etc. The service AphiaIDsByAttributeKeyID is a query to get a list of AphiaIDs (max 50) with attribute tree for a given attribute ID.