We assembled occurrence records (presence-only) for all four horseshoe crab species in Asia and Eastern America from our own observations, collaborators, scientific networks as well as through publishing a scratchpad site at http://horseshoecrabs.myspecies.info/. For many species, numerous distribution records exist in the literature, and we manually geo-referenced additional occurrence data from these sources.

Observadores del Mar is a marine citizen science platform launched in 2012 devoted to enhancing the understanding of the conservation status of marine ecosystems. The platform hosts 13 projects covering 8 main taxa: corals, jellyfishes, decapod crustaceans, fishes, seaweeds, seagrasses, seabirds and molluscs, in addition to two projects focused on marine litter and reporting information on two main topics: i) biodiversity data focusing mainly on species distribution and abundance, and ii) the impacts of anthropogenic activities (e.g. jellyfish blooms) and associated mid- to long-term changes (e.g. colonization of invasive species). Almost 5500 observations validated by scientists have been already collected resulting in more than 20 scientific papers and communications. The major findings have been new records of introduced and invasive species, tracking the spread of novel pen shell mortality outbreak in the Mediterranean Sea and monitoring microplastic concentration on beaches.

The dataset consists of abundance records of 43 marine species in the Mediterranean Sea from 2001 to 2020. Selected species include some protected and non-indigenous species, ecosystem engineers, species sensitive to climate change, and/or threatened by human activities. Taken together, these taxa represent key ecological aspects of Mediterranean subtidal habitats (Cerrano et al., 2017) and can be used to calculate the MedSens, an index of the environmental status of subtidal rocky coastal habitats based on species sensitivity (Turicchia et al., 2021). Observations were carried out by the EcoDivers (trained scuba divers, freedivers and snorkelers) according to a rigorous protocol (RCMed U-CEM) developed by the association Reef Check Italia onlus.