Uncovering the hidden amphipod biodiversity and its drivers in the Persian Gulf

The Persian Gulf (PG), an epicontinental sea at the northwestern corner of the Indian Ocean presents a challenging environment in which species confront the upper limits of their environmental tolerance. Previous hypotheses suggested that the PG is a homogeneous ecosystem characterized by low species diversity and a limited number of endemic species. We investigated these hypotheses by studying the amphipods' distribution pattern and environmental drivers, a dominant benthic group in the PG. We compiled an extensive database of amphipod distribution by integrating the open-access data including the Ocean Biodiversity Information System (OBIS) and Global Biodiversity Information Facility (GBIF), literature mining on amphipods, as well as the author's sampling database from the Persian Gulf and the Gulf of Oman. Following careful data cleaning and quality control, the final dataset comprised 1411 distribution records of 134 accepted marine amphipod species collected from depths ranging from 0 to 100 m. The environmental variables were extracted from the Bio-ORACLE database for the benthic layer with the maximum depth. Species richness per hexagonal cells (alpha species richness), and ES15 (expected number of species per 15 random samples) were calculated. Our findings revealed higher-than-expected species richness and non-homogeneous amphipod distribution across the region. Two biodiversity hotspots were identified in the northern and northwestern parts of the PG and a lowspot of amphipod species diversity in the southern half. Beta diversity cluster analysis exhibited three distinct compositions of amphipod assemblages: a northwestern community near the Arvand (Shat AL-Arab) river, a northern assemblage along Iranian coasts and the Strait of Hormuz region, and a southern assemblage along Arabian coasts. Generalized Additive Models (GAMs) and General Linear Models (GLMs) outputs showed that all environmental variables, pH, and temperature were the most important drivers in delimiting the benthic species distributions and richness. Our findings emphasize the need for a detailed approach to understanding the distribution and diversity of marine organisms in the PG where data and knowledge are less shared openly. This region should not be treated as a homogeneous ecosystem, as it harbors many endemic and rare species threatened by anthropogenic activities such as oil extraction and ocean warming.