This study improves our understanding of the global distribution, biodiversity patterns, and endemicity of the brachyuran crabs in the Persian Gulf. Moreover, it addresses the effects of different environmental factors on the distribution, and biodiversity patterns. We used open-access data in combination with our new distribution dataset in the Persian Gulf and Gulf of Oman in order to fill knowledge gaps in the available open-access databases. The highest sampling effort, species richness and ES50 along with 21 endemic species of brachyuran crabs were observed in the Persian Gulf, the focus of this study. In addition, brachyuran crabs sampling effort and gamma species richness peaked at shallow water depth, supporting the paradigm that species richness decreases with increasing depth (Brandt et al., 2009; Costello and Chaudhary, 2017). Furthermore, we found a significant positive relationship between biodiversity measurements and temperature as the key driver, followed by calcite, nitrate, and dissolved oxygen, supporting previous findings (Sato et al. 2011; Romano and Zeng 2013; Saeedi et al. 2018; Sharifian et al. 2020; Knauber et al. 2023).
The Indo-West Pacific is the most species-rich region, with 16,508 species, most of which are benthic crustaceans (Costello et al. 2017). Prior patterns of high marine species richness at tropical latitudes suggested that this region is the hotspot of global species richness and high endemicity region of the world's ocean for crustaceans (Neiber et al. 2011; Costello and Chaudhary 2017; Saeedi et al. 2019b; Saeedi et al. 2022; Knauber et al. 2023), and for many marine taxa such as bivalves (Crame 2000; Jablonski et al. 2006; Krug et al. 2008; Saeedi and Costello 2012; Saeedi et al. 2017a; Saeedi et al. 2017b; Saeedi et al. 2019a; Saeedi and Costello 2019a), sharks (Lucifora et al. 2011), bony fish (Carpenter and Springer 2005; Briggs and Bowen 2012), ascidians (Shenkar and Swalla 2011), benthic marine algae (Kerswell 2006), anemones and corals (Bellwood et al. 2012; Fautin et al. 2013; Reimer et al. 2019). Our findings showed that the highest species richness and ES50, along with 21 endemic species and seven endemic species, were observed in the Persian Gulf and Gulf of Oman, respectively, as parts of the IWP. It appears that the IWP's great habitat heterogeneity and moderate temperatures have created ideal conditions for the speciation of marine organisms in this area (Brown 2014).
Several previous studies on the latitudinal patterns of species richness, revealed that the diversity of different marine taxa followed unimodal distributions with tropical peaks (Razouls et al., 2000; Rosa et al., 2008; Macpherson et al., 2010; Brown 2014). We observed that sampling effort was greatest at latitudes 30° N and 10° S, gamma species richness peaked around latitudes 25° and 30° N and ES50 predicted high species richness at latitudes 30° N without a peak at the equator in agreement with other findings (Powell et al., 2012; Chaudhary et al., 2016; Saeedi et al. 2017b; Saeedi and Costello 2019b). In a review of 27 published studies, similar bimodal patterns of species richness were found for most other marine taxa, with a dip near the equator latitudes (Chaudhary et al., 2016). These findings suggested that the latitudinal patterns of species richness may influenced by different factors such as local climate and oceanography (Snelgrove et al., 2016). Speciation may occurs at the tropic margins in response to greater temperature variations around the equator, leading many species to migrate north- or southward, and the locations of mid-latitude peaks varied between taxa.
According to the present study, brachyuran crabs richness decreased from shallow to deep waters, and the highest sampling effort and gamma species richness were correlated with most samples reported in depths above 200 m. The species richness is generally expected to decrease with increasing depth (Brandt et al., 2009; Costello and Chaudhary 2017; Saeedi et al. 2019b). Other studies, however, found a variable correlation between depth and species richness. For example, polychaete richness decreased with depth, whereas the richness of isopods peaked around 3000 m, and bivalves showed no clear correlation with depth (Brandt et al., 2007), suggesting that patterns of depth distribution might be taxon-specific. The variable relationship between species richness and depth can be explained by oxygen concentration, sediment and habitat (Levin and Gage 1998; Levin et al. 2001; Saeedi et al. 2020).
Sea-surface temperature, salinity, primary productivity, latitude, calcite, dissolved oxygen, phytoplankton, chlorophyll, current velocity and nutrients are the environmental variables most often proposed to affect the species richness and global biogeography of marine organisms such as crustaceans and molluscs (Fortes and Absalão 2004; Goetze 2005; Sato et al. 2011; Soucek and Dickinson 2012; Romano and Zeng 2013; Valentine and Jablonski 2015; Schiffer and Herbig 2016; Saeedi et al. 2017b; Knauber et al. 2023). Based on our findings, mean temperature, calcite, dissolved oxygen, and nitrate were highly correlated with the brachyuran crab species richness and other biodiversity measurements, which is consistent with previous studies (Sato et al. 2011; Romano and Zeng 2013; Saeedi et al. 2018; Sharifian et al. 2020; Knauber et al. 2023). In our dataset, 17,450 of total occurrence records and the high biodiversity measurements such as species richness peaked around latitudes 25 and 30°N in tropical and subtropical areas, and were lower in temperate and polar areas can explain the positive linear relationship between temperature and biodiversity measurements. Fewer species are found above 29°C which is at the equator and beyond the thermal optima for the brachyuran crabs. Therefore, it shows that the equator may already be approaching the thermal limitations of some species. Temperature as the main driver influences species distributions and may promote a greater population size and extinction resistance, higher metabolism and mutation rates, higher speciation potential and shorter generation times, and the growth speed and development of shallow-water and tropical species which ultimately affects species richness and endemicity (Pörtner 2001; Thatje et al. 2005; Chaudhary et al. 2017; Costello and Chaudhary 2017; Sharifian et al. 2020). Our study showed that calcite as a biomineral correlated with the biodiversity measurements and influenced the distribution of brachyuran crabs. Crustaceans use biominerals in the exoskeleton and gastroliths for growth, molting and fecundity (Sato et al. 2011; Costello and Chaudhary 2017). Based on our findings, nitrate exhibited negative correlations with biodiversity measurements, which is in agreement with other studies that have mentioned the toxicity of nitrate for crustaceans (Soucek and Dickinson 2012; Romano and Zeng 2013).
New shallow-water habitats and islands are formed during the early stages of collisions between tectonic plates that correspond to the locations of marine species richness hotspots (Renema et al. 2008; Provoost 2020). This process causes increasing habitat heterogenities, and opportunities for population isolation, which have been identified as drivers of species richness (Bellwood et al. 2005; Leprieur et al. 2016). These regions are influenced by recent and past historical and geological events (such as glaciation and deglaciation) resulted in repeated episodes of sea level expansion and contraction (Renema et al. 2008; Yasuhara et al. 2012; Leprieur et al. 2016; Yasuhara et al. 2017). The northwestern region of the Indian Ocean, including the Persian Gulf, Gulf of Oman, Arabian Sea, the Gulf of Aden and the Red Sea, has a high percentage of endemics in some marine groups such as crustaceans, fishes, echinoderms, and hard corals (Campbell 1987; Sheppard et al. 1992; Burt et al. 2011; DiBattista et al. 2013; DiBattista et al. 2016b). This rate of endemism is related to the hydrodynamic and geological history of this region, which has caused severe changes in temperature and salinity, especially in the two regions of the Red Sea and the Persian Gulf that play a key role in the formation of marine biodiversity through the accumulation and expansion of populations in association with the evolution of isolated marginal endemic species (Samyn and Tallon 2005; Barth and Khan 2008). The Red Sea, as a marginal sea at the northwestern corner of the Indo-Pacific region, despite its marginal location relative to the Indo-Pacific, has high biodiversity, including 300 corals and 1,078 fishes. (Sheppard 1991; Golani and Bogorodsky 2010). It is also a refuge for endemic marine species that evolved and survived glacial periods consisting of 33% crustaceans, 14% fishes, and 25% corals (DeVantier et al. 2000; Tsang et al. 2012; DiBattista et al. 2016a). The Persian Gulf has a similar position to the Red Sea and has been characterized by periods of desiccation and intermittent connection to the Indian Ocean during the Pleistocene glacial and interglacial periods, which along with changing environmental conditions that play a key role in shaping the current species distribution has been severely affected by the narrow and shallow Strait of Hormuz, seasonal temperature fluctuations and superior salinity because of low rainfall, high evaporation, and low inflow of freshwater environments (Yao 2008; Naderloo and Schubart 2009; Sale et al. 2011; DiBattista et al. 2016b). These factors are limitations for this region’s biodiversity, which may also cause evolutionary divergence and speciation of marine organisms such as brachyuran crabs by providing different environmental conditions. However, the rate of endemic species in the Persian Gulf is lower than in the Red Sea, where only 7% of crustaceans (Naderloo and Tuerkay, 2012) and 13% of the polychaete (Wehe and Fiege, 2002) are endemic to the Persian Gulf, and from other groups, no endemics have been reported. In general, additional research is required to understand biodiversity patterns, particularly analyses of species richness that are taxon-specific, in order to determine how distribution and species richness patterns vary among different taxa at local and large geographical scales. This basic knowledge is required for species management and habitat restorations, in order to estimate the best strategies to preserve both the species and their habitats.