A global analysis of the biogeography of species richness in razor clams (Solenidae) found the number of species was highest in the northern hemisphere, and dipped at the equator with a smaller peak in the southern hemisphere (Saeedi et al. 2016). Thus Chaudhary et al. (2016) reviewed previous studies and found that almost all latitudinal gradients in marine species richness peaked in the northern hemisphere, with a smaller southern hemisphere peak, and dip at the equator. This contradicted the prevailing paradigm that biodiversity peaks at the equator.

A response to this paper suggested that the pattern could be affected by sampling bias (Fernandez and Marques 2016). Thus Chaudhary et al. (2017) used data from OBIS to show that indeed sampling bias influenced the gradient. However, this effect was reduced when using gamma (total species in a latitudinal band) over alpha (average species in latitude-longitude cells in a latitudinal band). Furthermore, when adjusted for sampling effort using the ES50 index, the pattern was still bimodal with a dip at the equator, but the peaks in richness were equal in both hemispheres. We suggest that this may be because temperature is the main cause of the gradient and is getting too hot at the equator for some species.

These analyses would not have been possible without the integration of data across all taxa and geographic locations by OBIS.

Chaudhary C., Saeedi H., Costello 2016. Bimodality of latitudinal gradients in marine species richness. Trends in Ecology and Evolution, 31 (9), 670-676. http://dx.doi.org/10.1016/j.tree.2016.06.001

Fernandez, M.O. and Marques, A.C. 2017. Diversity of diversities: a response to Chaudhary, Saeedi, and Costello. Trends Ecology and Evolution 32 (4), 232–234. http://dx.doi.org/10.1016/j.tree.2016.10.013

Chaudhary C., Saeedi H., Costello 2017. Marine species richness is bimodal with latitude: a reply to Fernandez and Marques. Trends in Ecology and Evolution 32 (4), 234–237. http://dx.doi.org/10.1016/j.tree.2017.02.007

Saeedi, H, Dennis TE, Costello 2017. Bimodal latitudinal species richness and high endemicity in razor clams (Mollusca). Journal of Biogeography 44 (3), 592–604. DOI: 10.1111/jbi.12903

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