Abstract
Sea otters are a keystone species with cultural significance to Indigenous populations. The increased mortality of Northern sea otters (E. lutris kenyoni) near Kachemak Bay, Alaska, led to a surveillance study to determine the cause of the increased deaths. Samples EL1562 and EL1411 were obtained from two dead-stranded sea otters. Whole genomes of two different orthoreoviruses with all ten segments were obtained. All EL1562 genome segments show the highest similarity to Phocid orthoreovirus 1, a new species isolated from harbor seals. However, the σ1 protein exhibits high divergence (34% amino acid identity) that warrants re-evaluation of the species demarcation criteria. Cell culture and microscopy are consistent with EL1562 being similar to Phocid orthoreovirus 1. Based on binomial naming conventions, we suggest the species name Orthoreovirus marinarum, which encompasses both EL1562 and PhRV1. Characterization of EL1411 sequences shows that it belongs to the species Orthoreovirus mammalis, which includes members from terrestrial mammals. This study represents the first orthoreovirus cultured and genome sequenced from sea otters, and is only the second report of a mammalian orthoreovirus in marine mammals. This study provides new insights into the genetic diversity of orthoreovirus in marine mammals and the evolutionary linkage between terrestrial and marine orthoreoviruses. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1038/s41598-025-33400-0.