First record of mussel larvae occurring in the mantle cavity of littoral-dwelling Mytilus spp. in Svalbard, Arctic

After a millennium, the blue mussels Mytilus have returned to the high Arctic region of Svalbard in the beginning of 21st century. Later it was found that not only M. edulis arrived in Svalbard, but a whole complex of sympatric species including M. edulis, M. galloprovincialis , M. trossulus and their hybrids, which may have a wide range of origins from both sides of the North Atlantic. The mussels, which first were found in the sublittoral at the mouth of Isfjorden on the west coast of Spitsbergen island, now are spreading in other areas, and recently were also found in the intertidal zone. Here we report on the finding of viable larvae at the late stages of development (up to pediveliger) that were recovered from the mantle cavity of mussels collected from the lower intertidal shore in late July 2019 and 2021 in Trygghamna, Isfjorden, Spitsbergen. Both the identity of the adult mussels and the larvae was confirmed using by sequencing of barcode region. Larvae ranged in size from 178 to 376 µm, averaging 295 ± 35 µm. They were recovered from mussel specimens ranging from 21 to 57 mm in length, and their ages were estimated, based on the number of rings, from 3 to 12 years. It remains unclear whether the retention of larvae within the mantle cavity results from the intake of near shore concentrations and whether it leads to larviphagy.