|Left: Dorsal and ventral view of adult Halarachne halichoeri mite, Right: Mite in situ in the nasal passage of a seal.
Photos from Fig. 1 and 2 of the paper.
Due to a variety of human-related factors, including pollutants, habitat alteration, and excessive hunting, the number of grey seals and harbour seals had been dwindling in the Baltic and Wadden Sea since the start of 1960s. By the late 1970s, the number of Baltic grey seals waters were down to less than 4000 individuals. This seems to have had an effect on H. halichoeri population since no cases of these mites have been recorded from German waters since 1901, even though the mite continues to be reported from other areas where grey seals are found. In 1988, seal hunting was banned in the Baltic Sea, and the grey seal population started making a comeback - and it seems so has their nasal mites.
In a recent study, researcher examined the carcasses of six seals - four grey seals and two harbour seals - that were collected as a part of a wildlife monitoring network which screen marine mammal carcasses for various parasites. During this routine examination, they discovered that the seals were host to these nasal mites, with one of them found to have over 60 mites in its nasal passage. This was the first time that H. halichoeri has been recorded from German waters in over a century, though the authors also suggested that cases of these nasal mites are often under-reported, since the mites are very quick to escape from the nasal passage of a dead host, so many of them could have been lost while the carcasses were in transit.
Since H. halichoeri is a generalist parasite, it was able to maintain a viable population in the nasal passages of other marine mammals such other seals, sea lions, and otters during the period when the Baltic grey seals number dwindled, and were poised to make a comeback when its host population recovered. But that's not always the case for other species of parasites and symbionts. In the last decade or so, conservation biologists are starting to recognise that symbionts like parasites should also be targeted for conservation efforts, and co-extinction of symbionts along with their hosts is a major concern.
A recent report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) found that one million species are at risk of extinction due to environmental changes caused by human activities - however, that number is a vast underestimate given that all the animal and plants included that report are themselves host to a vast array of parasites and symbionts which have not been accounted for.
In this case, Halarachne halichoeri was able to remain in circulation in other marine mammals even as one of their hosts was being severely depleted, but that option might not be available for many others parasites that require multiple specific hosts to complete their life cycles, or just stick to the one host species for life - their fates are tied with that of their hosts, whether that means prosperity or extinction.
Reckendorf, A., Wohlsein, P., Lakemeyer, J., Stokholm, I., von Vietinghoff, V., & Lehnert, K. (2019). There and back again–The return of the nasal mite Halarachne halichoeri to seals in German waters. International Journal for Parasitology: Parasites and Wildlife 9: 112-118.