Evidence of avian-mediated long distance dispersal in American tardigrades

Abstract

Terrestrial tardigrades, commonly known as “water bears”, are part of a phylum of microscopic, aquatic invertebrates famous for cryptobiosis and space travel, but little is known about their modes of dispersal on Earth. Wind is assumed, but not truly demonstrated, to be the major method of global dispersal. Yet, some water bear distribution patterns cannot be explained by patterns of prevailing winds. Mammals and birds have been proposed as potential animal vectors. Importantly, most nearctic-neotropical migrant birds move north and south, with many crossing the equator, whereas prevailing winds move west to east or east to west but do not cross the equator. When multiplied by billions of birds over tens of millions of years, if the ectozoochory of tardigrades by birds is true then both regional and intercontinental patterns can be better explained. To test for the potential role of birds in tardigrade dispersal, the nests of 10 species for birds were examined. Seventy percent of nests were positive for tardigrades, demonstrating that some birds are in a position for transference. The carcasses of eight birds (six species) found dead from window strikes and a Sandhill Crane (Grus canadensis) found dead during routine surveys were also examined. Of the birds examined, 66% yielded tardigrades from two classes, three orders, and five species, including juveniles, adults, and eggs, suggesting that many bird species are potential vectors for many species of tardigrades. Our data support the hypothesis of avian-mediated long distance dispersal of tardigrades and provide evidence that further investigation is warranted.

“Everything is not everywhere”: Time‐calibrated phylogeography of the genus Milnesium (Tardigrada)

“…Given that low altitude tropical regions (which constitute the majority of the tropics) are characterised by similar climate conditions and habitats (mainly tropical and temperate climate types), tardigrade species exhibiting tropical preferences are likely to proliferate and dwell on any of the continents and islands along the equator provided they had a chance to arrive there. In contrast, longitudinal dispersal may be much more difficult, as the latitudinally arranged subtropical and tropical areas may constitute a significant barrier, sort of a firewall, for species with temperate climate preferences that could potentially be dispersed by birds migrating between the north and south hemisphere (Mogle et al, 2018 ; but note that direct evidence for avian dispersal is missing).…”

‘Everything is not everywhere’: time-calibrated phylogeography of the genus Milnesium (Tardigrada)

“…Given that low altitude tropical regions (which constitute the majority of the tropics) are characterised by similar climate conditions and habitats (mainly tropical and temperate climate types), tardigrade species exhibiting tropical preferences are likely to proliferate and dwell on any continents and islands along the equator provided they had a chance to arrive to there. In contrast, longitudinal dispersal may be much more difficult, as the latitudinally arranged subtropical and tropical areas may constitute a significant barrier, sort of a firewall, for species with temperate climate preferences that could potentially be dispersed by birds migrating between the north and south hemisphere (Mogle, Kimball, Miller, & McKown, 2018; but note that direct evidence for avian dispersal is missing).…”

Long-distance passive dispersal in microscopic aquatic animals

“…The only mobile animals that could allow meiofauna to disperse across large distances are migratory birds [109]: indeed microscopic animals have been found associated to birds. Since Darwin's time researchers placed feet of ducks in water and performed other experiments to find viable microscopic animals attached to or in the gut of migratory birds [7,[110][111][112]]. Yet, dispersal of meiofauna through birds may not be as effective as for seeds of plants [105], and can be considered plausible only for those microscopic animals that live in shallow water or in other habitats that are frequently visited by birds.…”